Charles M Falco

Interests

Research

Growth and Structure of Thin Films; X-Ray Optics; Metallic Superlattices; Magnetism; Superconductivity; Computerized Imaging

Teaching

Optical Sciences and Physics

Courses

Scholarly Contributions

Books

• Falco, C. M., Robinson, C. J., & Suzuki, T. (1989). Materials for Magneto-Optic Data Storage. Materials Research Society, Pittsburgh, 1989.
• Falco, C. M., Deaver, Jr., B. S., Harris, J. H., & Wolf, S. A. (1978). Future Trends in Superconductive Electronics.. American Institute of Physics, New York.

Chapters

• Falco, C. M. (2017). Light-Based Science : Technology and Sustainable Development, The Legacy of Ibn Al-Haytham. In Ibn al-Haytham and His Influence on Post-Mediaeval Western Culture(pp Chapter 8). CRC Press.
• Falco, C. M. (2008). Use of Optics by Renaissance Artists. In McGraw-Hill Yearbook of Science & Technology 2008(pp chapter). McGraw-Hill, 2008.
• Falco, C. M., & Keavney, D. J. (2002). Deposition Techniques for Magnetic Thin Films and Multilayers. In Spintronics—Spin Transport Electronics(pp Chapter). Plenum, New York, 2002.
• Falco, C. M., & Kasper, E. (1994). Electron Beam Evaporator Based MBE. In Advanced Silicon and Semiconducting Silicon-Alloy Based Materials & Devices(pp chapter 3). IOP, London, 1994.
• Falco, C. M. (1993). Metallic Multilayers and Superlattices. In Encyclopedia of Applied Physics(pp chapter). VCH Publishers, New York, 1993.
• Falco, C. M. (1988). Growth of Metallic and Metal-Containing Superlattices. In Physics Fabrication and Applications of Multilayered Structures(pp chapter 1). Plenum, New York, 1988.
• Falco, C. M., & Schuller, I. K. (1985). Electronic and Magnetic Properties of Metallic Superlattices. In Synthetic Modulated Structures(pp Chapter 9).
• Falco, C. M., & Schuller, I. K. (1982). Metallic Superlattices. In Microstructure Science and Engineering/VLSI(pp Chapter5, Vol. 4, p. 183).
• Falco, C. M. (1981). Thermoelectric Effects. In Superconductors in Non-Equilibrium Superconductivity, Phonons and Kapitza Boundaries(pp Chaper 16, p. 521). Plenum Press, New York, 1981.

Journals/Publications

• Guo, L., Liu, S., Chen, D., Zhang, S., Liu, Y., Zhong, Z., & Falco, C. M. (2016). Fabrication and Performance of Micron Thick CsI(Tl) Films for X-Ray Imaging Application. IEEE TRANSACTIONS ON NUCLEAR SCIENCE, 63(3), 1827-1831.
• Guo, L., Liu, S., Chen, D., Zhang, S., Liu, Y., Zhong, Z., & Falco, C. M. (2016). Structure and scintillation properties of CsI(Tl) films on Si single crystal substrates. APPLIED SURFACE SCIENCE, 384, 225-229.
• Shikoh, A. S., Ahmad, Z., Touati, F., Shakoor, R. A., Al-Thani, N. J., Zhu, Z., Mankowski, T., Benammar, M. A., Mansuripur, M., & Falco, C. M. (2016). Integration of the inexpensive CuNWs based transparent counter electrode with dye sensitized photo sensors. RSC ADVANCES, 6(58), 53123-53129.
• Zhu, Z., Mankowski, T., Balakrishnan, K., Shikoh, A. S., Touati, F., Benammar, M. A., Mansuripur, M., & Falco, C. M. (2016). Hybrid transparent conductive electrodes with copper nanowires embedded in a zinc oxide matrix and protected by reduced graphene oxide platelets. JOURNAL OF APPLIED PHYSICS, 119(8).
• Zhu, Z., Mankowski, T., Balakrishnan, K., Shikoh, A. S., Touati, F., Benammar, M. A., Mansuripur, M., & Falco, C. M. (2016). Sol-gel deposited aluminum-doped and gallium-doped zinc oxide thin-film transparent conductive electrodes with a protective coating of reduced graphene oxide. JOURNAL OF NANOPHOTONICS, 10(2).
• Zhu, Z., Mankowski, T., Shikoh, A. S., Touati, F., Benammar, M. A., Mansuripur, M., & Falco, C. M. (2016). Ultra-high aspect ratio copper nanowires as transparent conductive electrodes for dye sensitized solar cells. THIN FILMS FOR SOLAR AND ENERGY TECHNOLOGY VIII, 9936.
• Zhu, Z., Mankowski, T., Balakrishnan, K., Shikoh, A. S., Touati, F., Benammar, M. A., Mansuripur, M., & Falco, C. M. (2015). Ultrahigh Aspect Ratio Copper-Nanowire-Based Hybrid Transparent Conductive Electrodes with PEDOT:PSS and Reduced Graphene Oxide Exhibiting Reduced Surface Roughness and Improved Stability. ACS APPLIED MATERIALS & INTERFACES, 7(30), 16223-16230.
• Zhu, Z., Mankowski, T., Balakrishinan, K., Touati, F., Benammar, M. A., Mansuripur, M., Falco, C. M., Zhu, Z., Mankowski, T., Balakrishinan, K., Touati, F., Benammar, M. A., Mansuripur, M., & Falco, C. M. (2013). Hybrid thin-films of graphene materials and metallic nanowires for next generation transparent electrodes. Optics InfoBase Conference Papers.
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  Abstract: We report advances in the fabrication and characterization of transparent conductive electrodes based on thin-films of metallic nanowires (copper and silver) encapsulated with graphene, graphene oxide, and reduced graphene oxide, using solution-processed approaches. Renewable Energy and the Environment Congress. © 2013.
• Falco, C. M., & L., A. (2012). The optics of Jan van Eyck and Ibn al-Haytham. Proceedings of SPIE - The International Society for Optical Engineering, 8481.
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  Abstract: Ibn al-Haytham (Latinized as Alhazen or Alhacen) wrote nearly one hundred works on topics as diverse as poetry and politics. With his landmark seven-volume Kitāb al-Manāzir [Book of Optics], published sometime between 1028 [418 A.H.] and 1038 [429 A.H.], he made intellectual contributions that subsequently were incorporated throughout the core of post-Medieval Western culture, including its optics and art. We have used material from both the writings of Ibn al-Haytham and the paintings of Jan van Eyck in a workshop teaching modern optical principles to a variety of audiences. © 2012 SPIE.
• Hockney, D., & Falco, C. M. (2012). The science of optics: Recent revelations about the history of art. Proceedings of SPIE - The International Society for Optical Engineering, 8480.
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  Abstract: We have discovered a variety of types of optical evidence that demonstrate artists as early as Jan van Eyck and Robert Campin (c1425) used optical projections as aids for producing certain elements in their paintings. We also found optical evidence within works by well-known later artists, including Bermejo (c1475), Lotto (c1525), Caravaggio (c1600), de la Tour (c1650), Chardin (c1750) and Ingres (c1825), showing that the use of optical projections by artists continued up to the development of photography and beyond. However, it is important to emphasize this does not mean that paintings are effectively photographs. The mind as well as the hand of the artist is intimately involved in the creation process, so these complex images are much more than simply traced images that have been projected. © 2012 SPIE.
• Shuang, L., Falco, C. M., & Zhong, Z. (2011). Study of the micro-structure of Pt_xSi ultra-thin film. International Journal of Modern Physics B, 25(21), 2925-2929.
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  Abstract: Ultra-thin platinum (Pt) films were deposited on Si(100) substrates at 160°C by magnetron sputtering and subsequently annealed to form silicides. The thickness of the PtxSi films was found to be approximately 4 nm as determined by transmission electron microscopy (TEM). X-ray photoelectron spectroscopy (XPS) analysis shows that these films consist of PtSi and Pt 2Si phases, and a multi-layer configuration of SiO x/PtSi/Pt2 Si/Si was detected by angle-resolved XPS. However, the Pt3Si phase was not detected by X-ray diffraction (XRD). © 2011 World Scientific Publishing Company.
• Zeng, P., Liu, S., Wei, G., Zhang, S., Liu, Y., & Falco, C. M. (2011). Theoretical design and analysis: A ring resonator filter using er ³⁺/yb ³⁺-co-doped phosphate waveguides. Photonics Spectra, 45(7).
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  Abstract: An integrated optical waveguide ring resonator filter in which the ring resonator structure is fabricated on Er 2+/Yb 3+-co-doped phosphate glass using ion-exchanged techniques is proposed. This approach uses ion-exchange techniques to fabricate the integrated waveguide ring resonator filter, rendering a theoretical assessment of the filter characteristics and providing a new way to design a lossless ring filter. The bend loss changes with bend radius and the waveguide dimension, both of which dramatically affect the filter performance. By changing the round-trip optical round-trip length, a different free spectrum range (FSR) and spectrum width can be obtained. An ideal lossless ring filter can be fabricated on Er 3+/Yb 3+-codoped phosphate glass using ion-exchanged techniques to produce ideal loss cancellation.
• Falco, C. M. (2010). High-resolution infrared imaging. Proceedings of SPIE - The International Society for Optical Engineering, 7782.
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  Abstract: The hands and mind of an artist are intimately involved in the creative process of image formation, intrinsically making paintings significantly more complex than photographs to analyze. In spite of this difficulty, several years ago the artist David Hockney and I identified optical evidence within a number of paintings that demonstrated artists began using optical projections as early as c1425-nearly 175 years before Galileo-as aids for producing portions of their images. In the course of our work, Hockney and I developed insights that I have been applying to a new approach to computerized image analysis. Recently I developed and characterized a portable high resolution infrared for capturing additional information from paintings. Because many pigments are semi-transparent in the IR, in a number of cases IR photographs ("reflectograms") have revealed marks made by the artists that had been hidden under paint ever since they were made. I have used this IR camera to capture photographs ("reflectograms") of hundreds of paintings in over a dozen museums on three continents and, in some cases, these reflectograms have provided new insights into decisions the artists made in creating the final images that we see in the visible. © 2010 SPIE.
• Seu, K. A., Roy, S., Turner, J. J., Park, S., Falco, C. M., & Kevan, S. D. (2010). Cone phase and magnetization fluctuations in Au/Co/Au thin films near the spin-reorientation transition. Physical Review B - Condensed Matter and Materials Physics, 82(1).
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  Abstract: Using coherent soft x-ray scattering we have measured slow magnetization fluctuations in an Au/Co/Au heterostructure near a thermally driven spin-reorientation phase transition. The intermediate scattering function is well described by a stretched exponential, suggesting cooperative motion through the transition. The decay times were found to exhibit a pronounced maximum as a function of temperature. We argue that the transition proceeds through a cone phase in which the local magnetization evolves continuously from a perpendicular to longitudinal orientation. Our results demonstrate a different and fruitful way to probe the complex spatiotemporal dynamics that arise in unusual magnetic phases with competing anisotropies. © 2010 The American Physical Society.
• Falco, C. M. (2009). Invited Article: High resolution digital camera for infrared reflectography. Review of Scientific Instruments, 80(7).
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  PMID: 19655936;Abstract: This paper describes the characteristics of a high resolution infrared (IR) imaging system operating over the wavelength range of 830-1100 nm, based on a modified 8 Mpixels commercial digital camera, with which nonspecialists can obtain IR reflectograms of works of art in situ in a museum environment. The relevant imaging properties of sensitivity, resolution, noise, and contrast are characterized and the capabilities of this system are illustrated with an example that has revealed important new information about the working practices of a 16th century artist. © 2009 American Institute of Physics.
• Liu, S., Chen, X., Zhong, Z., & Falco, C. M. (2009). Influence of the crystal characterization of CsI thin film for X-ray image detectors. Proceedings of SPIE - The International Society for Optical Engineering, 7279.
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  Abstract: Powder CsI crystal has been deposited with vacuum thermal evaporation on three different kinds of substrates: Si, SiO2/Si and Pt/Si. We have analyzed and observed these CsI films with different depth and various preparation conditions by XRD measurement. Through analyzing, we find that in such process condition the crystal state of CsI film has a strong relationship with the crystal structure of substrate, and non-crystal substrate goes against crystallization. By contrasting standard XRD diagram of CsI(Tl), we discover that with the influence of the surface structure of substrate, CsI crystal film has a preferred orientation in (200) crystal face. We also notice that the preferred orientation of CsI film has a close relation with the depth of the film: the preferred orientation has been weakened as the depth of film turning from 707mu;m to 1007mu;m. copy; 2009 SPIE.
• Falco, C. M. (2008). Learning from monet: Renown artists and display design. IDW '08 - Proceedings of the 15th International Display Workshops, 1, 7-10.
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  Abstract: Recently the artist David Hockney and I identified optical evidence within a number of paintings that demonstrated painters as early as Jan van Eyck (c1425) used optical projections as aids for producing portions of their images. While making these discoveries, Hockney and I developed fundamentally new insights into image analysis. As discussed in this paper, I am now applying these new insights to problems in computerized image display and analysis.
• Falco, C. M. (2007). Computer vision and art. IEEE Multimedia, 14(2), 8-11.
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  Abstract: We routinely rely on art to understand science, but science is also used to better understand art. When David Hockney wrote in his book Secret Knowledge that he had "rediscovered" secret methods of the great painters, asserting that "from the early 15th century many Western artists used optics - by which I mean mirrors and lenses (or a combination of the two) - to create living projections," he sparked a debate that would motivate new lines of inquiry and research in a multitude of disciplines including art history, optics, image analysis, and more. Charles Falco later claimed he had proven Hockney's findings scientifically. In the last three "Artful Media" articles, David Stork presented applications of computer imaging for analyzing paintings, some of which challenge those claims. In this article, Falco defends his original findings. © 2007 IEEE.
• Falco, C. M. (2007). IBN al-Haytham and the origins of modern image analysis. 2007 9th International Symposium on Signal Processing and its Applications, ISSPA 2007, Proceedings.
• Falco, C. M. (2007). Ibn al-Haytham and the origins of computerized image analysis. ICCES'07 - 2007 International Conference on Computer Engineering and Systems, xx-xxiii.
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  Abstract: Ibn al-Haytham (Latinized as Alhazen or Alhacen) was born in Basra in 965 A.D. [354 A.H.], but produced nearly all of his work in Cairo's al-Azhar Mosque, where he wrote nearly one hundred works on topics as diverse as poetry and politics. Al-Haytham is primarily known for his writings on geometrical optics, astronomy, and mathematics, and for nearly four hundred years his treatment of the geometry of reflection from flat and curved surfaces has been known as "Alhazen's problem." However, as discussed in this paper, with his landmark seven-volume Kitāb al-Manāzir [Book of Optics], published sometime between 1028 [418 A.H.] and 1038 [429 A.H.], al-Haytham made intellectual contributions that subsequently were incorporated throughout the core of post-Medieval Western culture. His seminal work on the human vision system initiated an unbroken chain of continuous development that connects 21st century optical scientists with the 11th century Ibn al-Haytham. The noted science historian, David Lindberg, wrote that "Alhazen was undoubtedly the most significant figure in the history of optics between antiquity and the seventeenth century." Impressive and accurate as that characterization is, it significantly understates the impact that al-Haytham had on areas as wide-ranging as the theology, literature, art, and science of Europe.
• Falco, C. M. (2007). Novel expert-based approach to image analysis. 2007 9th International Symposium on Signal Processing and its Applications, ISSPA 2007, Proceedings.
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  Abstract: The hands and minds of all painters are intimately involved in the creative process, making paintings intrinsically complex to analyze. Although tracing projected images is known to have become a common technique by the 19th century, earlier use of optics has been difficult to identify and analyze, hindered also by the lack of interaction between art historians and scientists. In spite of this difficulty, the painter David Hockney and I recently identified optical evidence within a number of paintings demonstrating artists as early as Jan van Eyck (c1425) used optical projections as aids for producing portions of their images. While making these discoveries, Hockney and I developed fundamentally new insights into image analysis. As discussed in this paper, I am now applying these new insights to problems in computerized image display and analysis. ©2007 IEEE.
• Shaw, J. M., & Falco, C. M. (2007). Structure, spin dynamics, and magnetic properties of annealed nanoscale Fe layers on GaAs. Journal of Applied Physics, 101(3).
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  Abstract: We performed a detailed study of the effect of annealing (at temperatures up to 300 °C) on 0.2-3.0 nm thick epitaxial Fe layers deposited on GaAs(001). Using Brillouin light scattering, we studied the magnetic properties and spin dynamics of these layers and found a strong correlation between magnetic properties and the structure and chemical properties, as measured with electron diffraction, scanning tunneling microscopy, and x-ray photoemission spectroscopy. We found that significant changes in crystallinity and microstructure occur with annealing. Specifically, annealing of the thinnest layers results in the formation of a discontinuous magnetic layer with increased crystal order. At slightly larger thicknesses, faceted pits form in the Fe layer. This change in structure results in an earlier transition to a ferromagnetic phase, the creation of an additional higher frequency spin-wave mode, and a reduction in the magnetic uniaxial anisotropy constant. © 2007 American Institute of Physics.
• Shaw, J. M., Rippard, W. H., Russek, S. E., Reith, T., & Falco, C. M. (2007). Origins of switching field distributions in perpendicular magnetic nanodot arrays. Journal of Applied Physics, 101(2).
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  Abstract: We studied the reversal properties of perpendicularly magnetized CoPd nanodots from 100 to 50 nm in diameter fabricated using electron beam lithography. Polycrystalline CoPd multilayers show considerable differences in the switching field distribution (SFD) depending on the seed layer used. With a Ta seed layer, we reduced the SFD to approximately 5% of the average switching field. To rule out effects of grain boundaries, we also fabricated nanodot arrays from epitaxial CoPd superlattices. Although significant improvement in SFDs are obtained using epitaxial superlattices, our results indicate that grain boundary variation within nanodots is not the primary origin of SFD broadening that occurs with nanopatterning. © 2007 American Institute of Physics.
• Falco, C. M. (2006). No clear evidence to disprove optics thesis [2]. Nature, 439(7075), 392-.
• Reith, T., Shaw, J. M., & Falco, C. M. (2006). Effect of very thin Cr films on the magnetic behavior of epitaxial Co. Journal of Applied Physics, 99(8).
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  Abstract: A single monolayer (ML) of Cr was found to have a significant effect on the magnetic properties of subsequently deposited ultra-thin epitaxial Co layers. Brillouin light scattering (BLS) indicated that the Cr interlayer reversed the sign of the interface contribution to the total perpendicular magnetic anisotropy (PMA) of the Co. The negative value of the interfacial anisotropy term implied in-plane behavior. As a result, the samples with Cr became in-plane at about 11 ML of Co (1 ML of Co is about 0.2 nm), but the structures without Cr, while behaving qualitatively similar, exhibited higher total PMA and became in-plane at about 13 ML of Co. This behavior was also confirmed using perpendicular magneto-optic Kerr effect measurements. These results suggested that the in-plane nature of the interface anisotropy term for those samples with Cr was significant in determining their greater sensitivity to increased Co thickness. The impact of interdiffusion at the Cr/Co interface should not be neglected; by itself Cr is antiferromagnetic, and as an additive to Co and with no diffusion to grain boundaries (these films are single crystal), a greater effect from it is expected than from simple diluents such as Cu or Au. A monotonic decrease in magnetization with decreasing Co thickness was observed, but only for the samples with Cr, further suggesting the importance of interdiffusion. © 2006 American Institute of Physics.
• Shaw, J. M., Lee, S., & Falco, C. M. (2006). Overlayer-induced magnetic uniaxial anisotropy in nanoscale epitaxial Fe. Physical Review B - Condensed Matter and Materials Physics, 73(9).
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  Abstract: We used Brillouin light scattering to probe and quantify the effect Al overlayers have on the magnetic properties of epitaxial Fe layers grown on GaAs(001). In addition, we correlate the magnetic properties with structural properties obtained using reflection high-energy electron diffraction. We unexpectedly find that an epitaxial Al overlayer induces a significant volume component to the uniaxial magnetic anisotropy energy in 1.1-3.0 nm Fe layers of magnitude (2.5±0.2) × 105 erg cm3. Our data indicate that the origin of this volume component resides in the presence of an anisotropic strain and relaxation induced in the Fe layer. However, for thinner Fe layers, the overlayer suppresses the uniaxial magnetic anisotropy of Fe layers. We also find that the Al overlayer has no effect on the cubic magnetic anisotropy energy and effective magnetization as our measured values of these constants are consistent and in good agreement with previous reports of Fe layers with Au and Cu overlayers and those without an overlayer. © 2006 The American Physical Society.
• Tanner, C. E., Williams, T., Schwall, S., Ruggiero, S. T., Shaklee, P., Potashnik, S., Shaw, J. M., & Falco, C. M. (2006). Magneto-optic effects in ferromagnetic films: Implications for spin devices. Optics Communications, 259(2), 704-709.
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  Abstract: We present results on the magneto-optic properties of ferromagnetic films deposited on GaAs and SiO2 substrates. Using left- and right-circularly polarized light, we have measured the polarization-dependent photoresponse and reflectivity of Co/GaAs, Fe/GaAs and NiFe/GaAs Schottky diodes and the polarization-dependent reflection and transmission of NiFe/SiO 2 and Co/SiO2 structures as a function of ferromagnetic film thickness, reported here in the range of 7.5-15 nm. Films were prepared by sputtering and molecular-beam epitaxy. Measurements were made in the presence of magnetic fields ranging from -1.2 to +1.2 T both parallel and perpendicular to the sample surface. We find maximum polarization-dependent transmission and photoresponse effects (with respect to left- versus right-circularly polarized light) of 2-4% in magnitude. Taken together the work suggests that magneto-optic effects intrinsic to the films, rather than spin injection across the ferromagnetic/semiconductor interface, are responsible for the observed phenomenology. The work has direct implications for the interpretation of results in ferromagnetic/semiconductor spintronic systems. © 2005 Elsevier B.V. All rights reserved.
• Falco, C. (2005). Quantitative analysis of qualitative images. Proceedings of the Digital Imaging Computing: Techniques and Applications, DICTA 2005, 2005, 437-.
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  Abstract: Recently, renowned artist David Hockney observed that certain drawings and paintings from as early as the Renaissance seemed almost " photographic" in detail.[1] Following an extensive visual investigation of western art of the past 1000 years, he made the revolutionary claim that artists even of the prominence of van Eyck and Bellini must have used optical aids. However, many art historians insisted there was no supporting evidence for such a remarkable assertion. In this talk I will show a range of optical evidence for his claim that Hockney and I subsequently discovered during an unusual, and remarkably-productive, collaboration between an artist and a scientist.[2,3] These discoveries convincingly demonstrate optical instruments were in use - by artists, not scientists - nearly 200 years earlier than previously even thought possible, and account for the remarkable transformation in the reality of portraits that occurred early in the 15th century. As the examples in my talk will show, paintings are much more complex than if projected images simply had been traced. The new image analysis insights Hockney and I developed in our collaboration enabled us to overcome this complexity, allowing us to extract information that had eluded generations of scholars. [4] Because of this, these discoveries have significant implications for the fields of machine vision and computerized image analysis as well as for the histories of art and science. © 2005 IEEE.
• Falco, C. M., Shaw, J. M., & Reith, T. M. (2005). Spinwaves in spintronics structures. Proceedings - 2005 International Conference on MEMS, NANO and Smart Systems, ICMENS 2005, 41-.
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  Abstract: We have been applying in situ and ex situ spin-wave Brillouin Light Scattering (BLS) techniques to spinwave structures we are growing by Molecular Beam Epitaxy (MBE). The sensitivity of our BLS technique is illustrated by our ability to measure spin-wave excitations for samples as thin as 1 monolayer of Co buried by as much as 20 nanometers of Au. This talk will describe the use of in situ and ex situ BLS in combination with appropriate structural probes to determine the origin of several interesting and useful physical phenomena in spintronics structures. © 2005 IEEE.
• Hockney, D., & Falco, C. M. (2005). Optical instruments and imaging: The use of optics by 15 ^th century master painters. Proceedings of SPIE - The International Society for Optical Engineering, 5638(PART 1), 1-7.
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  Abstract: We discuss some of the details of the optical technology that was known at the dawn of the Renaissance. We shown that it is quite easy to fabricate concave mirrors of suitable focal length, diameter, and resolution for 15th century artists to have used to project images. The information in this paper complements information on the actual optical evidence that we have presented elsewhere.
• Hockney, D., & Falco, C. M. (2005). Quantitative analysis of qualitative images. Proceedings of SPIE - The International Society for Optical Engineering, 5666, 326-336.
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  Abstract: We show optical evidence that demonstrates artists as early as Jan van Eyck and Robert Campin (c1425) used optical projections as aids for producing their paintings. We also have found optical evidence within works by later artists, including Bermejo (c1475), Lotto (c1525), Caravaggio (c1600), de la Tour (c1650), Chardin (c1750) and Ingres (c1825), demonstrating a continuum in the use of optical projections by artists, along with an evolution in the sophistication of that use. However, even for paintings where we have been able to extract unambiguous, quantitative evidence of the direct use of optical projections for producing certain of the features, this does not mean that paintings are effectively photographs. Because the hand and mind of the artist are intimately involved in the creation process, understanding these complex images requires more than can be obtained from only applying the equations of geometrical optics. © 2005 SPIE and IS&T.
• Park, S., Zhang, X., Misra, A., Thompson, J. D., Fitzsimmons, M. R., Lee, S., & Falco, C. M. (2005). Tunable magnetic anisotropy of ultrathin Co layers. Applied Physics Letters, 86(4), 042504-1-042504-3.
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  Abstract: We prepared multilayers, consisting of an epitaxial Au/Co/Au/Cu(111) architecture on Si(111) substrates, to understand the magnetic anisotropy of an ultrathin Co layer in relation to its structure. The room-temperature magnetization of an 8-monolayer (ML)-thick Co layer is predominately either in-plane or out-of-plane depending upon the thickness of the Au underlayer. Specifically, for a Co film grown on a 2-ML-thick Au, the Co film has a distorted fee structure and in-plane magnetic anisotropy. For a Co film grown on a 6-ML-thick Au, the Co film has a distorted hcp structure and out-of-plane magnetic anisotropy. © 2005 American Institute of Physics.
• Shaw, J. M., & Falco, C. M. (2005). Thermal processing effects on ultra-thin ferromagnetic Fe layers grown on GaAs(0 0 1). Journal of Magnetism and Magnetic Materials, 286(SPEC. ISS.), 420-424.
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  Abstract: We used Brillouin light scattering and magneto-optical Kerr effect measurements to study the magnetic properties of ultra-thin 1.5 nm Fe layers undergoing various annealing and high temperature growth processes on GaAs(0 0 1). We found that the thermal processing has a profound effect on the form and magnitude of the magnetic anisotropy and value of the coercivity. In addition, in situ scanning tunneling microscopy revealed that high temperature post-deposition annealing results in the formation of rectangular pits, while high temperature growth results in a rough faceted microstructure. © 2004 Elsevier B.V. All rights reserved. © 2004 Elsevier B.V. All rights reserved.
• Falco, C. M. (2004). Optics and the old masters. Optics and Photonics News, 15(6), 7-.
• Park, S., Keavney, D. J., & Falco, C. M. (2004). Interdiffusion and thermal stability in magnetic tunnel junction ferromagnet/insulator/ferromagnet trilayer structures. Journal of Applied Physics, 95(6), 3037-3040.
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  Abstract: The air-oxidized magnetic tunnel junction structures grown by molecular beam epitaxy (MBE) were studied. The thermal stability of the trilayer structures was analyzed using x-ray photoelectron spectroscopy (XPS) and in situ scanning tunneling microscopy (STM). No oxidation of the bottom magnetic layer was shown in the unannealed trilayers for barriers thicker than 1.1 nm. The results upon annealing show that the metallic XPS peak of the top layers decrease dramatically, while that of the bottom and barrier layers increases.
• Ryan, P., Winarski, R. P., Keavney, D. J., Freeland, J. W., Rosenberg, R. A., Park, S., & Falco, C. M. (2004). Enhanced magnetic orbital moment of ultrathin Co films on Ge(100). Physical Review B - Condensed Matter and Materials Physics, 69(5), 544161-544166.
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  Abstract: Electronic and magnetic studies of Co films grown on Ge(100) are presented using two sample systems; incremental in situ Co depositions and a pre-made Co wedge structure. Both a magnetically inactive region and a corresponding Co-Ge intermixed region form at the interface of both systems. The Co grows in a uniform manner beyond this Co-Ge region with ∼2 Å perpendicular roughness. At low coverage of the in situ grown system, the Co exhibited an enhanced orbit to spin moment ratio, while, at higher Co coverages, we measured a total magnetic moment of 1.53 μB in agreement with previously published results of bcc Co grown on GaAs. The Co wedge indicated a constant but larger orbit-spin ratio along the wedge possibly due to the presence of an overlayer.
• Shaw, J. M., Park, S., & Falco, C. M. (2004). Structure and spin dynamics of annealed buried ultra-thin Fe layers on GaAs(001). Journal of Applied Physics, 95(11 II), 6552-6554.
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  Abstract: The spin dynamics in annealed ultra-thin Fe layers on GaAs(001) was analyzed using Brillouin light scattering (BLS). The samples used for investigation were grown by molecular beam epitaxy on GaAs(001). A critical annealing temperature existed at ≈225°C where Fe layer exhibited structural and magnetic behavioral change. The results show that annealing of room temperature deposited Fe layers to 300°C which had significant effect on magnetic properties and surface structure of Fe.
• You, C., Lee, S., Park, S., & Falco, C. M. (2004). Slant-perpendicular magnetic anisotropy axis induced by steps in 4°-miscut Si(111)/Cu/Au/Co/Au system. Physical Review B - Condensed Matter and Materials Physics, 69(13), 134402-1-134402-6.
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  Abstract: We find that the easy axis orientation of the Co layer in the 4°-miscut Si(111)/Cu/Au(2ML)/Co(5ML)/Au system is neither perpendicular nor in plane. Analysis of magneto-optical Kerr effect and Brillouin light scattering measurements for various angles of in-plane applied magnetic field with respect to the miscut direction reveals that easy axis orientation is slightly slanted from the film normal toward the direction of the miscut. Since there is no such slanted magnetic anisotropy axis in films grown on a nonmiscut Si substrate, and since the direction of incline is strongly correlated with the step direction, we conclude that the slanted magnetic anisotropy axis is induced by the steps in 4°-miscut Si(111) substrate.
• You, C., Lee, S., Park, S., & Falco, C. M. (2004). Step-induced slanted perpendicular magnetic anisotropy in 4°-miscut Si(111)/Cu/Au/Co/Au system. Journal of Applied Physics, 95(11 II), 7279-7281.
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  Abstract: The Brillouin light-scattering measurements and magneto-optic Kerr effect were performed on a 4°-miscut Si(111)/Cu/Au(2ML)/Co(5ML)/Au system. The correlation between slanted angle and magnetic anisotropy of the ultrathin Co layer was discussed. It was found that the easy axis orientation was neither in-plane nor perpendicular with respect to the sample surface. The direction of incline was also found to be correlated with the step direction. The results show that the steps in the 4°-miscut Si(111) substrate were the cause of the slanted magnetic anisotropy axis.
• Falco, C. M. (2003). The art and materials science of 190-mph superbikes. MRS Bulletin, 28(7), 512-516.
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  Abstract: The following article is an edited transcript of a talk presented in Symposium X - Frontiers of Materials Research at the 2002 Materials Research Society Fall Meeting in Boston on December 2, 2002. From Bessemer steel used on the first motorized bicycle in 1871 to sintered aluminum ceramic composites and TIN thin-film coatings used on standard production machines today, motorcycles have been at the forefront of the use of high-performance materials. Thanks to developments In materials technology, relatively Inexpensive mass-produced motorcycles are now capable of achieving speeds of >190mph.
• Falco, C. M., & Yencharis, L. (2003). Picture imperfect? (multiple letters). Advanced Imaging, 18(7), 5-.
• Hyomi, K., Murayama, A., Oka, Y., & Falco, C. M. (2002). Effects of perpendicular magnetic anisotropy on a large enhancement of elastic light scattering in ultrathin Co films. Journal of Magnetism and Magnetic Materials, 240(1-3), 386-388.
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  Abstract: The effects of perpendicular magnetic anisotropy (PMA) on a large enhancement of elastic light scattering (ELS) in ultrathin Co films were studied. The equilibrium position of magnetization in a Co film with a strong PMA was explained. Results showed that the maximum ELS intensity increased with the increasing Co thickness and the strong PMA realized Hcrit in the thicker Co films.
• Hyomi, K., Murayama, A., Oka, Y., & Falco, C. M. (2002). Misfit strain and magnetic anisotropies in ultrathin Co films hetero-epitaxially grown on Au/Cu/Si(1 1 1). Journal of Crystal Growth, 235(1-4), 567-571.
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  Abstract: The relation between misfit strain and uniaxial perpendicular magnetic anisotropy (PMA) has been studied in ultrathin Co single films hetero-epitaxially grown on Au/Cu/Si(1 1 1), by means of spin-wave Brillouin light scattering. The misfit strain in Co was intentionally controlled by the thickness of the Au-interlayer. The interface PMA with Cu-overlayers is proportional to the misfit strain, while the volume PMA is equal to a bulk value. We show Co-thickness dependences of PMA in the atomic-scale thickness region to be thinner than several monolayers (ML). The experimental values of the PMA are smaller than the calculated ones, while assuming constant volume and interface PMAs, when the misfit strain is large. In addition, the interface PMA is largely affected by ultrathin Au-overlayers, which indicate a significant deformation of the Co lattice near the upper interface after forming those overlayers. The interface PMA increases monotonically with increasing Au-overlayer thickness ranging from 1 to 3ML. © 2002 Published by Elsevier Science B.V.
• Hyomi, K., Murayama, A., Oka, Y., Hiller, U., & Falco, C. M. (2002). Surface magnetic anisotropy of Co(0001) and effects of ultrathin Cu overlayers studied by in situ spin-wave Brillouin light scattering. Applied Physics Letters, 80(2), 282-284.
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  Abstract: We have developed an in situ spin-wave Brillouin light scattering system, for the study of surface magnetic anisotropies under an ultrahigh vacuum. The surface uniaxial anisotropy constant is determined as -1.1mJ/m2 for a bare surface of Co(0001), indicating a strong in-plane anisotropy. With only 0.5 monolayer (ML) of Cu deposited on, the strong perpendicular anisotropy is induced, which is followed by an intensity peak at 1 ML of the Cu. We attribute these results to a steep change in the surface electronic state due to the Cu-atom deposition. © 2002 American Institute of Physics.
• Hyomi, K., Murayama, A., Oka, Y., Kondoh, S., & Falco, C. M. (2002). MBE growth of ultrathin Co films on a Si(1 1 1) surface with ultrathin buffer layers. Journal of Crystal Growth, 237-239(1-4 II), 1383-1387.
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  Abstract: We have grown hetero-epitaxially ultrathin Co films on a 7 × 7-Si(1 1 1) surface using buffer layers of Au and Cu, where the thickness of each film is controlled to atomic-scale dimensions. The film structure and related magnetic properties are investigated. The hcp-(0 0 0 1) planes are identified in Co deposited on the Au/Cu surface, while the fcc-(1 1 1) planes are dominant with a Cu-buffer layer. Perpendicular magnetic anisotropy constants in Co indicate sharp thickness-independent interfaces and bulk-like crystallinity even in a monatomic thickness region of < 2 monolayers of Co. Structure related inhomogeneities are quantified as a function of Co thickness for various overlayers by the spin-wave spectrum width. In the results, we show a successful fabrication of epitaxial growth of ultrathin Co films on the Au/Cu/Si surface with a well-defined magnetic structure. © 2002 Elsevier Science B.V. All rights reserved.
• Keavney, D. J., Park, S., & Falco, C. M. (2002). Diffuse interface electron scattering in epitaxial Co/Cu bilayers. Journal of Applied Physics, 91(10 I), 8108-8110.
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  Abstract: We have examined the origin of diffuse electron scattering at Co/Cu interfaces using in situ resistance measurements and scanning tunneling microscopy (STM) on a model system consisting of epitaxial (111)-oriented Co on Cu and Cu on Co. We grew epitaxial Co/Cu bilayers on 40 Å Cu(111)/Si(111) substrates, and monitored the resistance and surface morphology during the initial stages of growth for both Co on Cu and Cu on Co. For the case of Co on Cu, the resistance initially increases by 10% at submonolayer coverage, and then drops after 1-1.5 ML coverage. In situ STM topographs taken at similar stages of growth reveal that 20-30 Å Co islands initially nucleate above Cu step edges at submonolayer coverage and then grow inward to cover the Cu terraces. These islands introduce new steps at the surface, dramatically reduce the lateral correlation length of the surface profile, and consequently increase its contribution to the surface scattering resistance. We find that the nucleation and percolation of these islands is strongly correlated with the resistance behavior. In contrast, for Cu deposited on Co, we observe no island nucleation, and no corresponding resistance increase. © 2002 American Institute of Physics.
• Lee, S., Park, S., & Falco, C. M. (2002). Magnetic properties of Au/Co/Au films grown on vicinal Si(111) substrates. Journal of the Korean Physical Society, 40(5), 913-917.
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  Abstract: In this paper, we compare the magnetic properties of Au/Co/Au thin films grown by molecular bern epitaxy (MBE) on 4°-miscut Si(111) with those of Au/Co/Au thin films grown by MBE on non-miscut Si(111). The hysteresis curves measured using the magneto-optic Kerr effect showed strong perpendicular magnetic anisotropies (PMA) for both films. However, the spin wave frequencies measured using Brillouin light scattering showed that only those on non-miscut Si had a PMA, with those on 4°-miscut Si revealing the unusual spin wave frequency behavior. We speculate that Co deposited on 4°-miscut Si(111) has multiple domains, one domain having PMA and the others without PMA, which could be due to the high step density of vicinal Si(111) substrates.
• Lee, S., Park, S., & Falco, C. M. (2002). Magnetic properties of ultra-thin MBE-grown multilayers. Journal of Magnetism and Magnetic Materials, 240(1-3), 457-460.
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  Abstract: Recently, we have combined measurements of the magneto-optic Kerr effect with Brillouin light scattering (BLS) to compare the magnetic properties of molecular beam epitaxy (MBE)-grown Au/Co/Au films on 4°-miscut Si(1 1 1) with those of films grown on non-miscut Si(1 1 1). In both types of samples, the Kerr measurements indicate strong perpendicular magnetic anisotropy. However, the spin wave frequencies determined by BLS for the samples on 4°-miscut wafers exhibit anomalous behavior. We will discuss such BLS measurements on a variety of ultra-thin MBE-grown layered samples, showing the relationship of the magnetic properties to the evolving structure of the films. © 2002 Elsevier Science B.V. All rights reserved.
• Murayama, A., Hiller, U., Hyomi, K., Oka, Y., & Falco, C. M. (2002). In situ spin-wave Brillouin light scattering for the study of Co surfaces. Journal of Magnetism and Magnetic Materials, 240(1-3), 355-357.
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  Abstract: We have developed an in situ spin-wave Brillouin light scattering system for the study of surface magnetism of magnetic thin films. The surface uniaxial anisotropy constant is determined for a bare surface of 10 monolayers (ML) of Co(0 0 0 1), indicating the in-plane anisotropy. With only 0.5 ML of Cu deposited on this Co(0 0 0 1) surface, the strong perpendicular anisotropy is immediately induced, which is followed by a weak peak at 1 ML of the Cu. © 2002 Elsevier Science B.V. All rights reserved.
• Park, S., Lee, S., & Falco, C. M. (2002). Modification of the perpendicular magnetic anisotropy of ultrathin Co films due to the presence of overlayers. Journal of Applied Physics, 91(10 I), 8141-8143.
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  Abstract: In order to understand the effect of interfacial strain on perpendicular magnetic anisotropy (PMA) of ultrathin Co films, ex situ Brillouin light scattering measurements were performed for various overlayer materials. The samples were prepared by molecular beam epitaxy and Cu, Al, and Au were used as overlayer materials. We observed a decrease in PMA of the Co with the Cu and Al overlayers. In addition, we found an unexpected result that the PMA is significantly reduced when an Au overlayer of 35 Å is covered by an extra Al capping layer. The amount of this reduction depends on the thickness of the Al layer. Our results lead us to speculate that misfit strain at the interface between the Al and the Au can be propagated through the Au layer to affect the magnetic properties of Co. © 2002 American Institute of Physics.
• Keavney, D. J., Park, S., Falco, C. M., & Slaughter, J. M. (2001). Thermal stability of magnetic tunnel junctions studied by x-ray photoelectron spectroscopy. Applied Physics Letters, 78(2), 234-236.
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  Abstract: We have studied the evolution of chemical state of the metallic layers in NiFe/Al oxide/NiFe tunnel junction structures in as-deposited films and after postdeposition annealing. Both top and bottom NiFe layers in as-deposited films show significant Fe oxidation, but no Ni oxidation. This Fe is reduced in annealed samples, implying that oxygen migrates from the FeNi layers, possibly into the Al oxide layer. We also find that both top and bottom electrodes are significantly oxidized even in optimally annealed films. © 2001 American Institute of Physics.
• Gerardi, C., Tagliente, M. A., Vecchio, A. D., Tapfer, L., Coccorese, C., Attanasio, C., Mercaldo, L. V., Maritato, L., Slaughter, J. M., & Falco, C. M. (2000). Secondary ion mass spectrometry and x-ray analysis of superconducting Nb/Pd multilayers. Journal of Applied Physics, 87(2), 717-723.
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  Abstract: We report on accurate structural investigations of sputtered Nb/Pd multilayers by means of high-resolution secondary ion mass spectrometry and x-ray reflectivity. The combined use of secondary ion mass spectrometry and x-ray specular reflectivity techniques allows us to study the chemical configuration of the interfaces and to relate it to the observed superconducting properties. Secondary ion mass spectrometry analyses reveal a distinct Nb and Pd modulation and very sharp profiles with abrupt interfaces indicating a negligible interdiffusion of Nb and Pd at the interfaces. Moreover, analyzing the features in the Nb and Pd profiles and correlating them to the oxygen distribution in the multilayers and to the low-angle x-ray patterns, thin layers (3-4 nm thick) of niobium oxide were noticed at the Nb/Pd interfaces, while no oxide layers at the Pd/Nb interfaces could be detected. The role of this oxide layer in the determination of the crossover between three- and two-dimensional superconducting behavior in parallel external magnetic field, is discussed. © 2000 American Institute of Physics.
• Hockney, D., & Falco, C. M. (2000). Optical insights into renaissance art. Optics and Photonics News, 11(7), 52-59.
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  Abstract: The Hockney's observation about optical insights into renaissance art is presented. Some ancient photogrpahs contain enough information to make an estimate of the focal length of the lens used. The properties of the mirror lens, and introducing concepts of image analysis to the attention of art historians will open new areas of investigation understanding paintings of the past 600 years.
• Kikitsu, A., Murayama, A., Hyomi, K., & Falco, C. M. (2000). Micromagnetic study of CoPt-SiO₂ granular films by spin-wave Brillouin scattering. Journal of Applied Physics, 87(9 III), 6944-6946.
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  Abstract: A spin-wave Brillouin scattering study of a CoPt-SiO2 granular magnetic recording medium was made. This film contains ferromagnetic CoPt particles in a SiO2 matrix, and has an extremely low medium noise property due to little exchange coupling between magnetic grains. Spin waves of both the propagating surface mode and standing wave mode were found to be excited in granular magnetic films with various microstructures. A possible origin of the spin wave is a magnetostatic coupling between regularly ordered CoPt grains, as reported for artificially patterned magnetic thin films. This result shows two promising features of the CoPt-SiO2 granular film for high density recording medium: It is an ordered media obtained in a self-organizing manner, and it is less influenced by the thermal fluctuation effect, which is a serious problem for current high density magnetic recording. © 2000 American Institute of Physics.
• Murayama, A., Hyomi, K., & Falco, C. M. (2000). Large enhancement of elastic light scattering due to random fluctuations of the magnetization. Journal of Applied Physics, 87(9 II), 5505-5507.
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  Abstract: We have observed the field dependence and enhancement of the intensity of polarized elastic light scattering, from ultrathin magnetic films with perpendicular magnetic anisotropy (PMA). High-resolution Brillouin spectroscopy elucidates that this enhancement reaches its maximum at a critical field (Hcrit) where the magnetization starts to incline toward out-of-plane due to the strong PMA. We attribute this new effect to the microscopic instability due to random fluctuations of the magnetization near Hcrit, since this elastic scattering has no frequency shift from the excitation light and thus the correlation of each spin decays monotonically. The enhancement factor depends on the PMA strength as well as the film thickness, and we obtain an enhancement as large as a factor of 20 at room temperature with a low external field of 0.18 kOe. © 2000 American Institute of Physics.
• Murayama, A., Hyomi, K., Eickmann, J., & Falco, C. M. (2000). Brillouin study of long-wavelength spin waves in quasimonatomic Co films with uniaxial perpendicular magnetic anisotropy. Physical Review B - Condensed Matter and Materials Physics, 61(13), 8984-8992.
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  Abstract: We have observed spin-wave Brillouin light scattering from ultrathin Co/Au/Cu(111) films with Co thicknesses tCo down to 1 monolayer (ML) and with a 1-ML Au interlayer. The detection of a well-defined spin-wave spectrum and the field dependence of its frequency show directly long-range collective and ferromagnetic ordering in these films at room temperature. From the field dependence of the spin-wave frequency, we derive uniaxial perpendicular magnetic anisotropy constants as a function of tCo with various overlayer materials, including Cu, Pd, and Au. With a Cu overlayer, we observe that the first-order perpendicular anisotropy Ku(1) obeys well a linear relation between Ku(1)tCo and tCo for tCo≧1.5 ML, which indicates a constant contribution of the interface anisotropy of 0.16 mJ/m2 in addition to the volume anisotropy of 0.73 MJ/m3. With an Au or a Pd overlayer, we find that both the interface and volume anisotropies are significantly larger than those with the Cu overlayer. We quantify magnetic inhomogeneities from the field dependence of the spectrum width. With the Au or Pd overlayer, Ku(1) shows a steep decrease with decreasing tCo for tCo
• Murayama, A., Hyomi, K., Eickmann, J., & Falco, C. M. (2000). Effects of misfit strain due to epitaxial growth on interface perpendicular magnetic anisotropy in ultrathin Co/Au/Cu(111) films. Journal of Applied Physics, 87(9 III), 6107-6109.
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  Abstract: We observe a systematic increase in interface perpendicular magnetic anisotropy (PMA) with increasing Au-interlayer thickness tAu ranging from 1 to 5 monolayers (MLs) in Cu/Co/Au/Cu(111), where misfit strain in Co due to the epitaxial growth increases with increasing tAu. This result can be understood within the framework of the magneto-elastic contribution to the interface PMA. With a Cu overlayer, a constant contribution of interface PMA as well as of volume anisotropy is observed when the Co thicknesses are greater than 1.3 ML. However, we have found an unexpected suppression of this variation of interface PMA with the use of Au overlayers instead of Cu ones. With Au or Pd overlayers, the PMA starts to degrade as the Co thickness is reduced below 2.5 ML. © 2000 American Institute of Physics.
• Eremenko, V. V., Novosad, V. A., Pishko, V. V., Falco, C., & Rashkovan, V. M. (1999). Microwave transmittance of a high-T_c superconductor film in a magnetic field. Low Temperature Physics, 25(5), 384-385.
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  Abstract: The results of microwave transmittance measurements of crystalline high-Tc YBaCuO film under the influence of an external magnetic field are presented. Generally, in high-Tc superconductors dissipation mechanisms different from those in conventional superconductors may take place as well as transport current. Josephson-junction and anisotropy resistance-connected processes. Measurements of transmittance induced by the magnetic field demonstrate the dominance of flux flow dissipation mechanism and nonlinear transmittance dependence when approaching Tc. This makes possible to detect and characterize all the mechanisms mentioned above. © 1999 American Institute of Physics.
• Hyomi, K., Murayama, A., Eickmann, J., & Falco, C. M. (1999). Magnetic inhomogeneity in ultrathin Co films studied by spin-wave Brillouin scattering. Journal of Magnetism and Magnetic Materials, 198, 528-530.
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  Abstract: We have studied magnetic inhomogeneity by means of spin-wave Brillouin scattering in ultrathin Co films. The distribution of perpendicular anisotropy causes a field-dependent broadening of the spin-wave spectrum. The anisotropy distribution shows a minimum for 1 monolayer (ML) of Au-interlayer at Co/Cu(1 1 1). The Co-thickness dependence of the spectrum width shows a significant increase in the magnetic inhomogeneity below 2.5 ML of Co.
• Hyomi, K., Murayama, A., Eickmann, J., & Falco, C. M. (1999). Perpendicular magnetic anisotropy in Au/Co/Au(1 1 1) films: Interface anisotropy and effect of strain. Journal of Magnetism and Magnetic Materials, 198, 378-380.
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  Abstract: We have studied perpendicular anisotropy in ultrathin Au/Co/Au(1 1 1) films. Misfit strain in Co grown on the Au-underlayer increases monotonically with increasing Au thicknesses from 0 to 5 monolayer (ML). We find that the interface anisotropy shows an abrupt increase for 2 ML of Au-underlayer. The volume anisotropy is larger than the magneto-crystalline anisotropy.
• Keavney, D. J., Park, S., Falco, C. M., & Slaughter, J. M. (1999). Specular and diffuse electron scattering at interfaces in metal spin-valve structures. Journal of Applied Physics, 86(1), 476-479.
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  Abstract: We have made thickness dependent in situ resistivity measurements on sputtered metal films in real time during film deposition. These measurements allow the separation of the bulk and finite-size contributions to the resistance. The metals studied were Co, Cu, Ni80Fe20, and Ta, deposited in differing orders to isolate the scattering at interfaces relevant to common spin-valve structures. We see sources of excess diffuse scattering in bilayers of Ta with the 3d metals, regardless of the deposition order. We see a similar effect for Co on Cu, but not for Cu on Co. In some cases, we see significant changes in film resistance from overlayers as thin as 2 Å. These results show that the two Cu interfaces in a spin valve are not equivalent, and that other interfaces in the structure may be important. We discuss the origin of the diffuse scattering in terms of interdiffusion or a roughening transition during the initial deposition and island coalescence. © 1999 American Institute of Physics.
• Kikitsu, A., & Falco, C. M. (1999). New self-organized nanostructure: Co ON Au/Cu/Si. Digests of the Intermag Conference, HC-06.
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  Abstract: A new self-assembled nanostructured material was fabricated by electron beam deposition in ultra-high vacuum of less than 1.2×10-10 Toor. The resulting material exhibits interesting magnetic properties.
• Murayama, A., Hyomi, K., & Falco, C. M. (1999). Enhancement of elastic-light scattering due to the magnetization instability in ultrathin magnetic films. Physical Review B - Condensed Matter and Materials Physics, 60(9), 6281-6284.
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  Abstract: We have observed magnetic-field-dependent enhancements as large as a factor of 20 of the intensity of elastic-light scattering, from ultrathin magnetic films with perpendicular magnetic anisotropy. High-resolution Brillouin spectroscopy shows that this enhancement reaches its maximum at a critical field (Hcrit) where the magnetization starts to incline toward out-of-plane due to the perpendicular anisotropy. We attribute this effect to the microscopic instability due to random fluctuations of the magnetization near Hcrit. ©1999 The American Physical Society.
• Murayama, A., Hyomi, K., Eickmann, J., & Falco, C. M. (1999). Low-frequency excitations of spin-wave Brillouin scattering in ultrathin Co/Au(1 1 1) films. Journal of Magnetism and Magnetic Materials, 198, 372-374.
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  Abstract: We have observed low-frequency excitations of spin-wave Brillouin scattering in ultrathin Co/Au(1 1 1) films with a large perpendicular magnetic anisotropy. With several monolayers (ML) of Co, we observe diffuse scattering under an external field just below a critical field at which the magnetization starts to incline toward out-of-plane. With Co films thicker than 6 ML, we find a new mode at fields lower than the critical field, in addition to a surface spin wave.
• Murayama, A., Hyomi, K., Eickmann, J., & Falco, C. M. (1999). Spin-wave Brillouin scattering in quasimonatomic Co films. Journal of Applied Physics, 85(8 II A), 5051-5053.
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  Abstract: We have observed spin-wave Brillouin scattering in ultrathin Co/Au/Cu/Si(111) films with the Co thicknesses tCo down to one monolayer (ML). A graded Co-wedge sample with tCo=0-7 ML was prepared on 1 ML of Au interlayer by molecular beam epitaxy, where the thickness was calibrated by determining the number of deposited atoms using Rutherford backscattering spectroscopy. Observations of the spin-wave Brillouin excitation show directly collective and ferromagnetic ordering at room temperature. The tCo dependence of uniaxial perpendicular anisotropy is obtained from the field dependence of the spin-wave frequency. In addition, we quantify magnetic inhomogeneities from the field dependence of the spectrum width. With tCo≤2 ML, the perpendicular anisotropy shows a steep decrease with decreasing tCo, as well the structure-related magnetic inhomogeneity significantly increasing. © 1999 American Institute of Physics.
• Murayama, A., Hyomi, K., Eickmann, J., & Falco, C. M. (1999). Strain dependence of the interface perpendicular magnetic anisotropy in epitaxial Co/Au/Cu(111) films. Physical Review B - Condensed Matter and Materials Physics, 60(22), 15245-15250.
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  Abstract: We observe a systematic increase in interface perpendicular magnetic anisotropy (PMA) with increasing Au-interlayer thickness tAu ranging from 1 to 5 ML in Cu/Co/Au/Cu(111), where misfit strain in Co due to the epitaxial growth increases with increasing tAu. In addition, we have found an unexpected suppression of this variation of interface PMA with the use of Au overlayers instead of Cu. Also, the volume PMA significantly increases with the Au overlayer. ©1999 The American Physical Society.
• Attanasio, C., Coccorese, C., Mercaldo, L. V., Salvato, M., Maritato, L., Lykov, A. N., Prischepa, S. L., & Falco, C. M. (1998). Angular dependence of the upper critical field in Nb/CuMn multilayers. Physical Review B - Condensed Matter and Materials Physics, 57(10), 6056-6060.
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  Abstract: We present data related to the temperature (T) and angular (θ) dependencies of the upper critical field (Hc2) of Nb(superconductor)-CuMn(spin glass) multilayers as a function of CuMn layer thickness and Mn concentration. We observe two-dimensional (2D) behavior for large CuMn thickness. As the CuMn layer thickness is decreased, the Hc2(T) curves correspond to three-dimensional (3D) behavior, while the Hc2(θ) dependencies measured in the range [-1.5°,+1.5°] are more sensitive to the dimensionality of the system. In particular, the experimental data reveal a 3D-→2D crossover at very low angle when the Mn percentage is increased at fixed CuMn thickness. This behavior can be related to vortex dimensionality change in anisotropic superconductors.
• Coccorese, C., Attanasio, C., Mercaldo, L. V., Salvato, M., Maritato, L., Slaughter, J. M., Falco, C. M., Prischepa, S. L., & Ivlev, B. I. (1998). Vortex properties in superconducting Nb/Pd multilayers. Physical Review B - Condensed Matter and Materials Physics, 57(13), 7922-7929.
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  Abstract: We have investigated the upper critical magnetic field Hc2, the critical current density Jc, and the pinning force Fp in sputtered Nb/Pd multilayers, varying the temperatures T, the Pd thicknesses dpd and the magnetic field H orientation (parallel and perpendicular to the plane of the sample). In perpendicular fields, the vortex dynamics was strongly influenced by grain-boundary pinning. In parallel fields, a peak was observed in the JC(H) curves for samples with Pd thicknesses dPd>100 Å. After comparing the experimental results with the existing theories, we have related the presence of this peak effect to the matching of vortex kinks with the layered artificial structure.
• Kaneko, S., Hiller, U., Slaughter, J. M., Falco, C. M., Coccorese, C., & Maritato, L. (1998). Superconducting transition temperatures and structure of MBE-grown Nb/Pd multilayers. Physical Review B - Condensed Matter and Materials Physics, 58(13), 8229-8231.
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  Abstract: We have studied the structure and superconducting properties of molecular-beam-epitaxy-grown Nb/Pd multilayers. The resistivity of each layer was calculated from the multilayer resistivity by including size effect and bulk imperfection terms. The superconducting transition temperatures obtained using these resistivities together with the de Gennes-Werthamer theory showed good agreement with the experimental results.
• Kikitsu, A., Falco, C. M., & Mansuripur, M. (1998). Kerr effect enhancement by photon tunneling and possible application to a new scanning probe magnetic microscope. Journal of Applied Physics, 83(11), 6232-6234.
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  Abstract: Magneto-optical effects are calculated for the film stack consisting of hemisphere glass/magnetic film (10 nm)/air gap (d nm)/glass plate. Polarized light (wave length=800 nm) is irradiated through the hemisphere glass in the total internal reflection configuration. A typical amorphous rare earth-transition metal alloy is used for the magnetic layer. We find a large monotonic change in the figure of merit (product of the reflected amplitude of light and the Kerr rotation angle) as a function of the air gap, ranging from 1 to 800 nm. Similar results are obtained for a magnetic film with a 10 nm SiO2 protective layer and for a 1-nm-thin magnetic film. This phenomenon is mostly caused by a change in the reflectivity at magnetic film/air interface due to photon tunneling. The difference in the figure of merit between perpendicular and longitudinal magnetization is about 0.6°. These results imply that it might be possible to obtain an image of perpendicular magnetic moment with photon scanning tunnel microscopy (STM). This method can be combined simultaneously with a conventional atomic force microscope or STM. © 1998 American Institute of Physics.
• Murayama, A., Hyomi, K., Eickmann, J., & Falco, C. M. (1998). Effect of an Au imderlayer on perpendicular magnetic anisotropy in An/Co/Au(111) sandwich films. IEEE Transactions on Magnetics, 34(4 PART 1), 1198-1200.
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  Abstract: We have used spin-wave Brillouin scattering to study perpendicular magnetic anisotropy in Au/Co/Au(111) sandwiched films with varying thicknesses of Au-underlayers. By using 5-monolayer (ML) -thick Co films, the transition of spin-wave frequency between out-of-plane and in-plane magnetizations can be observed by varying the in-plane field. We find that the field-dependence of the spin-wave energy is explained by a calculation including both the first-order (second power) and the second-order uniaxial perpendicular anisotropies depending on the Au-underlayer. We observe saturation of the anisotropy at 5 ML of Au-underlayer thickness, which corresponds to the saturation of in-plane lattice expansion of Co on that Au-undcrlayer. The contribution of interface anisotropy is evaluated from the Cothickness dependence of the anisotropy observed. We also have observed a field-dependent broadening of the spin-wave Brillouin spectra at a critical field between out-of-plane and inplane magnetizations, which we attribute to a distribution of the first-order anisotropy. -. © 1998 FREE.
• Murayama, A., Hyomi, K., Eickmann, J., & Falco, C. M. (1998). Effect of an ultrathin Au interlayer at a Co/Cu(111) interface on the magnetic properties. Journal of Applied Physics, 83(1), 613-615.
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  Abstract: We have found from spin-wave Brillouin spectra that a half-monatomic thick Au interlayer at a Co/Cu(111) interface causes a significant improvement in magnetic uniformity in the Co ultrathin overlayer film deposited. The field-independent Brillouin spectrum width abruptly decreases with the insertion of the Au-interlayer, and an inhomogeneous transition at a critical field between out-of-plane and in-plane magnetizations also is suppressed. From the field dependence of the spin-wave energy, we find that the second-order uniaxial perpendicular anisotropy increases with the Au interlayer. © 1998 American Institute cf Physics.
• Murayama, A., Hyomi, K., Eickmann, J., & Falco, C. M. (1998). Underlayer-induced perpendicular magnetic anisotropy in ultrathin Co/Au/Cu(111) films: A spin-wave Brillouin-scattering study. Physical Review B - Condensed Matter and Materials Physics, 58(13), 8596-8604.
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  Abstract: We have used spin-wave Brillouin scattering to study the perpendicular magnetic anisotropy of ultrathin Co/Au/Cu(111) films with various thicknesses of Au underlayer. From the field dependence of the spin-wave frequency we find that the second-order (fourth power) uniaxial perpendicular anisotropy increases monotonically with increasing Au-underlayer thickness ranging from 0 to 5 monolayers (ML), while the first-order (second power) anisotropy shows a nonmonotonic increase with a minimum at 1 ML Au. The ratio of the second-order anisotropy constant to the first one also increases with increasing Au thickness, from 0.01 to 0.07. We observe saturation for both the perpendicular anisotropy constants for a 5 ML Au underlayer, which coincides with saturation of expansion of the in-plane Co lattice due to the coherent growth of Co at the interface between the Co and Au underlayer. When the thickness of Au is further increased beyond 5 ML, we find an anisotropy-independent increase in coercivity on the polar-Kerr hysteresis curves. We also have observed a field-dependent broadening of the spin-wave Brillouin spectrum around a critical field between out-of-plane and in-plane magnetizations, which we explain by assuming a distribution of the first-order perpendicular anisotropy. As the result, the normalized distribution of the first-order anisotropy is shown to have a minimum at 1 ML Au thickness. We believe the observed Au-thickness dependences of the perpendicular anisotropy and of the distribution are characteristic behaviors due to an atomic-scale transition of the underlayer materials from Cu to Au.
• Tarrio, C., Watts, R. N., Lucatorto, T. B., Slaughter, J. M., & Falco, C. M. (1998). Optical constants of in situ-deposited films of important extreme-ultraviolet multilayer mirror materials. Applied Optics, 37(19), 4100-4104.
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  PMID: 18285847;Abstract: We have performed angle-dependent reflectance measurements of in situ magnetron sputtered films of B4C, C, Mo, Si, and W. The Fresnel relations were used to determine the complex index of refraction from the reflectance data in the region of approximately 35-150 eV. In the cases of Si, C, and B4C we found excellent agreement with published data. However, for Mo and W we found that the optical properties from 35 to 60 eV differed significantly from those in the literature.
• Murayama, A., Hyomi, K., Eickmann, J., & Falco, C. M. (1997). Spin-wave Brillouin study of ultra-thin Co/Au(111) films with perpendicular magnetic anisotropy. Journal of Applied Physics, 82(12), 6186-6192.
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  Abstract: We have studied ultra-thin Co/Au(111) films with perpendicular magnetic anisotropy by means of spin-wave Brillouin scattering. The ultra-thin Co films with thicknesses from 1 to 7 monolayer (ML) were grown by molecular beam epitaxy on 1 ML Au underlayers without intentional substrate heating. With 2 or more MLs of Co, spin-wave excitations are clearly observed. We find the magnetic field dependence of the spin-wave energy can be quantitatively explained by a calculation including out-of-plane magnetization due to uniaxial perpendicular magnetic anisotropy. The first-order perpendicular anisotropy constant systematically increases as the Co thickness decreases from 7 to 3 ML. The second-order perpendicular anisotropy constant is much smaller than that expected in Co(0001) bulk and also increases as the Co thickness decreases. The physical origin of the thickness dependence of the perpendicular anisotropy is discussed. We find the field dependence of the scattering intensity is different between Stokes and anti-Stokes processes. Finally, with 2 ML of Co, significant broadening of the spin-wave spectrum is observed, as well as the degradation of the perpendicular anisotropy. We believe this is the result of the effect of magnetic inhomogeneities due to the structural imperfections. © 1997 American Institute of Physics.
• Tagliente, M. A., Vecchio, A. D., Tapfer, L., Coccorese, C., Mercaldo, L., Maritato, L., Slaughter, J. M., & Falco, C. M. (1997). Glancing-incidence X-ray characterization of Nb/Pd multilayers. Nuovo Cimento della Societa Italiana di Fisica D - Condensed Matter, Atomic, Molecular and Chemical Physics, Biophysics, 19(2-4), 473-480.
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  Abstract: The study of periodic metallic multilayers in which one of the two constituent layers is a superconductor has attracted considerable interest. The structural configuration and quality of the interfaces is of fundamental importance because it influences the phases of the superconductive wave function in the interface region and, hence, the coupling between the nearest superconductive layers. In this work we present a structural investigation of Nb/Pd multilayers by using high- and low-angle X-ray diffraction measurements. All the samples were grown on Si(lOO) substrates by dc-triode sputtering. We investigated two samples consisting of 10 stacks of nominally 18 nm Nb, 4 nm Pd and 18 nm Nb, 8 nm Pd, respectively. The high-angle analyses reveal that the Nb layer is oriented in the [110] direction and the Pd in the [111] direction. Off-specular reflectivity measurements show the presence of a (partially) con-elated roughness across the interfaces. From specular reflectivity it was found how the rms roughness increases from the substrate to the surface.
• Attanasio, C., Coccorese, C., Maritato, L., Mercaldo, L. V., Salvato, M., Vecchio, A. D., Tapfer, L., Eickmann, J., Slaughter, J. M., & Falco, C. M. (1996). Superconducting and structural properties of Nb/Pd(Mn) multilayers. Czechoslovak Journal of Physics, 46(SUPPL. 2), 717-718.
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  Abstract: Nb/Pd(Mn) multilayers have been fabricated by sputtering and Molecular Beam Epitaxy (MBE). The samples were characterized by X-Ray analyses which showed in the case of sputtered samples oxygen contamination at the Nb interfaces. The MBE films exhibited higher residual resistivity ratios (RRR) and superconducting critical temperatures versus the Pd thicknesses in agreement with the de Gennes-Werthamer theory. Critical magnetic field measurements were also performed on these samples. Nb/PdMn multilayers were analyzed to study the superconductivity and the magnetism interplay.
• Attanasio, C., Coccorese, C., Maritato, L., Prischepa, S. L., Salvato, M., Engel, B., & Falco, C. M. (1996). Quantum vortex melting in Nb/CuMn multilayers. Physical Review B - Condensed Matter and Materials Physics, 53(3), 1087-1090.
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  Abstract: We have measured the electrical resistance R vs temperature T in different perpendicular magnetic fields on Nb/CuMn multilayers. The samples had constant Nb layer thickness ( ∼ 230 Å) and CuMn layer thicknesses in the range 4-50 Å. Below the zero-field superconducting critical temperature Tc, the (InR vs 1/T) curves showed a sharp change in the slope at magnetic-field-dependent temperatures T*(H). The T* vs H curves are consistent with vortex-lattice melting, mainly due to quantum fluctuations.
• Marliere, C., Engel, B. N., & Falco, C. M. (1996). Influence of a non-magnetic interlayer on the magnetic anisotropy of ultra-thin Co sandwiches. Journal of Magnetism and Magnetic Materials, 156(1-3), 209-210.
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  Abstract: We have studied the magnetic anisotropy of MBE-grown X/Y/Co/Au thin film structures. For an interlayer wedge of Y = Au on a X = Cu buffer, we find a minimum in the perpendicular anisotropy (PA) at approx. 1 atomic layer (ML). In the reverse case (Y = Cu on a X = Au buffer), we observed an exponential decrease of PA with increasing thicknesses of the Cu interlayer.
• Montcalm, C., Kearney, P. A., Slaughter, J. M., Sullivan, B. T., Chaker, M., Pépin, H., & Falco, C. M. (1996). Survey of Ti-, B-, and Y-based soft x-ray-extreme ultraviolet multilayer mirrors for the 2- to 12-nm wavelength region. Applied Optics, 35(25), 5134-5147.
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  PMID: 21102948;Abstract: We have performed an experimental investigation of Ti-, B4C-, B-, and Y-based multilayer mirrors for the soft x-ray-extreme ultraviolet (XUV) wavelength region between 2.0 and 12.0 nm. Eleven different material pairs were studied: Ti/Ni, Ti/Co, Ti/Cu, Ti/W, B4C/Pd, B/Mo, Y/Pd, Y/Ag, Y/Mo, Y/Nb, and Y/C. The multilayers were sputter deposited and were characterized with a number of techniques, including low-angle x-ray diffraction and normal incidence XUV reflectometry. Among the Ti-based multilayers the best results were obtained with Ti/W, with peak reflectances up to 5.2% at 2.79 nm at 61° from normal incidence. The B4C/Pd and B/Mo multilayer mirrors had near-normal incidence (5°) peak reflectances of 11.5% at 8.46 nm and 9.4% at 6.67 nm, respectively, whereas a Y/Mo multilayer mirror had a maximum peak reflectance of 25.6% at 11.30 nm at the same angle. The factors limiting the peak reflectance of these different multilayer mirrors are discussed. © 1996 Optical Society of America.
• Attanasio, C., Maritato, L., Prischepa, S. L., Salvato, M., Engel, B. N., & Falco, C. M. (1995). Superconducting properties of Nb-CuMn multilayers. Journal of Applied Physics, 77(5), 2081-2086.
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  Abstract: The superconducting properties of spin-glass superconducting multilayers made of Nb (superconducting) and CuMn (spin glass) have been studied. The superconducting critical temperature Tc of the multilayers was strongly dependent on the thickness of the spin-glass layers. The Radovic et al. theory [Phys. Rev. B 44, 759 (1991)], which foresees a phase difference 0≤φ≤π between neighboring superconducting layers, has given a qualitative description of these experimental data. The parallel and perpendicular critical magnetic-field measurements have shown many interesting effects related to the reduced dimensionality of the samples. © 1995 American Institute of Physics.
• Boufelfel, A., & Falco, C. M. (1995). Thickness variation effects on X-ray scattering of multilayers. Thin Solid Films, 258(1-2), 26-33.
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  Abstract: A cosine-squared flux distribution from a disk-shaped source was used to calculate the spatial deposition profiles for films sputtered onto rotating substrates. Depositions were made by a stable and reproducible d.c. sputtering machine and thicknesses measured by Rutherford backscattering spectroscopy to compare with this calculation. The measured and calculated profiles were compared at a fixed value of target-substrate distance. We showed for the first time that the thickness variation is largely responsible for the broadening of the X-ray Bragg peaks at low angles of Fe Pd multilayers. These results have important implications for the classical interpretation of X-ray scattering from multilayered structures. © 1995.
• Celinski, Z., Pardo, D., Engel, B. N., & Falco, C. M. (1995). Magneto-optical properties of MnBi_1-xSb_x films. IEEE Transactions on Magnetics, 31(6 pt 1), 3233-3238.
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  Abstract: We have grown MnBi1-xSbx alloy films on mica and glass substrates under UHV conditions, and performed a systematic study of the structural, magnetic and magneto-optic properties on films in the concentration range 0 ≤ × ≤ 0.4. Sharp Reflection High Energy Electron Diffraction (RHEED) patterns were observed for samples grown on mica substrates, indicating epitaxial growth. X-ray diffraction patterns of MnBi1-xSbx grown on glass substrates show these films to be highly textured, with the c-axis parallel to the substrate normal. We found that only samples with Mn concentration above 50 at.% exhibited good magneto-optical properties. Perpendicular magnetization and Kerr rotation larger than 0.5° were observed for samples with Sb concentration less than 8 at.% in mica and 4 at.% on glass. For larger concentrations of Sb, the coercivity increased rapidly while Kerr rotation decreased. Samples grown on both types of substrates showed an increase in the Kerr rotation and ellipticity with decreasing light wavelengths. The off-diagonal E-field reflectivity, rxy, which describes the magneto-optic efficiency of the sample, is larger than that reported for TbFeCo layers.
• Engel, B. N., Marliere, C., & Falco, C. M. (1995). Influence of a non-magnetic underlayer on the magnetic anisotropy of ultra-thin Co sandwiches. IEEE Transactions on Magnetics, 31(6 pt 2), 4080-4084.
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  Abstract: We have used in situ polar Kerr effect measurements to study the magnetic anisotropy of MBE-grown Cu/Au/Co/Au sandwich structures. We reported earlier that the magnitude of the Au/Co/Au perpendicular anisotropy is strongly peaked at approximately 1 atomic layer Au overlayer coverage. We report here extensions of this work to investigate the influence on anisotropy of a confined ultra-thin non-magnetic underlayer placed at one of the Co interfaces. Sandwich structures were deposited in a stepped-wedge geometry providing multiple underlayer thicknesses for each sample. For the case of an underlayer wedge of Au, e.g. Cu/Au(x)/Co/Au, we find a sharp minimum in the perpendicular anisotropy at approximately 1 atomic layer of Au. This anisotropy behavior is opposite to that of a Au overlayer deposited on a Co film.
• Herbert, S. T., Gomez, L. B., Mast, D. B., Newrock, R. S., Wilks, C., & Falco, C. M. (1995). Fabrication of 3D proximity coupled Josephson junction arrays. Superlattices and Microstructures, 18(1), 53-57.
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  Abstract: Experimentally realizable 3D arrays of Josephson junctions have been a goal of researchers since 2D Josephson junctions (JJ) arrays were first introduced. In the past, it has proven to be technically impossible to manufacture 3D proximity-coupled arrays. Recent advancements in etching technology have now made fabrication more feasible. In this paper, we present details of our fabrication process.
• Kearney, P. A., Slaughter, J. M., Shen, D. H., & Falco, C. M. (1995). Growth modes of Pd, Ag, and Si thin films on B. Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films, 13(1), 78-82.
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  Abstract: We have studied the growth of Pd, Ag, and Si on amorphous B films at room temperature to determine the growth modes, the chemical sharpness of the interfaces, and the structure of the layers formed. All films were deposited in ultrahigh vacuum and studied in situ with Auger electron spectroscopy, x-ray photoelectron spectroscopy, and reflection high-energy electron diffraction. Scanning tunneling microscopy characterization and Rutherford backscattering calibrations were done ex situ. In the Pd case, we find that it reacts with the B to form a smooth, amorphous PdxB1-x layer, with x≈0.85. This reaction occurred for all Pd coverages studied, from 2.3 to 130 angstrom, with only small variations in x. Depositing Ag on B results in a polycrystalline layer composed of large Ag islands. We find that Si-on-B forms a sharp interface and an amorphous overlayer of pure Si. The applicability of these materials to multilayer extreme ultraviolet optics is discussed.
• Lee, S., Stegeman, G. I., Kim, J., Falco, C. M., Askarpour, V., & Manghnani, M. H. (1995). Effect of thermal annealing on elastic properties of Ag/Pd superlattice films. Solid State Communications, 94(9), 691-693.
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  Abstract: We report a 50 % increase in the shear elastic constant c55 in Ag/Pd superlattice films upon thermally annealing as measured by Brillouin light scattering. X-ray diffraction studies show that this increase is due to intermixing of the Ag and Pd layers. © 1995.
• Marliere, C., Engel, B. N., & Falco, C. M. (1995). Unusual behavior in the magnetic anisotropy of ultra-thin Co sandwiches: the role of Au underlayers. Materials Research Society Symposium - Proceedings, 384, 221-226.
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  Abstract: We have used in situ polar Kerr effect measurements to study the magnetic anisotropy of X/Co/Y sandwich structures grown by MBE on Cu(111) buffers, where X and Y are variable thicknesses of Au. For fixed values of Y and in the case of an underlayer wedge, e.g. variable X value, we have found a sharp minimum in both coercive field and perpendicular anisotropy at ≈ 1 atomic layer of the Au underlayer. This anisotropy behavior is opposite to that of an Au overlayer deposited on a Co film, i.e. variable Y and fixed X.
• Wiedmann, M. H., Marliere, C., Engel, B. N., & Falco, C. M. (1995). Perpendicular magnetic behavior of ultra-thin Co sandwiches. Journal of Magnetism and Magnetic Materials, 148(1-2), 125-126.
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  Abstract: In situ polar Kerr effect measurements have been used to study the properties of MBE-grown X/Co/Y trilayers, where X and Y are combinations of the non-magnetic metals Au, Cu or Pd. Polar hysteresis curves were measured in situ for systematically-varied Co and overlayer Y layer thicknesses 2 Å ≤ tCo ≤ 20 Å and 0 Å ≤ tY ≤ 100 Å. We recently reported that for base layers of Pd or Au that induce a large perpendicular anisotropy, the magnitude of the X/Co/Y perpendicular coercivity is strongly peaked at {all equal to} 1 atomic layer overlayer coverage. However, in Pd overlayers the peak in coercivity is much less pronounced than for the other materials. Here we report further measurements on Pd overlayers confirming the existence of a small peak in coercivity. In addition, measurements have been performed on a 'reversed' layered Cu/Co/Au sandwich where the Cu base layer induces only a weak perpendicular anisotropy. We find similar peaked behavior similar to that in our earlier measurements on Au/Co/Cu. © 1995.
• Attanasio, C., Maritato, L., Engel, B., & Falco, C. M. (1994). Superconducting spin-glass multilayers. Physica B: Physics of Condensed Matter, 194-196(PART 2), 1721-1722.
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  Abstract: Superconducting spin-glass multilayers of Nb and CuMn have been fabricated by using a sputtering deposition technique. In such a system critical temperatures, strongly dependent on the thickness of the spin-glass, have been observed. This behavior could be tentatively explained in terms of a pair breaking parameter related to both the proximity effect and the magnetic interaction. © 1994.
• Borchers, J. A., Ankner, J. F., Majkrzak, C. F., Engel, B. N., Wiedmann, M. H., Leeuwen, R. V., & Falco, C. M. (1994). Spatial modulation of the magnetic moment in Co/Pd superlattices observed by polarized neutron reflectivity. Journal of Applied Physics, 75(10), 6498-6500.
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  Abstract: We have measured the specular reflectivity of spin-polarized neutrons from two (111) superlattices, [Pd 80 Å∥Co 60 Å]×20/Pd 300 Å/Co 10 Å/GaAs and [Pd 53 Å∥Co 32 Å]×10/Pd 30 Å/Cu 354 Å/Si, grown by molecular-beam epitaxy. In a saturating field of 2300 Oe, we extract for the first superlattice a Co moment value of corresponding to 78% of the bulk Co moment. At temperatures ranging from 38 to 295 K, no excess magnetic moment from polarization of the Pd atoms is evident in the nominal Pd layer. Comparable reflectivity data from the second sample support these conclusions.
• Engel, B. N., Wiedmann, M. H., & Falco, C. M. (1994). Overlayer-induced perpendicular anisotropy in ultrathin Co films (invited). Journal of Applied Physics, 75(10), 6401-6405.
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  Abstract: We have used in situ polar Kerr effect measurements to study the magnetic anisotropy of MBE-grown X/Co/Y trilayers, where X and Y are combinations of the nonmagnetic metals Ag, Au, Cu, or Pd. The competition between the perpendicular anisotropy of the initial underlayer X/Co interface and the in-plane shape anisotropy of the Co film allows us to adjust the total anisotropy of the uncovered Co to be in plane and of moderate strength. In this way, we can measure hard-axis (perpendicular) polar hysteresis curves in situ as a function of overlayer Y coverage, and directly deduce the anisotropy field. Polar hysteresis curves were measured in situ for systematically varied Co and overlayer Y layer thicknesses 2 Å≤tCo≤20 Å and 0 Å≤tY≤100 Å. We find, for particular combinations, the magnitude of the X/Co/Y perpendicular anisotropy is strongly peaked at ∼1 atomic layer overlayer Y coverage.
• Falco, C. M., Eickmann, J., Ruffner, J. A., & Slaughter, J. M. (1994). Growth and surface structure of epitaxial Be thin films. Proceedings of SPIE - The International Society for Optical Engineering, 2364, 338-344.
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  Abstract: We have investigated the growth of beryllium thin films on α- Al2O3, Si (111), and Ge (111). In all cases, epitaxial Be films were obtained under the proper conditions. The effects of substrate temperature T on crystalline quality and surface structure were also studied. Samples were analyzed in situ using reflection high energy electron diffraction and ex situ with ion beam analysis, scanning electron microscopy, atomic force microscopy, and a variety of x-ray diffraction techniques. Studies showed an increase in crystalline quality with increased T, as well as the presence of a surface superstructure, probably √3 × √3, R30°, for films deposited on Si at T ≥ 300°C and films on Ge at T ≥ 200°C. To date, the 2ighest quality Be films are those grown on Ge (111) at T = 300°C.
• Gibson, G. A., Lange, D. A., & Falco, C. M. (1994). Growth of MBE-codeposited IrSi₃ on Si(111) and Si(100). Materials Research Society Symposium Proceedings, 299, 303-308.
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  Abstract: We have used Molecular Beam Epitaxy (MBE) to successfully grow films that are predominantly IrSi3 on both Si(111) and Si(100) substrates by codeposition of Si and Ir in a 3:1 ratio. Bragg-Brentano and Seemann-Bohlin x-ray diffraction reveal that polycrystalline IrSi3 films form as low as 450 °C. This is the lowest temperature yet reported for growth of this iridium silicide phase. These x-ray diffraction techniques, along with Transmission Electron Microscope (TEM) diffraction and in situ Low Energy Electron Diffraction (LEED), show that at higher deposition temperatures codeposition can form IrSi3 films on Si(111) that consist predominantly of a single epitaxial growth orientation. Ion beam channeling and x-ray rocking curves show that the epitaxial quality of IrSi3 films deposited on Si(111) is superior to that of IrSi3 films deposited on Si(100). We also present evidence for several new epitaxial IrSi3 growth modes on Si(111) and Si(100).
• Lange, D. A., Gibson, G. A., & Falco, C. M. (1994). Growth and structure of IrSi₃ on Si(111). Journal of Applied Physics, 75(6), 2917-2923.
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  Abstract: Molecular-beam epitaxy has been used to grow films that are almost entirely IrSi3 by codeposition of Si and Ir in a 3:1 ratio on Si(111) substrates. Bragg-Brentano and Seemann-Bohlin x-ray diffraction reveal that polycrystalline IrSi3 films form as low as 490°C, the lowest temperature yet reported for growth of this iridium silicide phase. Above 580°C this hexagonal phase becomes textured, with as many as seven preferred growth orientations on Si(111). Samples codeposited on Si(111) between 680 and 780°C consist almost entirely of IrSi3 with its c axis perpendicular to the substrate's surface. At higher substrate temperatures, near 830°C growth of IrSi3 with its c axis in the plane of the substrate dominates. Atomic force microscopy shows that there is a difference in the surface morphology of the c-axis in-plane and perpendicular growth modes. Transmission electron microscope diffraction and in situ low-energy electron diffraction verify that both of these IrSi3 growth modes are epitaxially registered with their substrates. To check the quality of this epitaxy ion-beam channeling and x-ray rocking curves were used. The data from the epitaxial samples have channeling χmin ratios as low as 0.64. The x-ray rocking curves for these IrSi3 films are narrow, with full-width-at- half-maxima of as little as 0.07°.
• Montcalm, C., Sullivan, B. T., Ranger, M., Slaughter, J. M., Kearney, P. A., Falco, C. M., & Chaker, M. (1994). Mo/Y multilayer mirrors for the 8-12 nm wavelength region. Optics Letters, 19(15), 1173-1175.
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  Abstract: This paper describes the deposition and measurement of peak reflectances of 34% at 11.5 nm and 22% at 8.1 nm for Mo/Y multilayers, which are the best for wavelengths below 13 nm. The reflectances are improved by reducing to a great extent the effect of contamination. The other factor that influences the performance is the interface roughness.
• Montcalm, C., Sullivan, B. T., Ranger, M., Slaughter, J. M., Kearney, P. A., Falco, C. M., & Chaker, M. (1994). Mo/Y multilayer mirrors for the 8-12-nm wavelength region. Optics Letters, 19(13), 1004-1006.
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  Abstract: This paper describes the deposition and characterization of Mo/Y multilayer mirrors. Reflectance measurements at normal incidence show peak reflectances of 34% at 11.5 nm and 22% at 8.1 nm respectively. It is observed that the base pressure of the dc magnetron sputter deposition effects the contamination of the layers and the interface roughness, although the partial pressure of the different gas species is more important than the base pressure.
• Ruffner, J. A., Slaughter, J. M., Eickmann, J., & Falco, C. M. (1994). Epitaxial growth and surface structure of (0001) Be on (111) Si. Applied Physics Letters, 64(1), 31-33.
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  Abstract: We report the growth of epitaxial single-crystal (0001) hcp-Be on (111) Si substrates using molecular beam epitaxy. The Be is oriented with Be[101̄0]∥Si[110] and Be[112̄0]∥Si[211]. Crystalline quality improves with increasing deposition temperature T, where T=100, 200, 300, 400, and 500°C for the results reported here. The films deposited at T≤300°C are smooth while those deposited at T≥400°C are rough. A superstructure was observed on the surface, probably 3×3, R30°, for films grown at T=300°C. These epitaxial beryllium films are of much better quality than those we previously reported on α-Al2O 3.
• Slaughter, J. M., Schulze, D. W., Hills, C. R., Mirone, A., Stalio, R., Watts, R. N., Tarrio, C., Lucatorto, T. B., Krumrey, M., Mueller, P., & Falco, C. M. (1994). Structure and performance of Si/Mo multilayer mirrors for the extreme ultraviolet. Journal of Applied Physics, 76(4), 2144-2156.
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  Abstract: This study aims to compare multilayer made by sputtering and evaporation and to show how each of the structural and compositional parameters that describe a multilayer, affects the EUV reflectivity. To accomplish these goals, the rigorous characterization routine described here is necessary. Through the use of various deposition parameters, multilayers are made, and they are characterized with three types of x-ray diffraction, TEM, Rutherford backscattering spectrometry, and Auger depth profiling. Their performance was evaluated by measuring the EUV reflectivity as a function of wavelength using a synchrotron-radiation source.
• Wiedmann, M. H., Engel, B. N., & Falco, C. M. (1994). Magnetic anisotropy of metal/Co/metal and metal/Co/insulator sandwiches. Journal of Applied Physics, 76(10), 6075-6077.
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  Abstract: In situ polar Kerr-effect measurements have been used to study the magnetic anisotropy of MBE-grown Au(111)/Co/X and Pd(111)/Co/X sandwiches, where X is the nonmagnetic metal Ag, Au, Cu, and Pd or the insulator MgO. For the metals it was recently found that the magnitude of the Co/X perpendicular interface anisotropy is strongly peaked at ∼1 atomic layer (1.5-2.5 Å) coverage. To investigate structural influences on the anisotropy, reflection high-energy electron diffraction (RHEED) and low-energy electron diffraction (LEED) have been used to measure changes resulting from overlayer coverage. Analysis of digitized RHEED images captured every ∼1 Å during metal overlayer coverage shows no abrupt change of the in-plane lattice constant. The out-of-plane lattice spacing has also been investigated as a function of nonmagnetic metal coverage by measuring LEED I-V curves along the (0,0) rod. In the case of Cu, where the LEED behavior is nearly kinematic, no evidence was seen of any abrupt structural changes at ∼1 atomic layer coverage. These results suggest the observed peak in magnetic anisotropy is not structural in origin. To further study this phenomenon, the influence of an insulating overlayer, MgO, on the perpendicular magnetic properties has been measured.
• Wilks, C. W., Engel, B. N., & Falco, C. M. (1994). Proximity effect in MBE-grown superconducting/spin-glass multilayers. Journal of Applied Physics, 76(10), 6959-6961.
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  Abstract: We have grown epitaxial superconductor/spin-glass multilayers, Nb/CuMn, as well as complimentary nonmagnetic Nb/Cu multilayers by molecular beam epitaxy. To probe the interaction of superconductivity and magnetism, we measured the resistivity and ac susceptibility as a function of temperature for multilayers of nominally constant Nb thickness and varying normal-metal thickness. The reduction of the transition temperature of the Nb/Cu multilayers with increasing Cu thickness is in excellent agreement with the de Gennes-Werthamer proximity effect theory. The inclusion of Mn in the Cu causes a significant additional suppression of the transition temperatures relative to the Nb/Cu multilayers. The extension of the de Gennes-Werthamer theory to include the effects of random magnetic impurities agrees well with the data from the Nb/CuMn multilayers for small CuMn layer thicknesses. However, deviations occur at the largest CuMn thicknesses studied. These deviations between the data and theory may be due to a decoupling of the Nb layers, as a result of the spin-glass ordering, causing a three-dimensional to two-dimensional crossover.
• Chason, E., Falco, C. M., Ourmazd, A., Schubert, E. F., Slaughter, J. M., & Williams, R. S. (1993). Interface roughness:what is it and how is it measured. Materials Research Society Symposium Proceedings, 280, 203-239.
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  Abstract: A panel discussion on interface roughness was held at the Fall 1992 Materials Research Society meeting. We present a summary of the results presented by the invited speakers on the application and interpretation of X-ray reflectivity, atomic force microscopy (AFM), scanning tunneling microscopy (STM), photoluminescence and transmission electron microscopy. A transcript of the moderated discussion is provided in the final section.
• Engel, B. N., Wiedmann, M. H., A., R., & Falco, C. M. (1993). Anomalous magnetic anisotropy in ultrathin transition metals. Physical Review B, 48(13), 9894-9897.
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  Abstract: We have used in situ polar Kerr effect measurements to probe directly the magnetic anisotropy of ultrathin MBE-grown Co films with and without overlayers of various nonmagnetic metals (X=Ag, Cu, Pd). To investigate the Co/X interface, we measured perpendicular hysteresis curves as a function of overlayer coverage. We find the magnitude of the perpendicular anisotropy is strongly peaked near ∼1 atomic layer (1.5-2) coverage for Ag and Cu overlayers. For Cu, the overlayer with the strongest effect, the total anisotropy energy rapidly decreases by a factor of 3 from its peak value after a total coverage of only ∼2 atomic layers (4) of Cu. © 1993 The American Physical Society.
• Engel, B. N., Wiedmann, M. H., A., R., & Falco, C. M. (1993). Effect of transition-metal overlayers on the perpendicular magnetism of MBE-grown, ultra-thin Co films. Journal of Applied Physics, 73(10), 6192-6194.
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  Abstract: We have used in situ polar Kerr ellipticity measurements to study the perpendicular magnetic behavior of MBE-grown Pd/Co/TM sandwich structures, where TM is the nonmagnetic transition metal overlayer Pd, Cu, or Ag. These structures are epitaxially deposited on thick Pd (111) buffer layers grown on Co-seeded GaAs (110) substrates. Hysteresis curves were measured in situ for systematically varied Co and TM layer thicknesses 2 Å≤t Co≤10 Å and 0 Å≤tTM≤200 Å. We observed perpendicular loops with a coercive field of Hc≤200 Oe for the uncovered Co films for tCo≤6 Å, becoming in-plane above this thickness. However, subsequent deposition of just one atomic layer (≊2 Å) of any of the TM over the Co resulted in strongly perpendicular, square hysteresis curves with Hc≥700 Oe for all films in the Co thickness range studied. Deposition of TM overlayers causes nonmonotonic behavior in Hc as a function of coverage. We find a peak in Hc at a TM coverage of tTM∼1.5 Å for all materials, with a subsequent monotonic increase above tTM≊4 Å to a limiting value by tTM≊100 Å. The peak is most pronounced for Cu, where Hc changes by more than 200%.
• Engel, B. N., Wiedmann, M. H., A., R., & Falco, C. M. (1993). Influence of transition metal overlayers on the perpendicular magnetism of MBE-grown Co films. Journal of Magnetism and Magnetic Materials, 126(1-3), 532-534.
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  Abstract: We have used in situ polar Kerr effect measurements to probe the magnetic behavior of MBE-grown Co films with and without overlayers of various non-magnetic transition metals (TM = Ag, Cu, Pd). We find a large peak in the coercivity, Hc, near 2 Å overlayer coverage for Ag and Cu. © 1993.
• Falco, C. M., & Slaughter, J. M. (1993). Characterization of metallic multilayers for X-ray optics. Journal of Magnetism and Magnetic Materials, 126(1-3), 3-7.
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  Abstract: X-ray reflectivity is a sensitive tool for characterizing interfaces in multilayers. Using an appropriate model, the low-angle θ-2θ spectra can be fit to yield the thickness and roughness of each layer. For Si/Mo, a model that includes an interlayer at the Mo on Si interface gives much better results than a simple bilayer model. Non-specular X-ray scattering measurements are particularly sensitive to roughness that is correlated from layer to layer because correlated roughness gives rise to reasonant scattering in particular non-specular directions. The non-specular behavior can be understood in terms of current theories of X-ray scattering from multilayers. Structural parameters, such as spatial frequencies of the correlated roughness and a coherence length for the correlations can be determined from these data. For the Si/Mo multilayers studied, spatial frequencies below 1/200 Å-1 are correlated throughout the multilayer. © 1993.
• Lianjun, W. u., Nakayama, N., Engel, B. N., Shinjo, T., & Falco, C. M. (1993). X-ray diffraction measurements of lattice strains in Co/Pd(001) superlattices films. Japanese Journal of Applied Physics, Part 1: Regular Papers and Short Notes and Review Papers, 32(10), 4726-4731.
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  Abstract: Lattice spacings in fcc-Co/Pd(001) superlattice films grown on GaAs(001) substrates with a Ag(001) buffer layer were measured using a four-circle diffractometer. For samples with Co layers thinner than 9 angstrom and Pd layers thinner than 12 angstrom, the observed in-plane (110) lattice spacings were all 2.71±0.01 angstrom, indicating 8.0% lattice expansion of Co layers and 1.5% lattice contraction of Pd layers. The (002) lattice spacings along the growth direction were estimated from a profile fitting method, and they were 1.60±0.02 angstrom for Co layers and 1.98±0.02 angstrom for Pd layers. This means that Co layers are contracted nearly 10%, while Pd layers are expanded 2% along the growth direction of the superlattice films. The estimated large lattice strains are successfully applied to explain the large magnetovolume contribution to the magnetic anisotropy reported previously.
• Nakayama, N., Wu, L., Dohnomae, H., Shinjo, T., Kim, J., & Falco, C. M. (1993). X-ray diffraction analysis of lattice strain in metallic superlattice films. Journal of Magnetism and Magnetic Materials, 126(1-3), 71-75.
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  Abstract: In-plane lattice spacings of [Au(xÅ)/Ni(xÅ)] and [Pd(xÅ)/Cu(xÅ)] superlattice films with [111]fcc textures and large lattice misfits have been measured by X-ray diffraction. With decrease of superlattice period (Λ = 2x), Au and Pd layers are compressed whereas Ni and Cu layers are expanded. The observed lattice spacings (d220) of the individual layers vary almost linearly depending on 1/Λ. The lattice spacings of two constituent layers become equal in a Au/Ni superlattice with Λ = 8 A ̊ (d220 = 1.371 A ̊) and a Pd/Cu superlattice with Λ = 14 A ̊ (d220 = 1.337 A ̊). © 1993.
• Parizek, V., Kielar, P., Krishnan, R., & Falco, C. M. (1993). Resistance and magnetoresistance studies in (Co/Nb) multilayers before and after ion-mixing. Journal of Magnetism and Magnetic Materials, 123(1-2), 147-152.
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  Abstract: (Co/Nb) multilayers have been prepared by sequential deposition under UHV conditions. Resistivity and magnetoresistance were measured in the temperature range 3-300 K and in a magnetic field of 0-7.2 T. The specific resistivity ρ{variant} is increasing with increasing temperature. The ratio ρ{variant}(300 K)/ρ{variant}(4.2 K) decreases with an increase in the number of bi-layers. Investigations were also made on some samples subjected to ion-mixing in order to find the specific effect of the layered structure. A qualitative model has been proposed. © 1993.
• Shen, D. H., Kearney, P. A., Slaughter, J. M., & Falco, C. M. (1993). Chemical interaction at the Pd-B interface. Journal of Magnetism and Magnetic Materials, 126(1-3), 25-27.
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  Abstract: We have investigated the interaction that takes place when palladium is deposited on amorphous boron. XPS and Auger measurements show that interdiffusion and a strong reaction occur between the deposited Pd and the B substrate, even at room temperature. Electron transfer from Pd to B occurs in the resulting Pd boride. © 1993.
• Wiedmann, M. H., Engel, B. N., A., R., Mibu, K., Shinjo, T., & Falco, C. (1993). Anomalous perpendicular anisotropy in ultra-thin co films. Materials Research Society Symposium Proceedings, 313, 531-536.
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  Abstract: We have used in situ polar Kerr effect measurements to study the magnetic coercivity and anisotropy of MBE-grown Pd(111)/Co/X and Au(111)/Co/X trilayers, where X is the non-magnetic noble or transition metal overlayers Ag, Cu or Pd. Polar hysteresis curves were measured in situ for systematically varied Co and overlayers thicknesses 2 Angstrom ≤ tCo≤20 Angstrom and 0 Angstrom ≤tx≤50 Angstrom. We find the coercivity and total anisotropy display a strongly peaked perpendicular contribution at ≈1 atomic layer (2 Angstrom) non-magnetic metal coverage. For Cu where the effect is strongest, the total anisotropy energy rapidly decreases by a factor of 3 from its peak value after a total coverage of ≈2 atomic layers (4 Angstrom) of Cu.
• Christen, D. K., Dynes, R. C., Emery, V. J., Falco, C. M., Gubser, D. U., Jin, S., Kroger, H., & Shaw, D. T. (1992). New research opportunities in superconductivity III. Cryogenics, 32(4), 338-352.
• Engel, B. N., Wiedmann, M. H., A., R., Falco, C. M., Lianjun, W. u., Nakayama, N., & Shinjo, T. (1992). Influence of structure on the magnetic anisotropy of Co/Pd (001) epitaxial superlattices. Applied Surface Science, 60-61(C), 776-780.
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  Abstract: We have used molecular beam epitaxy to grow Co/Pd superlattices oriented along the fcc [001] crystal direction. A series of samples of fixed Pd thickness (tPd = 9±1 A ̊) and varying Co thickness (2≤tCo≤8 A ̊) was prepared. The uniaxial magnetic anisotropy in these films displayed both a large perpendicular interface contribution and a large in-plane volume contribution. Using off-axial X-ray diffraction techniques to measure the fcc (113) Bragg reflection, the in-plane lattice spacing of three samples was directly measured. These measurements are consistent with a coherent strain model with the Co layers expanded in-plane by 8.0%±0.5% over that of bulk fcc Co. An estimate of the in-plane magnetoelastic energy calculated from this strain is in very good agreement with our observed volume anisotropy in these superlattices. © 1992.
• Falco, C. M., & Engel, B. N. (1992). MBE growth and magneto-optic properties of magnetic multilayers. Applied Surface Science, 60-61(C), 790-797.
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  Abstract: Recent interest in the magnetic and magneto-optic properties of transition metal/transition metal multilayers has been stimulated by the discovery of perpendicular magnetism in particular systems such as Co/Pd and Co/Pt. Due to their favorable magneto-optic wavelength dependence and enhanced corrosion resistance, these materials show promise as future data storage media. However, partially due to the large variety of thin-film deposition methods and growth conditions, it has been difficult to obtain a clear understanding of the mechanisms of magnetic anisotropy in these systems. In order to create controlled and well characterized model systems, we have grown a series of epitaxial Co/Pd superlattices oriented along the three high-symmetry crystal directions [001], [110], and [111] on single-crystal GaAs substrates by molecular beam epitaxy [MBE]. Simultaneously, we have deposited polycrystalline Co/Pd multilayers on Si substrates mounted alongside the GaAs for direct comparisons of epitaxial and non-epitaxial films produced under identical conditions. The structural properties of these multilayers were determined by low-and reflection high-energy electron diffraction (LEED and RHEED), low- and high-angle X-ray diffraction, and scanning tunneling microscopy (STM). The dependence of the uniaxial magnetic anisotropy energy on the Co thickness in these superlattices showed significant systematic differences for each of the three crystal orientations. A review of our work on the structural influences responsible for these differences is presented. © 1992.
• Ruffner, J. A., Slaughter, J. M., & Falco, C. M. (1992). Epitaxial growth of Be on α-Al₂O₃. Applied Physics Letters, 60(24), 2995-2997.
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  Abstract: We report the growth of epitaxial single-crystal (0001) hcp-Be on (0001) α-Al2O3 substrates using molecular beam epitaxy. Thin films were characterized in situ with reflection high energy electron diffraction, and ex situ with ion beam analysis, electron microscopy, atomic force microscopy, and a variety of x-ray diffraction techniques. The in-plane orientation of films grown at substrate temperatures T in the range 10°C
• Slaughter, J. M., & Falco, C. M. (1992). Advances in multilayer X-ray optics. Nuclear Inst. and Methods in Physics Research, A, 319(1-3), 163-169.
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  Abstract: We present an overview of advances in multilayer X-ray optics which have been achieved in the past few years. Advances in fabrication, characterization, and theory are described. These mirrors are being used in a variety of synchrotron-related applications such as monochromators and projection lithography, as well as nonsynchrotron applications like astronomy and X-ray laser cavities. Although some of these coatings are produced routinely, much work remains to be done in producing higher quality multilayers with very thin layers (less than ∼ 20 Å) and multilayers which are stable at high temperatures. Good multilayers with very thin layers are needed for near normal incidence optics at short wavelengths. Very stable multilayers are needed to withstand the high power loads which can be imposed by synchrotron radiation. New materials and deposition techniques are being applied to these problems. For example, molecular beam epitaxy is being applied to the growth of these multilayers in an effort to produce single-crystal superlattice mirrors. Since the performance of these mirrors is strongly dependent on atomic-scale interface quality, it is necessary to control the fabrication and perform characterization on this length scale. Characterization techniques which have made recent impact include high-resolution TEM, in situ ellipsometry, and energy dispersive X-ray diffraction. Surface analysis techniques such as STM, RHEED, LEED, AES, and XPS are being used to study interface formation and surface structure on a subnanometer scale. © 1992.
• Bennett, W. R., England, C. D., Person, D. C., & Falco, C. M. (1991). Magnetic properties of Pd/Co multilayers. Journal of Applied Physics, 69(8), 4384-4390.
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  Abstract: Measurements of the magnetic anisotropy of a series of Pd/Co multilayers grown by magnetically enhanced dc triode sputter deposition revealed an interface contribution of 0.53 erg/cm2 and a net volume anisotropy of -13.2×108 erg/cm3. A characteristic reduction of perpendicular anisotropy for ultrathin Co layers (
• Boufelfel, A., Emrick, R. M., & Falco, C. M. (1991). Magnetism of Fe/Pd superlattices. Physical Review B, 43(16), 13152-13158.
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  Abstract: A series of Fe/Pd superlattices of varying numbers of atoms per layer were fabricated. Structural properties were characterized by x-ray diffraction. Magnetic properties were studied by Mössbauer spectroscopy and transport properties by electrical-conductivity measurements. All properties studied could be explained by structural characteristics, such as grain size and interdiffusion, without the need of invoking superlattice effects. © 1991 The American Physical Society.
• Engel, B. N., England, C. D., A., R., Wiedmann, M. H., & Falco, C. M. (1991). Interface magnetic anisotropy in epitaxial superlattices. Physical Review Letters, 67(14), 1910-1913.
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  Abstract: We have studied the uniaxial magnetic anisotropy of Co/Pd superlattices grown under identical conditions by molecular-beam epitaxy along the three crystal axes: [001], [110], and [111]. Our measurements unambiguously demonstrate that the large systematic variations of the anisotropy energy with crystal orientation result solely from differences in the volume contribution to the anisotropy. We find the perpendicular interface anisotropy to be independent of the epitaxial orientation (0.63 ± 0.05 erg/cm2), and hence to be an intrinsic property of the Co/Pd interface.
• Engel, B. N., England, C. D., A., R., Wiedmann, M. H., & Falco, C. M. (1991). Magnetocrystalline and magnetoelastic anisotropy in epitaxial Co/Pd superlattices. Journal of Applied Physics, 70(10), 5873-5875.
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  Abstract: We have used molecular-beam epitaxy (MBE) to grow Co/Pd superlattices along the three high-symmetry crystal axes: [001], [110], and [111]. Identical conditions were maintained for all depositions, and a series of samples of fixed Pd thickness (tPd = 10 ± 1 Å) and varying Co thickness (2 Å ≤ tCo ≤ 22 Å) were prepared for each orientation. A variety of in situ and ex situ characterization studies were made, which confirm these superlattices are single crystalline for all growth directions. The dependence of the uniaxial magnetic anisotropy energy on the Co thickness in these superlattices showed significant systematic differences for each of the three crystal orientations. These variations result entirely from differences in the volume contribution to the anisotropy. Estimates of the magnetocrystalline and magnetoelastic contributions for the (111) and (001) samples are in good agreement with the measured anisotropy energies of these oriented superlattices.
• Engel, B. N., England, C. D., Leeuwen, R. V., Nakada, M., & Falco, C. M. (1991). Magnetic properties of epitaxial Co/Pd superlattices. Journal of Applied Physics, 69(8), 5643-5645.
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  Abstract: A series of Co/Pd superlattices with constant Pd layer thickness of d Pd = 11 Å were grown on single-crystal GaAs(110) substrates by molecular beam epitaxy (MBE). A buffer layer of 500 Å of Pd provided an atomically smooth, fcc(111) single-crystal starting surface for the superlattice deposition. The resulting superlattices maintain this crystal symmetry and smoothness throughout their epitaxial growth. Details of the structural characterization of these samples are given elsewhere in these proceedings. The magnetic properties of these films have been measured by vibrating sample magnetometry. A perpendicular easy axis is found for all seven of the superlattices presented (dCo = 2, 4, 8, 10, 12, 17, and 20 Å). An unusually high coercive field of Hc = 6.6 kOe is observed for dCo = 2 Å, and decreases monotonically with increasing d Co. The saturation magnetization agrees well with a model composed of bulk Co layers with an additional contribution from polarized Pd. The uniaxial magnetic anisotropy energy can be described by both an interface term and a large volume term favoring a perpendicular easy axis for dCo < 23 Å. Comparisons with a series of polycrystalline multilayers simultaneously deposited on Si substrates reveal significant differences in the magnetic behavior.
• England, C. D., Engel, B. N., & Falco, C. M. (1991). Preparation and structural characterization of epitaxial Co/Pd(111) superlattices. Journal of Applied Physics, 69(8), 5310-5312.
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  Abstract: High-quality epitaxial fcc Co/Pd(111) superlattices have been prepared on single-crystal GaAs(110) substrates using molecular-beam epitaxy. An ultrathin (6 Å) buffer layer of bcc Co(110) was used to promote epitaxial growth on the GaAs substrate. A thick (≊ 500 Å) buffer layer of fcc Pd(111) was then deposited to provide a smooth surface for superlattice growth. In addition, the Pd buffer layer isolates the superlattice from chemical interaction with the GaAs substrate. Co/Pd superlattices with ultrathin (
• Falco, C. M., & Engel, B. N. (1991). Magnetic properties of metallic multilayers and superlattices. Physica B: Condensed Matter, 169(1 -4 pt 3), 293-298.
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  Abstract: Metallic superlattices composed of magnetic and non-magnetic constituents have been of recent interest both for the fundamental physical phenomena they exhibit, as well as for potential technological applications. Modern deposition and characterization techniques provide well-controlled material systems that display a variety of phenomena such as two-dimensional behavior, interface-induced magnetic anisotropy, enhanced magnetization, and long-range coupling effects. Although the critical temperature of the magnetic constituents in these superlattices can be very high in their bulk form, large reductions in the Curie point often occur for systems composed of a few atomic layers. These dimensionality effects frequently require the use of cryogenic temperatures to reveal their limiting behavior. Recent studies of single-crystal magnetic superlattices grown by Molecular Beam Epitaxy (MBE) are discussed.
• Falco, C. M., & Engel, B. N. (1991). Magnetic properties of metallic multilayers and superlattices. Physica B: Physics of Condensed Matter, 169(1-4), 293-298.
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  Abstract: Metallic superlattices composed of magnetic and non-magnetic constituents have been of recent interest both for the fundamental physical phenomena they exhibit, as well as for potential technological applications. Modern deposition and characterization techniques provide well-controlled material systems that display a variety of phenomena such as two-dimensional behavior, interface-induced magnetic anisotropy, enhanced magnetization, and long-range coupling effects. Although the critical temperature of the magnetic constituents in these superlattices can be very high in their bulk form, large reductions in the Curie point often occur for systems composed of a few atomic layers. These dimensionality effects frequently require the use of cryogenic temperatures to reveal their limiting behavior. Recent studies of single-crystal magnetic superlattices grown by Molecular Beam Epitaxy (MBE) are discussed. © 1991.
• Harzer, J. V., Hillebrands, B., Stamps, R. L., Güntherodt, G., England, C. D., & Falco, C. M. (1991). Magnetic properties of Co/Pd multilayers determined by Brillouin light scattering and SQUID magnetometry. Journal of Applied Physics, 69(4), 2448-2454.
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  Abstract: Co/Pd multilayers with modulation wavelengths between 4 and 220 Å have been prepared by magnetically enhanced dc-triode sputtering on single-crystal sapphire substrates. Their saturation magnetization and volume and interface anisotropies have been investigated using Brillouin light scattering from collective spin waves and by SQUID magnetometry. The saturation magnetization of Co is found to be independent of the Co layer thickness and reduced by about 20% from the Co bulk value. From the comparison of the results of the two experimental methods, clear evidence for a Pd polarization is found and the polarization depth is estimated. Samples with Co thicknesses of 2 atomic layers and Pd thicknesses ≥5 atomic layers exhibit a perpendicular magnetization due to a large negative out-of-plane interface anisotropy. The properties of spin waves in Co/Pd multilayers with the direction of magnetization pointing out-of-plane are discussed with respect to an appropriate theoretical model.
• Hillebrands, B., Harzer, J. V., Stamps, R. L., Güntherodt, G., England, C. D., & Falco, C. M. (1991). Evidence for collective exchange modes in Co/Pd multilayers observed by Brillouin light scattering. Journal of Magnetism and Magnetic Materials, 93(C), 211-214.
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  Abstract: We report Brillouin light scattering experiments in Co/Pd multilayers with individual layer thicknesses between 1 and 64 atomic layers. Evidence is found for exchange-dominated collective spin-wave excitations when interlayer exchange coupling is active. © 1991.
• Kearney, P. A., Slaughter, J. M., & Falco, C. M. (1991). Selection, growth and characterization of materials for MBE-produced x-ray optics. Proceedings of SPIE - The International Society for Optical Engineering, 1343, 25-31.
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  Abstract: Molecular Beam Epitaxy (MBE) is capable of producing a variety of high purity, epitaxial multilayer films with well controlled interfaces. However, the conditions necessary for MBE growth somewhat restricts the choice of useful materials. We discuss our material selection procedure for MBE-deposited x-ray optical multilayer materials. This procedure takes into account factors such as chemical reactivity, thermal stability, and lattice match, as well as the maximum theoretical reflectivity obtainable with a given material pair. The present work consists of a comprehensive study of elemental films, and a more detailed consideration of a few select compound systems. Both the precise deposition control possible with MBE, as well as the many in situ characterization methods, combine to allow a high degree of control over the formation of interfaces. Our principle MBE system for x-ray optics contains in situ Reflection High Energy Electron Diffraction (RHEED), Low Energy Electron Diffraction (LEED), Auger Electron Spectroscopy (AES), X-Ray Photoelectron Spectroscopy (XPS) and Ion Scattering Spectroscopy (ISS). An overview of the techniques used to analyze our films be given, including data from our growth of epitaxial CoSi2 absorbing layers on Si (111) using a solid state reaction technique.
• Schwarzacher, W., Allison, W., Penfold, J., Shackleton, C., England, C. D., Bennett, W. R., Dutcher, J. R., & Falco, C. M. (1991). Polarized neutron reflection used to characterize cobalt/copper multilayers. Journal of Applied Physics, 69(7), 4040-4045.
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  Abstract: The polarized neutron reflectivity (PNR) of cobalt/copper multilayer films has been measured close to the critical edge for total reflection. Prominent features in the scattered neutron intensity, such as the superlattice diffraction peaks and the positions of the critical edge for total reflection, are sensitive to both the magnetic and structural properties of films, making PNR a useful tool for the characterization of magnetic metallic superlattices. The films were prepared by sputter deposition and the sample composition was measured by Rutherford backscattering spectroscopy. It has been found that while the density of the sputtered copper/cobalt multilayers is approximately 5% less than the bulk metals, the cobalt magnetic dipole moment per atom is little changed compared to the bulk. Evidence is also found for oxidation of the top cobalt layers.
• Slaughter, J. M., Kearney, P. A., Schulze, D. W., Falco, C. M., Watts, R. N., Hills, C. R., & Saloman, E. B. (1991). Interfaces in Mo/Si multilayers. Proceedings of SPIE - The International Society for Optical Engineering, 1343, 73-82.
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  Abstract: Mo/Si multilayer mirrors with high peak reflectivities have been fabricated in many laboratories. This material pair works very well for wavelengths between 125 angstrom and 250 angstrom, and is therefore very useful in optics for astronomy and microscopy. However, complete understanding of the properties of these structures is presently limited by lack of understanding of the details of the interfaces. We report results from a study of Mo/Si interfaces performed with state-of-the-art surface science instruments and electron microscopy. Mo films and Mo/Si multilayers were deposited in ultra-high vacuum on clean Si surfaces using feedback-controlled electron-beam evaporators in a Molecular Beam Epitaxy growth chamber. These films were characterized with in situ RHEED, LEED, Auger, and XPS. Surprisingly, peak shifts in the XPS spectra indicate silicide formation at the interface for growth temperatures as low as 50 C (the lowest temperature studied). RHEED and LEED indicate that the silicide layer formed at low temperatures is amorphous. A contraction in the period of Mo/Si multilayers is observed as the deposition temperature is raised from 50 C to 200 C. This contraction can be accounted for by the formation of an interfacial silicide. TEM micrographs show an interfacial layer at the Mo on Si interfaces but not at the Si on Mo interfaces. Low-angle x-ray diffraction and synchrotron radiation characterization results for multilayers prepared under a variety of conditions are reported.
• Slaughter, J. M., Shapiro, A., Kearney, P. A., & Falco, C. M. (1991). Growth of molybdenum on silicon: Structure and interface formation. Physical Review B, 44(8), 3854-3863.
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  Abstract: Investigations of interface formation in the Mo-Si system were carried out by depositing Mo onto Si(100)-(2×1) and Si(111)-(7×7) surfaces in ultrahigh vacuum, followed by characterization with in situ reflection high-energy electron diffraction, low-energy electron diffraction, Auger-electron spectroscopy, and x-ray photoelectron spectroscopy (XPS). Continuous growth of multiple Mo coverages on a single Si wafer was accomplished with a technique involving a movable sample shutter. The formation of an amorphous interfacial silicide was observed at all substrate temperatures studied: 50°C, 100°C, and 200°C. However, the composition quickly becomes Mo rich as the deposition continues. The data are consistent with a composition profile that has an atomically abrupt transition between Si and amorphous MoSix, where x=2 for the first 4 and then decays with an error-function form with increasing overlayer thickness. The error-function interface-width parameter was found to be 10.0 at 50°C and 12.1 at 200°C. Significant differences were seen between Auger intensities calculated by two standard methods: the derivative-amplitude method and the linear-background integrated-intensity method. We attribute these differences to peak-shape changes (due to the varying chemical environment in the interfacial region) that invalidate the use of the derivative method. The XPS measurements revealed shifts in the energies of the Mo 3d3/2 and Mo 3d5/2 lines due to the reaction with the Si substrate. The maximum peak shift was -0.4 eV and originated from the Mo nearest the Si substrate. © 1991 The American Physical Society.
• Dutcher, J. R., Lee, S., England, C. D., Stegeman, G. I., & Falco, C. M. (1990). Elastic properties of CuCo multilayers. Materials Science and Engineering A, 126(1-2), 13-18.
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  Abstract: The acoustic phonon modes of sputter-deposited CuCo multilayers were studied using Brillouin scattering. These are the first such measurements to be made on an f.c.c.-f.c.c. multilayer system. The multilayer samples had a fixed atomic percentage of cobalt (53 at.%). The effective elastic constants of the multilayer samples, which were obtained by comparing the data with calculated dispersion curves for the phonon modes, were independent of the multilayer modulation wavelength. © 1990.
• Dutcher, J. R., Lee, S., Kim, J., Bell, J. A., Stegeman, G. I., & Falco, C. M. (1990). Brillouin scattering studies of the elastic properties of metallic superlattices. Materials Science and Engineering B, 6(2-3), 199-204.
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  Abstract: The determination of the elastic properties of metallic superlattices using Brillouin scattering is discussed. Previous work by our group is reviewed and preliminary results for Ag/Pd superlattices are reported. © 1990.
• Dutcher, J. R., Lee, S., Kim, J., Stegeman, G. I., & Falco, C. M. (1990). Enhancement of the c₁₁ elastic constant of Ag/Pd superlattice films as determined from longitudinal guided modes. Physical Review Letters, 65(10), 1231-1234.
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  Abstract: In Brillouin-light-scattering studies of a series of Ag/Pd superlattice films we have observed seattering from longitudinal guided modes (LGMs) whose velocities are determined primarily by the c11 elastic constant. From the measured velocity of the first-order LGM. we have obtained the first accurate determination of c11 for metallic thin films. For the Ag/Pd superlattice films, large increases in both the longitudinal (c11) and shear (c55) elastic constants were observed as the superlattice modulation wavelength A was decreased below A =60 Å.
• Hillebrands, B., Harzer, J. V., Güntherodt, G., England, C. D., & Falco, C. M. (1990). Experimental evidence for the existence of exchange-dominated collective spin-wave excitations in multilayers. Physical Review B, 42(10), 6839-6841.
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  Abstract: We report Brillouin-light-scattering experiments in Co/Pd multilayers with individual layer thicknesses between 1 and 64 atomic layers. Evidence is found for exchange-dominated collective spin-wave excitations when interlayer exchange coupling is active. © 1990 The American Physical Society.
• Leeuwen, R. V., England, C. D., Dutcher, J. R., Falco, C. M., Bennett, W. R., & Hillebrands, B. (1990). Structural and magnetic properties of Ti/Co multilayers. Journal of Applied Physics, 67(9), 4910-4912.
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  Abstract: We have investigated the structural and magnetic properties of sputter-deposited Ti/Co multilayer thin films. Sample composition was determined by Rutherford backscattering spectrometry. The crystal structure and composition modulation were examined with several x-ray diffraction techniques. X-ray diffraction analysis showed that the Ti and Co layers grow in the hcp structure and that the films are strongly textured along the (002) direction and are structurally incoherent. Composition modulation was confirmed from low-angle x-ray diffraction measurements. Vibrating-sample magnetometry (VSM) and Brillouin light scattering (BLS) were used to determine the magnetic properties of the films. VSM measurements of samples with Co layer thicknesses less than ≊22 Å showed no measurable net magnetization, while those with greater Co layer thicknesses had both in-plane and perpendicular components. Using a simple model, the measured thickness dependence of the magnetization can be explained by assuming that 11.3 Å of Co at each Ti interface is nonmagnetic with bulk magnetization for the remaining Co. Perpendicular volume and interface anisotropy constants obtained from the VSM data were Ks=0.23 erg/cm2 and Kv=3. 5×106 erg/cm3, respectively. Excellent agreement was obtained between the best fit magnetic parameters for the VSM and the BLS data.
• Deeter, M. N., Sarid, D., England, C. D., Bennett, W. R., & Falco, C. M. (1989). Determination of effective optical constants of magnetic multilayers. Applied Physics Letters, 54(21), 2059-2061.
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  Abstract: The effective optical and magneto-optical constants of a series of Cu/Co multilayer films are determined experimentally and compared with a theoretical thin-film model based on the bulk optical constants of Cu and Co. In the multilayer series, the atomic percentages of Cu and Co were kept fixed and the period varied from 0.4 to 13.6 nm. Deviations from bulk-like behavior in the effective optical constants are observed for multilayers with periods less than 3 nm.
• Makous, J. L., & Falco, C. M. (1989). Elastic and structural properties of Mo/Ta superlattices. Solid State Communications, 72(7), 667-670.
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  Abstract: Previously1 we reported the fabrication, structure, and electrical transport properties of Mo/Ta superlattices. This present paper reports a more quantitative analysis used to explain an observed wavelength-dependent strain perpendicular to the layers in terms of the van der Merwe2,3 model for epitaxial superlattices. These results are correlated with an anomalous decrease in the c44 elastic stiffness constant of Mo/Ta for 20 Å < ∧ < 50 Å. © 1989.
• Smith, A., Riedel, C., Edwards, B., Savage, D., Lai, B., Ray-Chaudhuri, A., Cerrina, F., Lagally, M. G., Underwood, J., & Falco, C. (1989). A simple monochromator based on x-ray multilayer mirrors. Review of Scientific Instruments, 60(7), 2003-2005.
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  Abstract: A simple two-mirror monochromator based on multilayer films has been developed as a part of a reflectometer under construction at the University of Wisconsin Synchrotron Radiation Center. Measurements of the performance of this monochromator and the characterization of a typical component mirror have been made. At AlKα the monochromator has a peak transmission of 9%, which is 55% of the theoretical value. Its resolution, λ/Δλ, is 35.
• Bell, J. A., Zanoni, R., Seaton, C. T., Stegeman, G. I., Makous, J., & Falco, C. M. (1988). Elastic constants of, and Stonely waves in, molybdenum films measured by Brillouin scattering. Applied Physics Letters, 52(8), 610-612.
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  Abstract: The dispersion of Rayleigh, Stonely, and Sezawa acoustic waves guided by deposited thin molybdenum films has been used to evaluate selected elastic constants and film homogeneity. This work includes the first experimental observation of Stonely waves by Brillouin scattering.
• Bennett, W. R., & Falco, C. M. (1988). Fabrication and structure of Mo/Ta superlattices. Journal of Applied Physics, 63(6), 2176-2178.
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  Abstract: Mo/Ta superlattices with superlattice wavelengths in the range 20-180 Å were made by alternately sputtering these components. We present the results of a variety of x-ray diffraction studies used to structurally characterize these samples.
• Boudry, J., Riedel, C., Edwards, B., Lagally, M., Redaelli, R., Cerrina, F., Falco, C., Fernandez, F., Underwood, J. H., & Hettrick, M. (1988). A beamline for layered synthetic microstructure studies. Nuclear Inst. and Methods in Physics Research, A, 266(1-3), 351-355.
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  Abstract: Layered synthetic microstructures (LSMs) have become an important X-ray optics component. The high reflectivity shown at large incidence angles in the XUV region allows for the design of efficient optical elements. LSMs, however, may be exposed to exceedingly large power densities during normal operation. This is particularly true in the case of the new storage rings under development, where LSM front optics are necessary in order to alleviate thermal loading of other elements. High reflectivity and long term stability are both key issues for successful applications to synchrotron radiation optics. The production of LSMs with high reflectivity depends critically on growth conditions, substrate preparation, and metallurgical properties of the materials involved. It then becomes necessary to characterize the properties of LSMs from the point of view of the material properties in order to improve the fabrication process. These include problems such as film structure, interfacial roughness, and intermixing. Large radiation doses may enhance defect formation and interdiffusion processes, thus leading to decreased performance. For these reasons we have begun a series of experiments to determine (a) the effect of growth conditions on LSM reflectivities and (b) the effect of large radiation doses on the LSM reflectivities. In this paper we will report results of LSM characterization and describe the beamline that we are preparing for more extensive studies. © 1988.
• Boufelfel, A., Hillebrands, B., Stegeman, G. I., & Falco, C. M. (1988). Fe/Pd second-order superlattices. Solid State Communications, 68(2), 201-204.
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  Abstract: We report the structural and magnetic properties of a new class of Fe/Pd second-order superlattices, formed by alternating two bilayers with different repeat periods. Chemical and structural characterization were obtained using Rutherford backscattering spectroscopy (RBS) and several x-ray diffraction techniques, respectively. The properties of the collective spin wave spectrum were studied by Brillouin scattering. © 1988.
• England, C. D., Bennett, W. R., & Falco, C. M. (1988). Magnetic and structural characterization of copper/cobalt multilayers. Journal of Applied Physics, 64(10), 5757-5759.
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  Abstract: Copper/cobalt multilayers were deposited by magnetically enhanced dc triode sputtering onto single-crystal sapphire substrates. According to the binary-phase diagram, no stable alloys of copper and cobalt exist. Because the multilayers are not exposed to high temperatures, we expect no alloy formation at the interfaces. X-ray diffraction techniques demonstrate crystalline layering existing down to monolayer thicknesses of copper and cobalt. The saturation magnetization behavior of the multilayers was modeled using the perpendicular and in-plane hysteresis curves obtained using a vibrating sample magnetometer. A torque magnetometer was used to determine the net magnetic anisotropy. Here we discuss the correlation between the magnetic behavior and the structural properties.
• England, C. D., Falco, C. M., & Aboaf, J. A. (1988). X-ray diffraction determination of stress in magnetron-sputtered permalloy films. Journal of Applied Physics, 63(8), 3227-3229.
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  Abstract: The stresses in magnetron-sputtered films depend on the deposition parameters. Magnetron-sputtered Permalloy (Ni 81%, Fe 19%) films were deposited onto fused quartz substrates using varying deposition parameters, including argon pressure, substrate bias, deposition rate, and film thickness. A Bragg-Brentano diffractometer was used to find the average stress in the films. A Seemann-Bohlin diffractometer was used to measure the strain either parallel or perpendicular to the magnetic easy axis in the Permalloy film. Kroner x-ray elastic constants were used to calculate the stresses from the strains determined by x-ray diffraction techniques. The average stress in the magnetron-sputtered Permalloy films was highly tensile but could be reduced by the appropriate combination of deposition parameters. The strain difference could also be modified by changing the deposition parameters.
• Falco, C. M., Fernandez, F. E., Dhez, P., & Khandar-Shahabad, A. (1988). Properties of multilayers for soft X-ray optics. Superlattices and Microstructures, 4(1), 51-53.
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  Abstract: Appropriate physical description of multilayer structures to be used as soft x-ray optical elements is necessary to ensure agreement of predicted and actual performance. Deviations of the fabricated structures from an ideal design (interfacial roughness and diffusion, microvoids, impurities, thickness errors) degrade the reflectance properties. In addition, deviations of the physical properties of very thin films from those of the bulk materials can limit the validity of reflectance calculations. We describe these difficulties and how a particular fabrication-characterization procedure can help solve them. Characterization techniques used include a variety of x-ray diffraction techniques, Rutherford Backscattering Spectroscopy and Transmission Electron Microscopy. Examples of results obtained for samples prepared by triode magnetically confined dc sputtering will be given, as will a discussion of the implication of these results for other multilayer materials. © 1988.
• Fernandez, F. E., & Falco, C. M. (1988). Multilayers for soft X-ray optics.. IEEE Journal of Quantum Electronics, 24(8), 1758-1762.
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  Abstract: Normal-incidence figured reflectors for soft X-rays using multilayer coatings have been produced and successfully used in the last several years. Very recently, the first X-ray image of an astronomical object by a multilayer reflector has been obtained. Partially transmitting optics that are possible with current technology and may contribute to the development of short-wavelength lasers and interferometry are reviewed.
• Hillebrands, B., Boufelfel, A., Falco, C. M., Baumgart, P., Güntherodt, G., Zirngiebl, E., & Thompson, J. D. (1988). Brillouin scattering from collective spin waves in magnetic superlattices (invited). Journal of Applied Physics, 63(8), 3880-3884.
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  Abstract: We report on the observation and the analysis of collective magnetostatic spin-wave excitations in magnetic superlattices. The influence of interface anisotropies, which can become dominant for small modulation wavelengths, is discussed. For the system Fe/Pd we show that Brillouin spectroscopy experiments in combination with the measurement of the saturation magnetization by a SQUID magnetometer give evidence for a magnetic polarization of the Pd spacer layers, as well as for a small negative out-of-plane interface anisotropy constant of Ks =-0.15 erg/cm2.
• Makous, J. L., & Falco, C. M. (1988). Integer monolayer metallic superlattices. Solid State Communications, 68(4), 375-378.
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  Abstract: We report the fabrication of metallic superlattices modulated by integer number of atomic planes of Mo and Ta. The set of superlattices reported here retain structural coherence and exhibit metallic resistivity behavior over the entire range of wavelengths down to the monolayer limit. These results contrast with structural and transport behavior observed in other metal-metal superlattices previously studied. © 1988.
• Maritato, L., Cucolo, A. M., Vaglio, R., Noce, C., Makous, J. L., & Falco, C. M. (1988). Electronic and superconducting properties of Mo-Ta superlattices. Physical Review B, 38(18), 12917-12921.
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  Abstract: We obtain bcc-bcc Mo-Ta superlattices in the modulation wavelength range 16 450 A. The x-ray analysis shows long structural coherence even in the short-modulation-wavelength limit, and the resistivity data are in the metallic regime. We find the universal behavior of the Tc versus curve already seen in other metals, compounds, and alloys and, for superlattices in the range 50 250 A the dV/dI versus V characteristics show a double-peak structure. The temperature behavior of these peaks is fitted by a two-band model and by a proximity-effect theory. © 1988 The American Physical Society.
• Bell, J. A., Bennett, W. R., Zanoni, R., Stegeman, G. I., Falco, C. M., & Nizzoli, F. (1987). Elastic constants of Mo/Ta superlattices measured by Brillouin scattering. Physical Review B, 35(8), 4127-4130.
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  Abstract: The elastic properties of Mo/Ta superlattices and the dependences on bilayer wavelength over the range 7200 AÌ are investigated. These are the first such measurements for a bcc-bcc system, and significant differences from previously studied metal superlattices are found. Surface velocities of as many as 12 acoustic modes guided by the thin-film samples have been measured using Brillouin spectroscopy. The elastic constants c11,c13,c33 and c44 for the superlattices are determined by fitting the data to an acoustic model of supported hexagonal films. We compare these stiffnesses with those predicted from the bulk constituent properties. The relative Brillouin cross section is found to be accurately represented by a pure ripple scattering theory with no photoelastic contribution. © 1987 The American Physical Society.
• Bell, J. A., Bennett, W. R., Zanoni, R., Stegeman, G. I., Falco, C. M., & Seaton, C. T. (1987). Elastic constants of Cu/Nb superlattices. Solid State Communications, 64(11), 1339-1342.
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  Abstract: Brillouin scattering has been performed on Cu/Nb metal superlattices with a multipass tandem interferometer. Behavior similar to that reported by Kueny et al. [Phys. Rev. Lett., 48, 166 (1982)] has been observed for the Rayleigh wave dispersion with bilayer periodicity. Multiple guided acoustic waves have been observed and their dispersion measured in order to obtain the variation in the c44 and c33 elastic constants with bilayer thickness. © 1987.
• Bell, J. A., Zanoni, R. J., Seaton, C. T., Stegeman, G. I., Bennett, W. R., & Falco, C. M. (1987). Brillouin scattering from Love waves in Cu/Nb metallic superlattices. Applied Physics Letters, 51(9), 652-654.
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  Abstract: Love waves have been observed for the first time by Brillouin scattering. The c12 constant of the metallic superlattice Cu/Nb has been measured.
• Bennett, W. R., Leavitt, J. A., & Falco, C. M. (1987). Growth dynamics at a metal-metal interface. Physical Review B, 35(9), 4199-4204.
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  Abstract: A determination of the sticking coefficient of sputter-deposited metal films on freshly deposited metal surfaces is described in detail. The systems investigated were Mo deposited on Ta and Ta on Mo. Also described is a detailed determination of the structure of Mo-Ta interfaces using Rutherford backscattering spectrometry and a combination of x-ray diffraction techniques. Within a few angstroms of each interface we find that the lattice is stretched in the growth direction and has an excess of defects relative to the bulk lattice. We also find that Mo/Ta superlattices fabricated with wavelengths in the range 20 to 120 A exhibit structural coherence extending over a number of superlattice layers. © 1987 The American Physical Society.
• Bennett, W. R., Person, D. C., & Falco, C. M. (1987). MAGNETIC PROPERTIES AND STRUCTURAL CHARACTERIZATION OF FE/TB MULTILAYERS.. Array, 127-130.
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  Abstract: The fabrication, structural characterization, and magnetic properties of Fe/Tb multilayers grown by dc-triode sputter deposition are discussed. The thin films have been characterized by X-ray diffraction and ion beam analysis techniques. Magnetic measurements indicate ferrimagnetic coupling between the Fe and Tb layers.
• Falco, C. M., Fernandez, F. E., Dhez, P., Khandar-Shahabad, A., Nevot, L., Pardo, B., & Corno, J. (1987). NORMAL INCIDENCE X-UV MIRRORS.. Proceedings of SPIE - The International Society for Optical Engineering, 733, 343-352.
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  Abstract: We describe the fabrication procedure as well as results of an extensive series of characterization measurements, model-fitting and synchrotron tests on Si/W multilayers designed as normal-incidence reflectors for approximately equals 200 A radiation. Characterization techniques used were low-angle diffraction, Bragg-Brentano and Seemann-Bohlin diffraction, Read camera, transmission electron microscopy and Rutherford backscattering spectroscopy. To our knowledge, this is the first time such a comprehensive set of characterization techniques has been applied to a multilayer x-ray mirror. Reflectances for approximately equals 200 A calculated using the results from these various characterization techniques are found to agree very well with measurements obtained with synchrotron radiation.
• Fernandez, F. E., & Falco, C. M. (1987). MULTILAYER REFLECTORS FOR THE 200 A REGION.. Proceedings of SPIE - The International Society for Optical Engineering, 688, 104-109.
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  Abstract: Silicon/tungsten multilayer normal-incidence mirrors with maximum reflectance at 212 Angstrom have been designed and studied. Details of the fabrication and characterization techniques are given. Preliminary results of synchrotron measurements show agreement with calculations based on microscopic structure of these multilayers. The Si/W combination has desirable characteristics for use in X-UV or soft x-ray devices.
• Fernández, F. E., Falco, C. M., Dhez, P., Khandar-Shahabad, A., Névot, L., Pardo, B., Corno, J., & Vidal, B. (1987). Characterization of multilayers for extended ultraviolet optics. Applied Physics Letters, 51(12), 880-882.
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  Abstract: We describe an extensive characterization procedure developed to study multilayers for extended ultraviolet (XUV, 1 Å≲λ≲1000 Å) optics. We present results of this procedure applied to sputtered Si/W multilayers designed as normal-incidence XUV reflectors for ∼200 Å. Techniques used were low-angle x-ray diffraction, Bragg-Brentano and Seemann-Bohlin diffraction, wide-film Debye-Scherrer (Read) camera, Rutherford backscattering spectroscopy, and transmission electron microscopy. Reflectances at several incidence angles were measured with synchrotron radiation and found to agree very well with reflectance curves calculated without adjustable parameters. The information obtained from the different techniques forms a coherent picture of the structure of these materials.
• Hillebrands, B., Baumgart, P., Mock, R., Güntherodt, G., Boufelfel, A., & Falco, C. M. (1987). Collective spin waves in Fe/Pd and Fe/W multilayers. Journal of Applied Physics, 61(8), 4308-4310.
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  Abstract: In Fe/Pd and Fe/W multilayers the complete, theoretically predicted band of collective magnetostatic spin-wave excitations has been observed. Despite the large number of bilayers (49-90) discrete spin-wave modes have been found in the upper half of the band. For Fe/Pd multilayers the saturation magnetization shows a monotonic reduction of up to 30% with decreasing modulation wavelength (down to 33 Å), which is not found in the Fe/W multilayer.
• Maritato, L., Falco, C. M., Aboaf, J., & Paul, D. I. (1987). Ferromagnetic multilayers of Permalloy and TiN. Journal of Applied Physics, 61(4), 1588-1591.
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  Abstract: Ferromagnetic multilayers of Permalloy (Ni 81%, Fe 19%) alternated with TiN were prepared by magnetically confined plasma dc sputtering. Rutherford backscattering spectroscopy and x-ray diffraction analysis showed high structural quality for our samples. The easy- and hard-coercivity field values and the anisotropy field values were reduced by the lamination and were almost independent of the number of layers. Electrical resistivity versus temperature and resistivity ratio measurements were also made.
• Vaglio, R., Cucolo, A., & Falco, C. M. (1987). Electron density in artificial metallic superlattices. Physical Review B, 35(4), 1721-1727.
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  Abstract: Detailed studies of the pair tunneling to a series of superconductive Nb/Cu artificial metallic superlattices are described. We report the first measurements, to our knowledge, of the dependence of the magnetic field penetration depth on the layer thickness in these materials. © 1987 The American Physical Society.
• Baumgart, P., Hillebrands, B., Mock, R., Güntherodt, G., Boufelfel, A., & Falco, C. M. (1986). Localized phonon modes in Fe-Pd multilayer structures. Physical Review B, 34(12), 9004-9007.
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  Abstract: The Rayleigh mode and up to six higher-order localized acoustic (Sezawa) surface modes have been observed in Fe-Pd multilayer structures using Brillouin scattering. The effective elastic constants of the multilayer structures have been determined from a fit of the measured mode dispersions. With decreasing multilayer modulation wavelength from 87 to 33, the elastic constant c11 shows an interface-induced softening of up to 20%. © 1986 The American Physical Society.
• Hillebrands, B., Baumgart, P., Mock, R., Güntherodt, G., Boufelfel, A., & Falco, C. M. (1986). Collective spin waves in Fe-Pd and Fe-W multilayer structures. Physical Review B, 34(12), 9000-9003.
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  Abstract: The complete, theoretically predicted band of collective magnetostatic spin-wave excitations has been observed for the first time in Fe-Pd and Fe-W multilayer structures by Brillouin scattering. Despite the large number of bilayers (49-90) discrete spin-wave modes due to a quantization of the momentum perpendicular to the stack have been observed in the upper half of the band. For Fe-Pd multilayer structures the saturation magnetization shows a monotonic decrease of up to 30% from that of the bulk Fe for modulation wavelengths down to 33 A. © 1986 The American Physical Society.
• Vaglio, R., Cucolo, A., & Falco, C. M. (1986). Pair tunneling in a two-dimensional superconductor. Physics Letters A, 118(2), 89-92.
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  Abstract: Pair tunneling to a two-dimensional superconductor is reported for the first time. It is found that flux quantization disappears due to the nature of the two-dimensional superconductor, while the magnitude of the pair tunneling current is unaffected. Also due to the specific behavior of the system under consideration, a crossover to conventional three-dimensional behavior is observed at low temperatures. © 1986.
• Fernandez, F. E., & Falco, C. M. (1985). SPUTTER DEPOSITED MULTILAYER V-UV MIRRORS.. Proceedings of SPIE - The International Society for Optical Engineering, 563, 195-200.
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  Abstract: The design and fabrication of aluminum/niobium multilayers to be used as normal-incidence reflectors for 300 A radiation are presented. Preliminary characterization results are discussed showing the effects of interfacial roughness and diffusion.
• Falco, C. M. (1984). FIGURES OF MERIT FOR SPUTTERED SUPERLATTICES.. Journal of Applied Physics, 56(4), 1218-1219.
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  Abstract: Two figures of merit for sputtered superlattices (FMSS) are proposed which allow a comparison of the quality of artificial superlattices prepared in different sputtering systems. These FMSS also demonstrate several conditions for optimizing the sputtering parameters which are not widely appreciated.
• Falco, C. M. (1984). STRUCTURAL AND ELECTRONIC PROPERTIES OF ARTIFICIAL METALLIC SUPERLATTICES.. Journal de Physique (Paris), Colloque, 45(4), 499-507.
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  Abstract: Recent progress in thin film deposition techniques has motivated renewed interest in 'artificially prepared' metallic superlattices, exhibiting different physical properties than those in naturally occurring materials. This paper describes the sputtering technique used to prepare these metallic superlattices, as well as considerations affecting their purity, interfacial quality, and crystallographic orientation. The contribution of the interfaces to the electronic transport properties is described in terms of finite size effect theories.
• Banerjee, I., Yang, Q. S., Falco, C. M., & Schuller, I. K. (1983). Anisotropic critical fields in superconducting superlattices. Physical Review B, 28(9), 5037-5040.
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  Abstract: The temperature and angular dependence of critical fields (Hc) have been studied as a function of layer thickness for superconducting Nb/Cu superlattices. For layer thicknesses between 100 and 300, dimensional crossover has been observed in the temperature dependence of Hc. Associated with the crossover we find a change in the angular dependence of Hc to that given by the effective-mass theory. This is the first time that a relationship has been found between dimensional crossover observed in the temperature dependence and that in the angular dependence of critical fields. © 1983 The American Physical Society.
• Falco, C., Botineau, J., Azema, A., deMicheli, M., & Ostrowsky, D. B. (1983). OPTICAL PROPERTIES DETERMINATION AT 10. 6 mu m OF THIN SEMICONDUCTING LAYERS.. Applied Physics A: Solids and Surfaces, A30(1), 23-26.
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  Abstract: An optical characterization of thin semiconducting multilayers in the infrared range, using a combination of m-lines and reflection spectroscopy techniques is exposed. Such a method, non-destructive, allows to determine the thickness and the refractive index of each component of a multilayer multimodal planar waveguide.
• Khan, M. R., Chun, C. S., Felcher, G. P., Grimsditch, M., Kueny, A., Falco, C. M., & Schuller, I. K. (1983). Structural, elastic, and transport anomalies in molybdenum/nickel superlattices. Physical Review B, 27(12), 7186-7193.
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  Abstract: Metallic superlattices of nickel and molybdenum have been synthesized for a wide range of layer thicknesses by alternate sputtering. X-ray examination shows that they are composed of layers of fcc nickel and bcc molybdenum oriented along [111] and [110] directions, respectively. Anomalies have been observed in the lattice spacings, elastic moduli, and electrical resistivity versus modulation wavelength. These anomalies occur simultaneously at the same superlattice wavelength. The anomalies have been interpreted as due to strains in the nickel lattice. © 1983 The American Physical Society.
• Schuller, I. K., & Falco, C. M. (1983). Superconducting properties of metallic heterostructures. Synthetic Metals, 5(3-4), 205-216.
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  Abstract: We review our recent results on the superconducting properties of artificially prepared metallic heterostructures. Our sputtering system used to prepare these materials is described in detail along with a calculation of the thermalization of sputtered atoms. The structure of Nb/Cu and Nb/Ti heterostructures is discussed, as are Tc, energy gap, and phonon frequency measurements. © 1983.
• Yang, Q. S., Falco, C. M., & Schuller, I. K. (1983). Tunneling studies of a metallic superlattice. Physical Review B, 27(6), 3867-3870.
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  Abstract: Tunneling studies of artificially prepared Nb-Cu superlattices have been performed. Individual layers were in the range 8-5000 with total sample thicknesses 1 m. The coupling strength 2kTc shows a continuous decrease from values of 3.8 (thick layers) appropriate for a strong-coupled superconductor toward the weak-coupled value of 3.5 (thin layers). Contrary to earlier theoretical predictions and reports of indirect experimental evidence, we find no evidence for electronic states within the superconducting energy gap. The energies of the Nb longitudinal and transverse acoustic (LA and TA) phonons are found to be unaltered from their bulk values down to layer thicknesses of 32. At 10 the TA peak remains unaltered in energy but significantly broadens while the LA peak becomes too broad to measure. These results, in conjunction with earlier Brillouin scattering measurements, imply large changes in the phonon dispersion as a function of layer thickness. © 1983 The American Physical Society.
• Banerjee, I., Yang, Q. S., Falco, C. M., & Schuller, I. K. (1982). Superconductivity of Nb/Cu superlattices. Solid State Communications, 41(11), 805-808.
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  Abstract: The superconducting transition temperature Tc of Nb/Cu superlattices has been investigated as a function of layer thickness. The dependence of Tc above 300 Å layer thickness agrees well with proximity effect theory with no adjustable parameters. Below 300 Å, the data in conjunction with current proximity theory shows that Tc of Nb decreases with layer thickness. This is interpreted as changes in the electronic density of states due to a decrease in the mean-free path. © 1982.
• Falco, C. M., & Schuller, I. K. (1982). ARTIFICIAL METALLIC SUPERLATTICES.. Array, 21-34.
• Falco, C. M., & Schuller, I. K. (1982). SUPERCONDUCTIVITY IN ARTIFICIAL Nb/Cu HETEROSTRUCTURES.. Array, 283-289.
• Falco, C., Azema, A., Botineau, J., & Ostrowsky, D. B. (1982). INFRARED PRISM COUPLING CHARACTERIZATION AND OPTIMIZATION VIA NEAR-FIELD m-LINE SCANNING.. Applied Optics, 21(10), 1847-1850.
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  PMID: 20389949;Abstract: The use of a detailed analysis of th near-field m-line pattern to characteirze and optimize prism coupling into infrared waveguides is described. Experimental results at 10. 6 mu m are given which completely confirm the theoretical predictions.
• KHAN, M. R., SCHULLER, I. K., & FALCO, C. (1982). SPUTTER EPITAXY OF AG AND NI FILMS.. PHYS STATUS SOLIDI A, V 73(N 1), 23-29.
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  Abstract: THE PAPER REPORTS ON THE SPUTTERED EPITAXY OF AG ON MICA SUBSTRATES AT ELEVATED TEMPERATURE (278 D. C) AND NI ON (111) AG SINGLE CRYSTAL SUBSTRATES AT 275 D. C. OTHER SUBSTRATES, NAMELY SAPPHIRE AND MGO WERE ALSO TRIED BUT WITH NO SUCCESS.NI COULD NOT BE GROWN EPITAXIALLY ON MICA AT ROOM TEMPERATURE OR AT ELEVATED TEMPERATURES UP TO 300 D. C OR AT VARIOUS PRESSURES IN THE RANGE 5 TO 20 MTORR.
• Kueny, A., Grimsditch, M., Miyano, K., Banerjee, I., Falco, C. M., & Schuller, I. K. (1982). Anomalous behavior of surface acoustic waves in Cu/Nb superlattices. Physical Review Letters, 48(3), 166-170.
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  Abstract: Brillouin-scattering measurements show an anomalous dip in the elastic constants of Nb/Cu superlattices for superlattice wavelengths of 100. This dip is correlated with changes in the electrical resistivity of the samples giving the first evidence that electronic effects are responsible for phonon softening in these materials. © 1982 The American Physical Society.
• Schuller, I. K., & Falco, C. M. (1982). Structure and physical properties of sputtered metallic superlattices. Surface Science, 113(1-3), 443-453.
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  Abstract: The technique of preparing metallic superlattices by sputtering is described as are the results of a calculation of the energy distribution of sputtered atoms. Structural studies by X-ray diffraction, Ion Mill Auger Electron Spectroscopy and Ion Mill Ion Scattering Spectroscopy all indicate well defined layer structure for the Nb/Cu system. The resistivity of Nb/Cu and Nb/Ti and the superconductivity of Nb/Cu are all found to be dependent upon the layered nature of the material. © 1982.
• Schuller, I. K., & Falco, C. M. (1982). Superconductivity and magnetism in metallic superlattices. Thin Solid Films, 90(2), 221-227.
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  Abstract: The preparation, structural and physical properties (superconductivity and magnetism) of metallic superlattices prepared by a sputtering technique developed by us are presented. X-ray diffraction, ion mill Auger and ion scattering spectroscopy, superconducting Tc and Hc values, as well as magnetization measurements, are described. © 1982.
• Werner, T. R., Banerjee, I., Yang, Q. S., Falco, C. M., & Schuller, I. K. (1982). Localization in a three-dimensional metal. Physical Review B, 26(4), 2224-2226.
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  Abstract: The transition from a positive to a negative temperature coefficient of resistivity (TCR) has been studied in the artificially layered metallic system Nb-Cu. We find the TCR to undergo a rather sudden change in sign from positive to negative as the layer thickness is decreased below approximately 10. The temperature dependence of the conductivity in the nonmetallic regime can be explained by localization-type theories, and implies that both localization and Coulomb effects are important in this purely metallic three-dimensional system. © 1982 The American Physical Society.
• Werner, T. R., Falco, C. M., & Schuller, I. K. (1982). Impurity scattering time in aluminum. Physical Review B, 25(7), 4510-4514.
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  Abstract: Using Gorkov's extension of the Ginzburg-Landau equations, we derive a formula for determining the impurity scattering time for a metal in the normal state in terms of the critical current density and transition temperature of the metal in the superconducting state. When we apply this formula to aluminum weak-link data, we find that the impurity scattering time varies by three orders of magnitude in going from clean to granular aluminum films. This corresponds quite well to impurity scattering times obtained directly from resistivity measurements of the films using the classical Drude-model formula. © 1982 The American Physical Society.
• Zheng, J. Q., Ketterson, J. B., Falco, C. M., & Schuller, I. K. (1982). MAGNETIZATION AND CURIE TEMPERATURE OF COMPOSITIONALLY MODULATED Cu/Ni FILMS.. Journal of Applied Physics, 53(4), 3150-3155.
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  Abstract: Detailed measurements of the magnetization of Cu/Ni composition modulated foils as a function of temperature, magnetic field, composition wavelength, and composition amplitude are reported. A Curie temperature is found that initially increases rapidly with wavelength but quickly saturates. It is also found that the Curie temperature is independent of composition amplitude which we ascribe to the existence of disk-like Ni clusters.
• Cashion, J. D., Shenoy, G. K., Niarchos, D., Viccaro, P. J., & Falco, C. M. (1981). MOESSBAUER EFFECT STUDY OF THE ABSENCE OF FE MAGNETISM IN SUPERCONDUCTING Sc//2Fe//3Si//5 AND Th//7Fe//3.. Mat Res Soc Symp Proc, 3, 315-320.
• Lee, T. W., & Falco, C. M. (1981). Josephson effects in ultrashort mean-free-path superconductors. Applied Physics Letters, 38(7), 567-569.
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  Abstract: We show that it is possible to observe Josephson effects at temperatures up to 17 K in a superconductor with intrinsic mean free path
• Meyer, K., Schuller, I. K., & Falco, C. M. (1981). THERMALIZATION OF SPUTTERED ATOMS.. Journal of Applied Physics, 52(9), 5803-5805.
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  Abstract: The energy distributions of sputtered Nb and Cu atoms ejected from amorphous targets under low-energy Ar bombardment have been calculated. A formula based on elementary kinetic gas theory is used to calculate the subsequent energy loss of the ejected atoms due to collisions in the sputtering gas. The energy distributions of the sputtered atoms arriving at the substrate is compared with the distributions obtained using thermal evaporation techniques. This comparison indicates that the preparation of epitaxial metallic films, such as layered ultrathin coherent structures, using sputtering techniques may have fundamental advantages over thermal evaporation.
• Zheng, J. Q., Falco, C. M., Ketterson, J. B., & Schuller, I. K. (1981). Magnetization of compositionally modulated CuNi films. Applied Physics Letters, 38(6), 424-426.
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  Abstract: We report static magnetization measurements on the compositionally modulated ferromagnetic alloy Cu/Ni, which, contrary to earlier ferromagnetic resonance measurements, show the moment per Ni atom is reduced relative to pure Ni. The low-temperature magnetization is found to vary linearly with modulation amplitude, but, surprisingly, the Curie temperature is found to be almost amplitude independent.
• Zheng, J. Q., Ketterson, J. B., Falco, C. M., & Schuller, I. K. (1981). Superconducting and transport properties of NbTi layered metals. Physica B+C, 108(1-3), 945-946.
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  Abstract: We report data on the superconducting transition temperature and room temperature resistivity of layered NbTi for thicknesses in the range 7 - 70 Å. © 1981.
• Cashion, J. D., Shenoy, G. K., Niarchos, D., Viccaro, P. J., Aldred, A. T., & Falco, C. M. (1980). SUPERCONDUCTIVITY AND MAGNETISM IN THE SERIES R//2Fe//3Si//5 BY MOESSBAUER AND MAGNETIZATION MEASUREMENTS.. Journal of Applied Physics, 52(3 pt 2), 2180-2182.
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  Abstract: Members of the R//2Fe//3Si//5 series become either superconducting or magnetically ordered depending on whether the metal atom R( equals rare earth, Y or Sc) is diamagnetic or paramagnetic, respectively. Moessbauer effect and magnetization measurements have been made to determine the magnetic state of the iron and the nature of the superconducting electrons. Results are reported in detail for Dy//2Fe//3Si//5. The nature of the superconducting electrons in the R//2Fe//3Si//5 series is discussed in the light of available measurements.
• Falco, C. M., Werner, T. R., & Schuller, I. K. (1980). T//c ENHANCEMENT?. Solar energy, 34(7), 535-538.
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  Abstract: Experimental evidence is presented which indicates that recent experiments claiming observation of microwave radiation stimulated T//c enhancement in superconducting bridges may be incorrect. The results indicate that energy gap and T//c enhancement theories and experiments should be seriously reevaluated.
• Falco, C. M., Werner, T. R., & Schuller, I. K. (1980). T_c enhancement?. Solid State Communications, 34(7), 535-538.
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  Abstract: We present experimental evidence which indicates that recent experiments claiming observation of microwave radiation stimulated Tc enhancement in superconducting bridges may be incorrect. Our results indicate that energy gap and Tc enhancement theories and experiments should be seriously reevaluated. © 1980.
• Lee, T. W., & Falco, C. M. (1980). JOSEPHSON EFFECTS IN Nb//3Sn MICROBRIDGES.. IEEE Transactions on Magnetics, MAG-17(1), 85-87.
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  Abstract: Josephson effects in long narrow Nb//3Sn microbridges at temperatures up to 17 K were studied. These microbridges are formed by photolithographic techniques and subsequently subjected to controlled electrical discharges to modifY the intrinsic T//c of the bridge region. The bridges exhibit 10 GHz microwave steps in their I-V characteristics whose amplitudes are in agreement with the Resistively Shunted Junction (RSJ) model. I-V characteristics can be fit assuming an effective temperature approximately 15 K above the bath temperature. Structures in the I-V characteristics in the absence of microwaves were also investigated. It was demonstrated that phase-slip centers are induced at weak superconducting positions along the bridge when the S-N boundarY of an expanding hot spot reaches within a thermal healing distance. The critical current of the phase-slip center thus formed exhibits a temperature dependence (1-T/T//c) one-half insteady of the usual mean field result (1-T/T//c)**3**/**2.
• Pang, C. S., Falco, C. M., & Schuller, I. K. (1980). High input impedance wideband rf preamplifier for SQUIDs. Review of Scientific Instruments, 51(9), 1272-1273.
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  Abstract: We describe a wideband rf preamplifier with high input impedance designed for operation of superconducting quantum interference devices (SQUIDs).
• Postol, T. A., Falco, C. M., Kampwirth, R. T., Schuller, I. K., & Yelon, W. B. (1980). Structure of amorphous silicon and silicon hydrides. Physical Review Letters, 45(8), 648-652.
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  Abstract: Neutron scattering measurements have been made on pure, hydrogenated, and deuterated samples of amorphous silicon (a-Si) in the wave-vector range 0.007-8.75-1. Small-angle data indicate structures in the samples of average radius of gyration as large as 270. Large-angle data show that for the concentrations we have measured (14%), the structure of a-Si is not altered by the incorporation of large amounts of H or D. The silicon-hydrogen and silicon-deuterium partial structure factors have also been obtained. © 1980 The American Physical Society.
• Falco, C. M. (1978). SQUIDS: Applications outside the laboratory. Physics in Technology, 9(4), 148-153.
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  Abstract: After a brief review of the Josephson effect and principles of operation of a SQUID, several areas of research that have been made possible due to the development of SQUIDS are discussed.
• Falco, C. M., & Schuller, I. K. (1978). LASER IRRADIATION EFFECTS IN SUPERCONDUCTORS.. J Phys (Paris) Colloq, 39 Colloq C-6(8), 529-530.
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  Abstract: A study has been made of the behavior of a thin film Superconducting Quantum Interference Device (SQUID) and of a bi-metallic superconducting loop under laser light irradiation. It is found that the experimental data can be explained by simple thermal heating without assuming the existence of non-thermal non-equilibrium quasiparticle distribution. The authors also find that the observation of collective behavior at low temperature as suggested by Bari and Sukhanov will be hindered by thermal effects.
• Falco, C. M., & Wu, C. T. (1978). PROPERTIES OF HIGH TEMPERATURE SQUIDs.. AIP Conf Proc, 44(1), 28-36.
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  Abstract: We review the presnet status of weak links and dc and rf biased SQUIDs made with high temperature superconductros. A method for producing reliable, reproducible devices using Nb//3Sn is outlined and comments are made on directions future work should take.
• Wu, C. T., & Falco, C. M. (1978). FABRICATION AND PROPERTIES OF HIGH-TEMPERATURE WEAK LINKS AND SQUID'S.. Journal of Applied Physics, 49(1), 361-365.
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  Abstract: Results are presented on the fabrication and properties of high-T//c Nb//3Sn thin-film weak links and SQUID's prepared by radiation damage of localized regions. High-temperature thin-film weak links with current-voltage characteristics similar to those produced either by proximity effect or ion-implantation techniques have been successfully fabricated using radiation damage. High-temperature (operation temperature greater than 14. 5 K) Nb//3Sn thin-film SQUID's with sensing area approximately 3 mm**2 have been successfully fabricated using these weak links.
• Falco, C. M. (1977). Observation of thermally induced potential in a superconductor. Physical Review Letters, 39(10), 660-663.
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  Abstract: Potential differences have been measured between a normal probe and a superconducting probe attached to a superconductor held in a temperature gradient. These observations suggest that a thermoelectrically generated quasiparticle current is flowing in the superconductor and allow evaluation of the thermal transport coefficient for the quasiparticles. © 1977 The American Physical Society.
• Likes, R. S., & Falco, C. M. (1977). Power spectrum of radiation from a Josephson junction. Journal of Applied Physics, 48(12), 5370-5371.
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  Abstract: The harmonic content of the power generated by a Josephson junction is calculated as a function of the applied current. The effect of the pair-quasiparticle interference conductivity is considered, and the limiting form of the power is derived for large driving currents.
• Wu, C. T., & Falco, C. M. (1977). High-temperature Nb₃Sn thin-film SQUID's. Applied Physics Letters, 30(11), 609-611.
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  Abstract: We have fabricated thin-film Nb3Sn weak links and rf SQUID's operating at temperatures ≳14.5 K. The properties of the weak links as well as the fabrication technique and operating characteristics of the SQUID's are described.
• Abraham, B. M., & Falco, C. M. (1976). Demountable ⁴He film stopper for dilution refrigerators. Review of Scientific Instruments, 47(2), 253-254.
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  Abstract: We describe an easily assembled and demountable film stopper for use in either a pumped 4He pot or a 3He-4He dilution refrigerator. The orifice plate for the film stopper is fabricated from a Pyrex glass plate and is sealed to the metal body of the still with an indium O-ring. The advantage of glass over metal for the orifice plate is achieved without complicated construction.
• Falco, C. M. (1976). Acoustically generated magnetic flux in a bi-metallic superconducting loop. Physical Review B, 14(9), 3853-3855.
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  Abstract: A calculation of the magnetic flux induced in a bi-metallic superconducting loop by an acoustic wave is described. Estimates of the magnitude of the effect have been made which show that it is large enough to be detected by presently available superconducting quantum interference magnetometers. © 1976 The American Physical Society.
• Falco, C. M. (1976). Thermal conductivity of mica from 1.5 to 4.2 K. Journal of Applied Physics, 47(7), 3355-3356.
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  Abstract: The thermal conductivity of mica along the direction parallel to the cleavage planes has been measured in the temperature range 1.5 to 4.2 K.
• Falco, C. M. (1976). Thermally induced magnetic flux in a superconducting ring. Solid State Communications, 19(7), 623-625.
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  Abstract: The thermally induced magnetic flux in a bi-metallic superconducting loop of Nb and Ta has been measured and compared with theory. The predicted logarithmic divergence of the flux near the Tc of Ta is found, however the magnitude of the flux is larger than predicted. © 1976.
• Falco, C. M., & Parker, W. H. (1975). OPERATING CHARACTERISTICS OF THIN-FILM RF-BIASED SQUID'S.. Journal of Applied Physics, 46(8), 3238-3243.
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  Abstract: Operating characteristics of rf-biased thin-film SQUID's of diameter as large as 1 cm are reported. Observation on these SQUID's have experimentally verified several of their conventionally assumed properties such as the dependence of the quantum interference signal on critical current of the weak link, total inductance of the SQUID, inductive coupling between drive coil and the SQUID, and random magnetic flux noise. A simple technique of lowering the inductance, and hence increasing the signal, of large-diameter SQUID's is reported.
• Falco, C. M. (1974). EFFECT OF THE CURRENT-PHASE RELATIONSHIP ON THE I-V CHARACTERISTICS OF WEAK LINKS: NOISE EFFECTS.. Journal of Applied Physics, 45(9), 4134-4135.
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  Abstract: When noise effects are considered, the quasiparticle-pair interference current is shown to have an observable effect on the behavior of superconducting weak links even when the I-V characteristics calculated in the absence of noise are indistinguishable from those calculated without the interference current.
• Falco, C. M., Parker, W. H., Trullinger, S. E., & Hansma, P. K. (1974). Effect of thermal noise on current-voltage characteristics of Josephson junctions. Physical Review B, 10(5), 1865-1873.
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  Abstract: Measurements are reported of the current-voltage (I-V) characteristics of externally shunted oxide-barrier tunnel junctions at temperatures T and critical currents Ic such that the Josephson coupling energy (2e)Ic is comparable to the thermal energy k T. The measurements are in complete quantitative agreement with the theory of Ambegaokar and Halperin in the limit that the dimensionless parameter βc=(2e) (IcCσ2)1, where C is the capacitance and σ the total conductance of the shunted junction. Measurements for larger values of βc are in qualitative agreement with the theory of Kurkijärvi and Ambegaokar. In addition, the Ambegaokar-Halperin theory is generalized to include the cos pair-quasiparticle interference term. It is concluded that a measurement of the cos term using only the I-V characteristic will be difficult. © 1974 The American Physical Society.
• Falco, C. M., Parker, W. H., & Trullinger, S. E. (1973). Erratum: Observation of a phase-modulated quasiparticle current in superconducting weak links (Physical Review Letters (1973) 31, 24). Physical Review Letters, 31(24), 1476-.
• Falco, C. M., Parker, W. H., & Trullinger, S. E. (1973). Observation of a phase-modulated quasiparticle current in superconducting weak links. Physical Review Letters, 31(15), 933-936.
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  Abstract: Current-voltage characteristics of current-biased Josephson junctions with a phase-dependent dissipative current σ1Vcos have been calculated in the presence of thermal noise. Measurements on superconducting weak links indicate the existence of such a phase-dependent dissipative current of the same magnitude and sign observed in tunnel junctions. © 1973 The American Physical Society.

Proceedings Publications

• Zhu, Z., Mankowski, T., Balakrishnan, K., Shikoh, A. S., Touati, F., Benammar, M. A., Mansuripur, M., Falco, C. M., Eldada, L., & Heben, M. (2015, 2015). Sol-gel deposition and plasma treatment of intrinsic, aluminum-doped and gallium-doped zinc oxide thin films as transparent conductive electrodes. In THIN FILMS FOR SOLAR AND ENERGY TECHNOLOGY VII, 9561.
• Falco, C. M. (2014, 2014). Metal nanowire-graphene composite transparent electrodes. In Thin Films for Solar and Energy Technology VI, SPIE.
• Falco, C. M. (2014, 2014). Transparent conducting electrodes based on thin, ultra-long copper nanowires and graphene nano-composites. In Thin Films for Solar and Energy Technology VI, SPIE.