Weigang Wang

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• Khadka, D., Thapaliya, T. R., Hurtado Parra, S., Han, X., Wen, J., Need, R. F., Khanal, P., Wang, W., Zang, J., Kikkawa, J. M., Wu, L., & Huang, S. X. (2020). Kondo physics in antiferromagnetic Weyl semimetal Mn Sn films. Science advances, 6(35), eabc1977.
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  Topology and strong electron correlations are crucial ingredients in emerging quantum materials, yet their intersection in experimental systems has been relatively limited to date. Strongly correlated Weyl semimetals, particularly when magnetism is incorporated, offer a unique and fertile platform to explore emergent phenomena in novel topological matter and topological spintronics. The antiferromagnetic Weyl semimetal MnSn exhibits many exotic physical properties such as a large spontaneous Hall effect and has recently attracted intense interest. In this work, we report synthesis of epitaxial Mn Sn films with greatly extended compositional range in comparison with that of bulk samples. As Sn atoms are replaced by magnetic Mn atoms, the Kondo effect, which is a celebrated example of strong correlations, emerges, develops coherence, and induces a hybridization energy gap. The magnetic doping and gap opening lead to rich extraordinary properties, as exemplified by the prominent DC Hall effects and resonance-enhanced terahertz Faraday rotation.
• Li, P., Ding, J., Zhang, S. S., Kally, J., Pillsbury, T., Heinonen, O. G., Rimal, G., Bi, C., DeMann, A., Field, S. B., Wang, W., Tang, J., Jiang, J. S., Hoffmann, A., Samarth, N., & Wu, M. (2020). Topological Hall Effect in a Topological Insulator Interfaced with a Magnetic Insulator. Nano Letters, 21(1), 84-90.
• Xu, M., Li, M., Khanal, P., Habiboglu, A., Insana, B., Xiong, Y., Peterson, T., Myers, J. C., Ortega, D., Qu, H., Chien, C. L., Zhang, W., Wang, J. P., & Wang, W. G. (2020). Voltage-Controlled Antiferromagnetism in Magnetic Tunnel Junctions. Physical review letters, 124(18), 187701.
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  We demonstrate a voltage-controlled exchange bias effect in CoFeB/MgO/CoFeB magnetic tunnel junctions that is related to the interfacial Fe(Co)O_{x} formed between the CoFeB electrodes and the MgO barrier. The unique combination of interfacial antiferromagnetism, giant tunneling magnetoresistance, and sharp switching of the perpendicularly magnetized CoFeB allows sensitive detection of the exchange bias. We find that the exchange bias field can be isothermally controlled by magnetic fields at low temperatures. More importantly, the exchange bias can also be effectively manipulated by the electric field applied to the MgO barrier due to the voltage-controlled antiferromagnetic anisotropy in this system.
• Bapna, M., Parks, B., Oberdick, S., Almasi, H., Sun, C., Voyles, P., Wang, W., & Majetich, S. A. (2019). Effect of Mo capping in sub-100 nm CoFeB-MgO tunnel junctions with perpendicular magnetic anisotropy. JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, 483, 34-41.
• Cheng, Y., Wang, W., & Zhang, S. (2019). Amplification of spin-transfer torque in magnetic tunnel junctions with an antiferromagnetic barrier. PHYSICAL REVIEW B, 99(10).
• Li, P., Kally, J., Zhang, S. S., Pillsbury, T., Ding, J., Csaba, G., Ding, J., Jiang, J. S., Liu, Y., Sinclair, R., Bi, C., DeMann, A., Rimal, G., Zhang, W., Field, S. B., Tang, J., Wang, W., Heinonen, O. G., Novosad, V., , Hoffmann, A., et al. (2019). Magnetization switching using topological surface states. Science advances, 5(8), eaaw3415.
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  Topological surface states (TSSs) in a topological insulator are expected to be able to produce a spin-orbit torque that can switch a neighboring ferromagnet. This effect may be absent if the ferromagnet is conductive because it can completely suppress the TSSs, but it should be present if the ferromagnet is insulating. This study reports TSS-induced switching in a bilayer consisting of a topological insulator BiSe and an insulating ferromagnet BaFeO. A charge current in BiSe can switch the magnetization in BaFeO up and down. When the magnetization is switched by a field, a current in BiSe can reduce the switching field by ~4000 Oe. The switching efficiency at 3 K is 300 times higher than at room temperature; it is ~30 times higher than in Pt/BaFeO. These strong effects originate from the presence of more pronounced TSSs at low temperatures due to enhanced surface conductivity and reduced bulk conductivity.
• Li, X., Sasaki, T., Grezes, C., Wu, D., Wong, K., Bi, C., Ong, P. V., Ebrahimi, F., Yu, G., Kioussis, N., Wang, W., Ohkubo, T., Khalili Amiri, P., & Wang, K. L. (2019). Predictive Materials Design of Magnetic Random-Access Memory Based on Nanoscale Atomic Structure and Element Distribution. Nano letters, 19(12), 8621-8629.
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  Magnetic tunnel junctions (MTJs) capable of electrical read and write operations have emerged as a canonical building block for nonvolatile memory and logic. However, the cause of the widespread device properties found experimentally in various MTJ stacks, including tunneling magnetoresistance (TMR), perpendicular magnetic anisotropy (PMA), and voltage-controlled magnetic anisotropy (VCMA), remains elusive. Here, using high-resolution transmission electron microscopy and energy-dispersive X-ray spectroscopy, we found that the MTJ crystallization quality, boron diffusion out of the CoFeB fixed layer, and minimal oxidation of the fixed layer correlate with the TMR. As with the CoFeB free layer, seed layer diffusion into the free layer/MgO interface is negatively correlated with the interfacial PMA, whereas the metal-oxides concentrations in the free layer correlate with the VCMA. Combined with formation enthalpy and thermal diffusion analysis that can explain the evolution of element distribution from MTJ stack designs and annealing temperatures, we further established a predictive materials design framework to guide the complex design space explorations for high-performance MTJs. On the basis of this framework, we demonstrate experimentally high PMA and VCMA values of 1.74 mJ/m and 115 fJ/V·m with annealing stability above 400 °C.
• Bapna, M., Parks, B., Oberdick, S. D., Almasi, H., Wang, W., & Majetich, S. A. (2018). Spin-Orbit-Torque Switching in 20-nm Perpendicular Magnetic Tunnel Junctions. PHYSICAL REVIEW APPLIED, 10(2).
• Newhouse-Illige, T., Xu, Y. H., Liu, Y. H., Huang, S., Kato, H., Bi, C., Xu, M., LeRoy, B. J., & Wang, W. G. (2018). Temperature dependence of interlayer coupling in perpendicular magnetic tunnel junctions with GdOX barriers. APPLIED PHYSICS LETTERS, 112(7).
• Parks, B., Bapna, M., Igbokwe, J., Almasi, H., Wang, W., & Majetich, S. A. (2018). Superparamagnetic perpendicular magnetic tunnel junctions for true random number generators. AIP ADVANCES, 8(5).
• Tu, K., Fernandez, E., Almasi, H., Wang, W., Otero, D. N., Ntetsikas, K., Moschovas, D., Avgeropoulos, A., & Ross, C. A. (2018). Magnetic reversal and thermal stability of CoFeB perpendicular magnetic tunnel junction arrays patterned by block copolymer lithography. NANOTECHNOLOGY, 29(27).
• Williamson, M., de, R. M., Almasi, H., Chao, X., Wang, W., Wang, J. -., & Tsoi, M. (2018). Saturation of VCMA in out-of-plane magnetized CoFeB/MgO/CoFeB magnetic tunnel junctions. AIP ADVANCES, 8(5).
• Almasi, H., Sun, C. L., Li, X., Newhouse-Illige, T., Bi, C., Price, K. C., Nahar, S., Grezes, C., Hu, Q., Amiri, P. K., Wang, K. L., Voyles, P. M., & Wang, W. G. (2017). Perpendicular magnetic tunnel junction with W seed and capping layers. JOURNAL OF APPLIED PHYSICS, 121(15).
• Bi, C., Almasi, H., Price, K., Newhouse-Illige, T., Xu, M., Allen, S. R., Fan, X., & Wang, W. (2017). Anomalous spin-orbit torque switching in synthetic antiferromagnets. PHYSICAL REVIEW B, 95(10).
• Bi, C., Sun, C., Xu, M., Newhouse-Illige, T., Voyles, P. M., & Wang, W. (2017). Electrical Control of Metallic Heavy-Metal-Ferromagnet Interfacial States. PHYSICAL REVIEW APPLIED, 8(3).
• Newhouse-Illige, T., Liu, Y., Xu, M., Hickey, D. R., Kundu, A., Almasi, H., Bi, C., Wang, X., Freeland, J. W., Keavney, D. J., Sun, C. J., Xu, Y. H., Rosales, M., Cheng, X. M., Zhang, S., Mkhoyan, K. A., & Wang, W. G. (2017). Voltage-controlled interlayer coupling in perpendicularly magnetized magnetic tunnel junctions. NATURE COMMUNICATIONS, 8.
• Williamson, M., de, R. M., Almasi, H., Chao, X., Wang, W., Wang, J. -., & Tsoi, M. (2017). Probing VCMA in MTJs with in-plane magnetization. AIP ADVANCES, 7(11).
• Almasi, H., Xu, M., Xu, Y., Newhouse-Illige, T., & Wang, W. G. (2016). Effect of Mo insertion layers on the magnetoresistance and perpendicular magnetic anisotropy in Ta/CoFeB/MgO junctions. APPLIED PHYSICS LETTERS, 109(3).
• Bi, C., Xu, M., Almasi, H., Rosales, M., & Wang, W. (2016). Metal Based Nonvolatile Field-Effect Transistors. ADVANCED FUNCTIONAL MATERIALS, 26(20), 3490-3495.
• Tryputen, L., Tu, K., Piotrowski, S. K., Bapna, M., Majetich, S. A., Sun, C., Voyles, P. M., Almasi, H., Wang, W., Vargas, P., Tresback, J. S., & Ross, C. A. (2016). Patterning of sub-50nm perpendicular CoFeB/MgO-based magnetic tunnel junctions. NANOTECHNOLOGY, 27(18).
• Almasi, H., Hickey, D. R., Newhouse-Illige, T., Xu, M., Rosales, M. R., Nahar, S., Held, J. T., Mkhoyan, K. A., & Wang, W. G. (2015). Enhanced tunneling magnetoresistance and perpendicular magnetic anisotropy in Mo/CoFeB/MgO magnetic tunnel junctions. APPLIED PHYSICS LETTERS, 106(18).
• Piotrowski, S. K., Bapna, M., Almasi, H., Wang, W., Tryputen, L., Ross, C. A., Li, M., Chien, C., & Majetich, S. A. (2015). Conductive Atomic Force Microscopy of Small Magnetic Tunnel Junctions With Interface Anisotropy. IEEE TRANSACTIONS ON MAGNETICS, 51(11).
• Bi, C., Liu, Y., Newhouse-Illige, T., Xu, M., Rosales, M., Freeland, J. W., Mryasov, O., Zhang, S., te Velthuis, S. G., & Wang, W. G. (2014). Reversible Control of Co Magnetism by Voltage-Induced Oxidation. PHYSICAL REVIEW LETTERS, 113(26).
• Zhao, R., Boudou, T., Wang, W., Chen, C., & Reich, D. (2014). Magnetic approaches to study collective three-dimensional cell mechanics in long-term cultures. J. Appl. Phys, 115.
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• Wang, W. G., & Chien, C. L. (2013). Voltage-induced switching in magnetic tunnel junctions with perpendicular magnetic anisotropy. Journal of Physics D: Applied Physics, 46(8).
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  Abstract: Spintronic devices can be operated by either a magnetic field or a spin polarized current; however, the former is not site-specific, and the latter suffers from large current density issues. In this work, we show that voltage-controlled spintronic devices offer many attributes. Although a metallic ferromagnet responds only very weakly to an electric field if at all, under special circumstances an electric field can have a profound impact on its magnetic properties. An electric field can alter the interfacial perpendicular magnetic anisotropy (PMA) of CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) in a prescribed manner. By exploiting the voltage dependence of the PMA we have accomplished voltage-controlled MTJ for which the high- and low-resistance states can be accessed reversibly and repeatedly by voltage pulses associated with very low current density in the range of 104 A cm-2. This development opens up a new avenue to achieve ultra-low power consumption and ultra-fast operation in next-generation spintronic devices. © 2013 IOP Publishing Ltd.
• Wang, W., & Chien, C. (2013). Voltage induced switching in magnetic tunnel junctions with perpendicular magnetic anisotropy. J. Phys. D: Appl. Phys., 46.
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• Zhao, R., Boudou, T., Wang, W., Chen, C. S., & Reich, D. H. (2013). Decoupling cell and matrix mechanics in engineered microtissues using magnetically actuated microcantilevers. Advanced Materials, 25(12), 1699-1705.
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  PMID: 23355085;Abstract: A novel bio-magnetomechanical microtissue system is described for magnetic actuation of arrays of 3D microtissues using microcantilevers. This system enables both in situ measurements of fundamental mechanical properties of engineered tissue, such as contractility and stiffness, as well as dynamic stimulation of the microtissues. Using this system, cell and extracellular matrix contributions to the tissue mechanical properties are decoupled for the first time under both static and dynamic loading conditions. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
• Huang, S. Y., Fan, X., Qu, D., Chen, Y. P., Wang, W. G., Wu, J., Chen, T. Y., Xiao, J. Q., & Chien, C. L. (2012). Transport magnetic proximity effects in platinum. Physical Review Letters, 109(10).
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  Abstract: Platinum (Pt) metal, being nonmagnetic and with a strong spin-orbit coupling interaction, has been central in detecting the pure spin current and establishing most of the recent spin-based phenomena. Magnetotransport measurements, both electrical and thermal, conclusively show strong ferromagnetic characteristics in thin Pt films on the ferromagnetic insulator due to the magnetic proximity effects. The pure spin current phenomena measured by Pt, including the inverse spin Hall and the spin Seebeck effects, are thus contaminated and not exclusively established. © 2012 American Physical Society.
• Huang, S. Y., Wang, W. G., Qu, D., Lee, S. F., Kwo, J., & Chien, C. L. (2012). Thermal spin transport and applications. Technical Digest - International Electron Devices Meeting, IEDM, 11.2.1-11.2.4.
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  Abstract: Spin caloritronics, exploiting the interaction between spin with heat currents, offers a promising path to further reduction in both the size and power consumption of solid state devices. Despite recent observations of spin dependent thermal transport by several groups, the underlying physical mechanism remains unsettled.1-3 Our study has demonstrated the profound effect of substrate on the spin-dependent thermal transport by patterned ferromagnetic thin films.4 This unexpected behavior is due to an out-of-plane temperature gradient imposed by the thermal conduction through the substrate, resulting in a mixture of anomalous Nernst effects (ANE) and spin Seebeck effect (SSE)1-3. Only with substrate-free sample have we determined the intrinsic spin-dependent thermal transport with characteristics and field sensitivity similar to those of anisotropic magnetoresistance (AMR) effect and planer Hall effect (PHE).4 These effects are sensitive to magnetic fields, encouraging for future applications such as spin thermoelectric coating, and sensors.5 © 2012 IEEE.
• Rumaiz, A., Jaye, C., Woicik, J., Wang, W., Fischer, D., Jordan-Sweet, ,. J., & Chien, C. (2012). Boron migration due to annealing in CoFeB/MgO/CoFeB tunnel junctions: A combined HAXPES and NEXAFS study. Appl. Phys. Lett., 99.
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• Stearrett, R., Wang, W. G., Kou, X., Feng, J. F., Coey, J. M., Xiao, J. Q., & Nowak, E. R. (2012). Influence of exchange bias on magnetic losses in CoFeB/MgO/CoFeB tunnel junctions. Physical Review B - Condensed Matter and Materials Physics, 86(1).
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  Abstract: The strength of the exchange bias field is found to influence the low-frequency magnetoresistive noise associated with the magnetic reference layer in sputtered-deposited and electron-beam-evaporated CoFeB/MgO/CoFeB tunnel junctions. The noise is due to magnetic losses arising in the reference layer. The losses are parameterized by a phase lag ε which exhibits a nontrivial dependence on the externally applied field. The general trend found among all devices is that the losses are largest in the antiparallel state. The effect of exchange bias on the reference layer's noise is investigated at a field corresponding to maximum resistance susceptibility, H ref. Higher values for the phase lag at H ref, ref, are found in devices having a large exchange bias field. We also observed that H ref and ref are larger in devices having thicker seed layers. This characteristic is also evident in double-barrier magnetic tunnel junctions. Prolonged thermal annealing is found to decrease ref, reduce H ref, and alter the field profile of the resistance susceptibility of the reference layer to resemble that of a more magnetically soft behavior. In addition to its impact on the magnetoresistive noise, the incorporation of exchange bias layers into the materials stack also affects the tunneling magnetoresistance ratio with higher values found at smaller exchange bias fields. We attribute the magnitude of the magnetic losses, and hence the magnetoresistive noise, from the reference layer to disorder in its magnetic microstructure. Our results indicate that the nature and degree of disorder are correlated to the strength of the exchange bias coupling. The origin of this correlation may be due to a competition between different microstructures among various layers, one that leads to coherent tunneling (large tunneling magnetoresistance) in MgO-based tunneling devices and the other which promotes strong exchange bias coupling. A decrease in the exchange bias either through degradation from thermal treatments or by varying the thickness of the underlying seed layer will lead to less magnetic disorder in the system. We show that the magnetoresistive noise can be used to probe magnetic disorder in exchange-biased devices through the determination of the magnetic losses. © 2012 American Physical Society.
• Wang, W., Mingen, L. i., Hageman, S., & Chien, C. L. (2012). Electric-field-assisted switching in magnetic tunneljunctions. Nature Materials, 11(1), 64-68.
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  PMID: 22081084;Abstract: The advent of spin transfer torque effect accommodates site-specific switching of magnetic nanostructures by current alone without magnetic field. However, the critical current density required for usual spin torque switching remains stubbornly high around 10 6 g10 7 A cm -2. It would be fundamentally transformative if an electric field through a voltage could assist or accomplish the switching of ferromagnets. Here we report electric-field-assisted reversible switching in CoFeB/MgO/CoFeB magnetic tunnel junctions with interfacial perpendicular magnetic anisotropy, where the coercivity, the magnetic configuration and the tunnelling magnetoresistance can be manipulated by voltage pulses associated with much smaller current densities. These results represent a crucial step towards ultralow energy switching in magnetic tunnel junctions, and open a new avenue for exploring other voltage-controlled spintronic devices.
• Huang, S. Y., Wang, W. G., Lee, S. F., Kwo, J., & Chien, C. L. (2011). Intrinsic spin-dependent thermal transport. Physical Review Letters, 107(21).
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  PMID: 22181905;Abstract: Most studies of spin caloritronic effects to date, including spin-Seebeck effect, utilize thin films on substrates. We use patterned ferromagnetic thin film to demonstrate the profound effect of a substrate on the spin-dependent thermal transport. With different sample patterns and on varying the direction of temperature gradient, both longitudinal and transverse thermal voltages exhibit asymmetric instead of symmetric spin dependence. This unexpected behavior is due to an out-of-plane temperature gradient imposed by the thermal conduction through the substrate and the mixture of anomalous Nernst effects. Only with substrate-free samples have we determined the intrinsic spin-dependent thermal transport with characteristics and field sensitivity similar to those of the anisotropic magnetoresistance effect. © 2011 American Physical Society.
• Rumaiz, A. K., Jaye, C., Woicik, J. C., Wang, W., Fischer, D. A., Jordan-Sweet, J., & Chien, C. L. (2011). Boron migration due to annealing in CoFeB/MgO/CoFeB interfaces: A combined hard x-ray photoelectron spectroscopy and x-ray absorption studies. Applied Physics Letters, 99(22).
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  Abstract: We report the hard x-ray photo-electron spectroscopy (HAXPES) and near edge x-ray absorption fine structure (NEXAFS) of CoFeBMgOCoFeB interfaces as a function of annealing time. Upon annealing, the oxidation state of B changes from predominantly elemental (0 valence) boron in the as deposited sample to higher oxidation in annealed samples as evident from HAXPES spectra. The NEXAFS spectroscopy results showed that upon heating, B species migrate towards the MgO and interact with it. A comparison of the tunnel junction NEXAFS signature with some standards suggests that the B forms a 3-fold coordinated boron compound in the MgO environment and 4-fold coordinated boron resembling Kotoite mineral in the CoFe/MgO interface. © 2011 American Institute of Physics.
• Wang, W., Hageman, S., Mingen, L. i., Huang, S., Kou, X., Fan, X., Xiao, J. Q., & Chien, C. L. (2011). Rapid thermal annealing study of magnetoresistance and perpendicular anisotropy in magnetic tunnel junctions based on MgO and CoFeB. Applied Physics Letters, 99(10).
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  Abstract: The tunneling magnetoresistance and perpendicular magnetic anisotropy in CoFeB(1.1-1.2 nm)/MgO/CoFeB(1.2-1.7 nm) junctions were found to be very sensitively dependent on annealing time. During annealing at a given temperature, decay of magnetoresistance occurs much earlier compared to junctions with in-plane magnetic anisotropy. Through a rapid thermal annealing study, the decrease of magnetoresistance is found to be associated with the degradation of perpendicular anisotropy, instead of impurity diffusion as observed in common in-plane junctions. The origin of the evolution of perpendicular anisotropy as well as possible means to further enhance tunneling magnetoresistance is discussed. © 2011 American Institute of Physics.
• Zhang, Y., Fan, X., Wang, W., Kou, X., Cao, R., Chen, X., Chaoying, N. i., Pan, L., & Xiao, J. Q. (2011). Study and tailoring spin dynamic properties of CoFeB during rapid thermal annealing. Applied Physics Letters, 98(4).
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  Abstract: We studied the real-time evolution of magnetic dynamic and static properties of 20 nm CoFeB thin film during annealing at 380 °C. The ferromagnetic resonance linewidth quickly reduces by 30% within 300 s annealing, and monotonically increases upon longer annealing. The magnetic static coercivity shows similar trend. The underlying physical relation between linewidth and anisotropy can be connected by the two-magnon scattering theory. By doping of Nb into CoFeB films, the damping was maintained at a low value within 2000 s annealing. This method to tailor the dynamic properties of CoFeB may benefit the development of magnetics and spintronics based microwave devices. © 2011 American Institute of Physics.
• Guo, L., Hung, D., Wang, W., Shen, W., Zhu, L., Chien, C., & Searson, P. C. (2010). Tunnel barrier photoelectrodes for solar water splitting. Applied Physics Letters, 97(6).
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  Abstract: Tunnel barrier photoelectrodes, with a thin inorganic tunnel barrier layer that isolates the semiconductor electrode from the electrolyte while allowing current flow across the interface, are a possible solution to the problem of photocorrosion in solar water splitting. In this approach, selection of the semiconductor for the light absorber is decoupled from selection of the tunnel barrier material that provides chemical stability. Here we demonstrate a proof-of-principle of this approach with TiO2/MgO tunnel barrier photoelectrodes. © 2010 American Institute of Physics.
• Kou, X., Wang, W., Fan, X., Shah, L. R., Tao, R., & Xiao, J. Q. (2010). High temperature annealing induced superparamagnetism in CoFeB/MgO/CoFeB tunneling junctions. Journal of Applied Physics, 108(8).
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  Abstract: We have investigated the evolution of the magnetic transport properties as a function of short annealing time in CoFeB/MgO/CoFeB based magnetic tunnel junctions (MTJs) with a free layer of 2 nm. It is found that the hysteresis behaviors in magnetoresistance (MR) loops disappear in samples annealed for 17 min. The linear region between MR and the applied field gradually increases. The MR loops without hysteresis can be well fitted by using the superparamagnetism theory, suggesting the formation of superparamagnetic particles in the free layer during the high temperature annealing. The control of MTJ properties with annealing time is desirable in magnetic field sensor productions. © 2010 American Institute of Physics.
• Qi, L. u., Mellinger, Z. J., Wang, W., Wanfeng, L. i., Chen, Y., Chen, J. G., & Xiao, J. Q. (2010). Differentiation of bulk and surface contribution to supercapacitance in amorphous and crystalline NiO. ChemSusChem, 3(12), 1367-1370.
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  PMID: 20957719;Abstract: The Incredible Bulk: We investigate and quantify the different contributions in both amorphous and crystalline electrode materials. The results provide a scientific basis for future NiO electrode design. The conclusions and the characterization techniques can also be helpful in understanding other candidate materials for pseudocapacitors. Copyright © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
• Rumaiz, A. K., Woicik, J. C., Wang, W. G., Jordan-Sweet, J., Jaffari, G. H., Ni, C., Xiao, J. Q., & Chien, C. L. (2010). Effects of annealing on the local structure of Fe and Co in CoFeB/MgO/CoFeB tunnel junctions: An extended x-ray-absorption fine structure study. Applied Physics Letters, 96(11).
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  Abstract: Extended x-ray-absorption fine structure (EXAFS) was used to study the evolution of the local structure of Fe and Co as a function of annealing time in CoFeB/MgO/CoFeB magnetic tunnel junctions. EXAFS indicates B depletion and crystallization of the CoFeB layers within a few seconds of the post growth high temperature anneal. The decrease in first-shell Debye-Waller factor and hence the increase in structural order during annealing explains the increase in tunnel magnetoresistance observed as a result of post deposition annealing. © 2010 American Institute of Physics.
• Shah, L. R., Zhu, H., Wang, W. G., Ali, B., Zhu, T., Fan, X., Song, Y. Q., Wen, Q. Y., Zhang, H. W., Shah, S. I., & Xiao, J. Q. (2010). Effect of Zn interstitials on the magnetic and transport properties of bulk Co-doped ZnO. Journal of Physics D: Applied Physics, 43(3).
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  Abstract: We have demonstrated that the bound magnetic polaron model is responsible for ferromagnetism in Co-ZnO semiconductors, where the carriers are provided by the interstitial zinc (Zni). Our experiment is unique since by changing the temperature, we are able to cross the carrier concentration threshold above which a long-range ferromagnetic order is established. Consequently, the ferromagnetic order is observed at room temperature but is weakened at temperatures below 100 K. To support our conclusion we have performed a systematic investigation on the structural, magnetic and transport properties which all give consistent results in the context of our proposed two-region model, i.e. (a) a Zni layer where carriers are sufficient to couple Co ions ferromagnetically and (b) a region with little carriers that remain in a paramagnetic state. © 2010 IOP Publishing Ltd.
• Stearrett, R., Wang, W. G., Shah, L. R., Gokce, A., Xiao, J. Q., & Nowak, E. R. (2010). Evolution of barrier-resistance noise in CoFeB/MgO/CoFeB tunnel junctions during annealing. Journal of Applied Physics, 107(6).
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  Abstract: The low-frequency resistance noise in sputtered-deposited magnetic tunnel junctions with MgO barriers has been measured as a function of annealing time at different annealing temperatures. The noise has a 1/f spectrum and it is quantified by a Hooge-like parameter α given in units of μ m 2. Unannealed devices have the highest noise levels and their α parameters exhibit a pronounced dependence on the voltage bias across the junction. A significant increase in tunneling magnetoresistance (TMR) is observed for short annealing times (on the order of minutes) at high temperatures and it is correlated with a large reduction in noise and in its bias dependence. The maximum TMR and minimum noise levels are reached at a later time that depends on temperature, being shorter at higher annealing temperatures. Devices annealed at 380 and at 430 °C exhibit the same minimum noise levels, α≈2× 10-10 μ m2. The origin of the resistance noise, its annealing time evolution, and its bias dependence are discussed and they are attributed to vacancy defects in the MgO barriers. © 2010 American Institute of Physics.
• Stearrett, R., Wang, W. G., Shah, L. R., Xiao, J. Q., & Nowak, E. R. (2010). Magnetic noise evolution in CoFeB/MgO/CoFeB tunnel junctions during annealing. Applied Physics Letters, 97(24).
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  Abstract: We report on the evolution of equilibrium magnetoresistive (MR) 1/f noise due to the exchange-biased magnetic layer in MgO-based magnetic tunnel junctions as a function of annealing time at 380 and 430 °C. The resistance susceptibility and MR noise are observed to increase rapidly with annealing time at a fixed temperature. The magnetic losses responsible for MR noise are not significantly affected by the structural crystallization at the CoFeB/MgO interface during short annealing times. After prolonged annealing, the decrease in magnetic losses is attributed to reduced disorder in the magnetic layers that result in thermally driven fluctuations in local micromagnetic structure. © 2010 American Institute of Physics.
• Wang, W. G., Ni, C., Miao, G. X., Weiland, C., Shah, L. R., Fan, X., Parson, P., Jordan-Sweet, J., Kou, X. M., Zhang, Y. P., Stearrett, R., Nowak, E. R., Opila, R., Moodera, J. S., & Xiao, J. Q. (2010). Understanding tunneling magnetoresistance during thermal annealing in MgO-based junctions with CoFeB electrodes. Physical Review B - Condensed Matter and Materials Physics, 81(14).
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  Abstract: The competition between the interface crystallization and diffusion processes, their influence on the onset of symmetry-filtering coherent tunneling of Δ1 band electrons in the MgO-based magnetic tunnel junctions is investigated. Systematic study of the transport and magnetoresistance during thermal annealing of these junctions shows a unique behavior of the tunneling conductance in the parallel state. The optimal annealing time for achieving giant tunneling magnetoresistance at different temperatures is determined. The evolution of magnetoresistance consists of three distinct regions, responsible by different contributions from CoFeB electrodes and the MgO barrier. The whole phenomenon can be understood through an empirical model based on the Landauer tunneling picture. © 2010 The American Physical Society.
• Zhang, Y., Pan, L., Zhu, H., Wang, W., Shah, L. R., Fan, X., & Xiao, J. Q. (2010). Enhancement of ferromagnetism in Mn-doped Si via B codoping. Journal of Applied Physics, 107(9).
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  Abstract: Room temperature ferromagnetic Mn0.026 BX Si0.974-X bulk samples with X=0.001, 0.005, and 0.01 were fabricated by arc melting and followed by thermal annealing at 1000 °C. The effects of carrier density on the ferromagnetism were studied. Due to the high carrier densities, all samples showed metallic behavior. Kondo effect was observed when the temperature was below 10 K. Weakly localized carriers monotonously increased with increasing boron concentration, resulting in the enhancement of exchange coupling among Mn ions. The study indicated that the ferromagnetism originates from the hole mediated Ruderman-Kittel-Kasuya-Yosida mechanism. © 2010 American Institute of Physics.
• Cao, R., Moriyama, T., Wang, W. G., Fan, X., Kolodzey, J., Chen, S. H., Chang, C. R., Tserkovnyak, Y., Nikolic, B. K., & Xiao, J. Q. (2009). Spin-polarized transport and dynamics in magnetic tunneling structures. IEEE Transactions on Magnetics, 45(10), 3434-3440.
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  Abstract: In the first part of this paper, we report a systematic study on the structural evolution under rapid thermal annealing and the corresponding transport properties in magnetic tunnel junctions (MTJs) with a crystalline MgO barrier. The results clearly indicate that high tunneling magnetic resistance can be achieved by annealing MTJs at avery short time, and it is directly related to the formation of (001) crystalline structures. In the second part, we report the spin dynamics in tunneling structure through direct electrical detection. A surprisingly large voltage generation in F/I/N and F/I/Fjunctions was observed, which is contradictory to the prediction from the standard spin-pumping theory. We proposed a theoretical formalism to study spin-pumping effects in ferromagnetic multilayer structures. The formalism can yield a remarkably cleanphysical picture of the spin and charge pumping in tunneling structures. The calculated values are consistent with experimental results. © 2009 IEEE.
• Fan, X., Cao, R., Moriyama, T., Wang, W., Zhang, H. W., & Xiao, J. Q. (2009). Magnetic tunnel junction based microwave detector. Applied Physics Letters, 95(12).
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  Abstract: We investigated the tunneling magnetoresistance change in magnetic tunnel junctions in the presence of external microwaves. The changing relative angle between the free layer and the pinned layer results in a rectification of the average resistance change. Due to its miniature size and its sensitivity to the microwave magnetic field, the magnetic tunnel junction could be utilized as a microwave power sensor with the ability to detect microwave frequencies. Studying microwave power and bias current dependencies reveals desired sensor features with linear responses and enhanced signal levels. © 2009 American Institute of Physics.
• Shah, L. R., Ali, B., Zhu, H., Wang, W. G., Song, Y. Q., Zhang, H. W., Shah, S. I., & Xiao, J. Q. (2009). Detailed study on the role of oxygen vacancies in structural, magnetic and transport behavior of magnetic insulator: Co-CeO₂. Journal of Physics Condensed Matter, 21(48).
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  PMID: 21832536;Abstract: Room temperature ferromagnetism in polycrystalline CoxCe 1-xO2-δ (0.001≤x≤0.10) bulk samples has been investigated. Annealing in the forming gas transformed the as-prepared paramagnetic into a ferromagnetic insulating material with over two orders of magnitude enhancement (from 3.7 × 10-2 to 1.24μB/ Co) in the magnetization. Structural characterization of both the as-prepared and H2-treated samples showed a single phase material. The incorporation of Co with the formation of oxygen vacancies in the oxide lattice was revealed by x-ray photoelectron spectroscopy (XPS). The presence of oxygen vacancies is indicated by the existence of mixed valence states of cerium (Ce4+ and Ce3+) in the high resolution XPS 3d spectrum. The role of the donor defects (oxygen vacancies) has been verified through the removal of oxygen vacancies. The ferromagnetic insulating ground state has been explained in terms of the interaction of the F+ center and 3d magnetic cations. The connection between magnetic properties, electronic structure of the magnetic impurity and donor defect has been established. First principle calculations have been performed using the full potential linearized augmented plane wave method within the density functional theory (DFT) framework; these support our experimental findings. Both the experiment and calculations reinforced the crucial role of oxygen vacancies. © 2009 IOP Publishing Ltd.
• Shah, L. R., Wang, W., Zhu, H., Ali, B., Song, Y. Q., Zhang, H. W., Shah, S. I., & Xiao, J. Q. (2009). Role of dopant, defect, and host oxide in the observed room temperature ferromagnetism: Co-ZnO versus Co- CeO2. Journal of Applied Physics, 105(7).
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  Abstract: A comparative study on the room temperature ferromagnetism of Co doped ZnO and CeO2 bulk samples is presented. Co-ZnO system has been observed to switch between ferromagnetic and paramagnetic by controlling the donor defects, interstitial zinc: Zni. On the other hand, Co- CeO2 is always ferromagnetic. However, ferromagnetism increases/decreases with increase/decrease in donor defects, oxygen vacancies: VO. Systemic structural, magnetic, and transport analyses reveal that the nature of donor defects and host oxide plays a vital role in establishing ferromagnetism. This study provides an insight into the underlying mechanisms that are responsible for the ferromagnetism in Co-ZnO and Co- CeO2. Moreover, the proposed mechanisms are supported by the electronic structure of magnetic impurity ions and defects. © 2009 American Institute of Physics.
• Wang, W. G., Jordan-Sweet, J., Miao, G. X., Ni, C., Rumaiz, A. K., Shah, L. R., Fan, X., Parsons, P., Stearrett, R., Nowak, E. R., Moodera, J. S., & Xiao, J. Q. (2009). In-situ characterization of rapid crystallization of amorphous CoFeB electrodes in CoFeB/MgO/CoFeB junctions during thermal annealing. Applied Physics Letters, 95(24).
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  Abstract: We report the crystallization study of CoFeB/MgO/CoFeB magnetic tunnel junctions using in-situ, time-resolved synchrotron-based x-ray diffraction and transmission electron microscopy. It was found that the crystallization of amorphous CoFeB electrodes occurs on a time scale of seconds during the postgrowth high temperature annealing. The crystallization can be well fit by the Johnson-Mehl-Avrami model and the effective activation energy of the process was determined to be 150 kJ/mol. The solid-state epitaxy mode of CoFeB was found to involve separate crystallization at different locations followed by subsequent merging of small grains, instead of layer-by-layer growth of CoFeB film along the MgO template. © 2009 American Institute of Physics.
• Wang, W. G., Han, K. J., Yee, K. J., Ni, C., Wen, Q., Zhang, H. W., Zhang, Y., Shah, L., & Xiao, J. Q. (2008). Properties of (Zn,Cr)Te semiconductor deposited at room temperature by magnetron sputtering. Applied Physics Letters, 92(10).
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  Abstract: We report the fabrication of (Zn,Cr)Te films at room temperature by magnetron sputtering. Various structural and elemental characterizations revealed there was only a zinc blende phase from the ZnTe host and Cr atoms were distributed uniformly in these films. The magnetization measurement by superconducting quantum interference device magnetometer clearly showed that the samples were ferromagnetic at low temperatures with Curie temperature around 150 K. The magnetic circular dichroism measurements confirmed that the observed ferromagnetism was originated from the interaction of substitutional Cr ions and ZnTe host. Transport measurement revealed typical semiconductor behaviors with the large negative magnetoresistance observed. © 2008 American Institute of Physics.
• Wang, W. G., Ni, C., Rumaiz, A., Wang, Y., Fan, X., Moriyama, T., Cao, R., Wen, Q. Y., Zhang, H. W., & Xiao, J. Q. (2008). Real-time evolution of tunneling magnetoresistance during annealing in CoFeBMgOCoFeB magnetic tunnel junctions. Applied Physics Letters, 92(15).
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  Abstract: We report the study of the real-time evolution of tunneling magnetoresistance (TMR) in CoFeBMgOCoFeB junctions during annealing at 380 °C. The TMR quickly developed at the early stage of the annealing, with 200% magnetoresistance observed in less than 10 min, followed by a slow approach to saturation. This evolution of TMR was correlated with the structural changes, including crystallization of amorphous CoFeB electrodes and improvement of barrier quality during the annealing. © 2008 American Institute of Physics.
• Wang, W. G., Yee, K. J., Kim, D. H., Han, K. J., Wang, X. R., Ni, C., Moriyama, T., Mathew, A., Opila, R., Zhu, T., & Xiao, J. Q. (2008). Microstructure, magnetic, and spin-dependent transport properties of (Zn,Cr)Te films fabricated by magnetron sputtering. Physical Review B - Condensed Matter and Materials Physics, 77(15).
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  Abstract: Chromium doped zinc telluride thin films with various doping concentrations are fabricated by magnetron sputtering. These films are ferromagnetic and the Curie temperature increases with Cr concentration. X-ray diffraction, transmission electron microscopy, and magnetic circular dichroism characterizations show that the films are free of Cr1-x Tex impurities and the ferromagnetism is intrinsic. The transport study shows that the resistivity and magnetoresistance are governed by variable range hopping at low temperatures. A negative magnetoresistance as large as -57% is observed at 5 K. The magnetoresistance and its temperature dependence can be well explained by a model involving an increase in the localization length of carriers with a magnetic field in the hopping region. An anomalous Hall effect is also observed and a possible origin of the sign difference between the anomalous Hall resistance and ordinary Hall resistance is discussed. © 2008 The American Physical Society.
• Wen, Q., Zhang, H., Song, Y., Yang, Q., Zhu, H., Wang, W., & Xiao, J. Q. (2008). Room-temperature ferromagnetism in Co doped La₂O₃. Journal of Applied Physics, 103(7).
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  Abstract: Single-phase La2 (1-x) Co2x O3-δ polycrystalline samples with x=0%-8% were synthesized by the conventional ceramic method, and the effect of Co content on the magnetic behaviors has been systemically investigated. X-ray diffraction and x-ray photoelectron spectroscopy studies indicate no Co metal clusters or secondary magnetic phases in any samples in this study. It is found that the undoped or slightly doped samples show no ferromagnetic signal, while samples with x in the range of 0.5%-2% exhibit an exponential increase of saturation magnetization (Ms) as a function of Co concentration. When x increases beyond 2%, an inverse correlation between the magnetization and Co content was observed. We reported an Ms as large as 0.05 emug and a Curie temperature above RT in our samples, rendering Co: La2 O3 a candidate diluted magnetic oxide for RT applications. Our results also strongly support the oxygen vacancy (F -center) mediated mechanism for RT ferromagnetism in transition-metal doped high- k oxides © 2008 American Institute of Physics.
• Liu, Y., Wang, W. G., Moriyama, T., Xiao, J. Q., & Zhang, Z. (2007). Direct measurement of barrier asymmetry in Al Ox Zr Oy magnetic tunnel junctions using off-axis electron holography. Physical Review B - Condensed Matter and Materials Physics, 75(13).
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  Abstract: Off-axis electron holography was used to investigate the barrier profile of the Py Al Ox Zr Oy Py magnetic tunnel junctions with different Zr Oy thicknesses. The tunneling magnetoresistance (TMR) has a strong dependence on bias voltage and the bias voltage for maximum TMR is shifted from zero. This shift increases with Zr Oy barrier thickness due to the increasing barrier asymmetry in the junctions. The evolution of barrier asymmetry was directly observed by the phase change of the off-axis electron holography, which unambiguously shows the barrier profile changes from triangular to trapezoidal shape as increasing of Zr Oy thickness. © 2007 The American Physical Society.
• Moriyama, T., Ni, C., Wang, W. G., Zhang, X., & Xiao, J. Q. (2006). Tunneling magnetoresistance in (001)-oriented FeCoMgOFeCo magnetic tunneling junctions grown by sputtering deposition. Applied Physics Letters, 88(22).
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  Abstract: Magnetic tunneling junctions (MTJs) with MgO tunnel barrier have been fabricated on both oriented and nonoriented buffer layers on Si(001) substrate by magnetron sputtering. FeCoMgOFeCo MTJs fabricated on oriented buffer layers show larger tunneling magnetoresistance (TMR) value up to 84% without high temperature postannealing, whereas those MTJs on nonoriented buffer layers show 45% of TMR. The high-resolution transmission electron microscopy images reveal an excellent morphology and very coherent crystal structure with FeCo (001) [110] MgO (001) [100] FeCo (001) [110] orientation. The results indicate that high TMR can be achieved without high temperature postannealing by sputtering deposition on appropriate oriented buffer layers. © 2006 American Institute of Physics.
• Moriyama, T., Wang, W., & Xiao, J. Q. (2006). Tunneling magnetoresistance in FeCo/MgO/FeCo tunneling junctions depending on FeCo/MgO interface state. INTERMAG 2006 - IEEE International Magnetics Conference, 866-.
• Wang, W. G., Ni, C., Moriyama, T., Wan, J., Nowak, E., & Xiao, J. Q. (2006). Spin-polarized transport in hybrid (Zn,Cr)Te/Al ₂O ₃/Co magnetic tunnel junctions. Applied Physics Letters, 88(20).
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  Abstract: Tunnel magnetoresistance (TMR) of 21% is observed at low temperature in hybrid magnetic tunnel junctions (MTJs) composed of a magnetic semiconductor (Zn,Cr)Te and Co electrodes separated by an alumina barrier. The TMR is observed up to 250 K, which is a considerable improvement over previous work on MTJs with semiconductor electrodes. The temperature and bias dependence of the TMR are consistent with a transport model involving spin-polarized tunneling and spin-independent hopping through impurity states. © 2006 American Institute of Physics.
• Wang, W., Chaoying, N. i., Zhu, T., Zhang, H., & Xiao, J. Q. (2006). Structural, magnetic, and transport properties of (Zn,V)Te semiconductors. Journal of Applied Physics, 99(8).
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  Abstract: Vanadium-doped ZnTe has been predicted to be one of the candidates for ferromagnetic semiconductors with a high Curie temperature [K. Sato and H. Katayama-Yoshida, Semicond. Sci. Technol. 17, 367 (2002)]. In this paper, we report the structural, magnetic, and transport properties of (Zn,V)Te films prepared by magnetron sputtering. Samples were fabricated on both GaAs and thermally oxidized silicon substrate at elevated temperature. Oriented sample (100) can be achieved on GaAs substrates and only polycrystalline samples are observed on Si substrates. X-ray diffraction (XRD) and transmission electron spectroscopy (TEM) show no magnetic precipitates in the (Zn,V)Te film. The magnetization measurement shows that the oriented sample is paramagnetic at 5 K, while films on Si substrate shows weak ferromagnetism at 5 K. The sign of magnetoresistance (MR= [R(H)-R(0)] R(0)) gradually changes from negative to positive with temperature, and positive MR at high temperatures shows H2 dependence, indicating ordinary MR effect. It is believed the observed negative MR corresponds to the ferromagnetic ordering at lower temperature. © 2006 American Institute of Physics.
• Zhu, T., Zhan, W. S., Wang, W. G., & Xiao, J. Q. (2006). Room temperature ferromagnetism in two-step-prepared Co-doped ZnO bulks. Applied Physics Letters, 89(2).
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  Abstract: We have prepared Co-doped ZnO bulks with a two-step method. The bulk samples are sintered at the temperature above 1000°C to get pure single phase and then treated by Zn vapor in a vacuum. Based on such two-step preparation, clear carrier induced ferromagnetism is found in Zn treated Co-doped ZnO bulks, which suggests that the magnetism mechanism can be of codoping effect. The onset of ferromagnetism is associated with the further increase of the hybridization of the impurity band with 3d states near the Fermi level when additional carrier induced to let the Stoner criterion for ferromagnetism to be satisfied. © 2005 American Institute of Physics.
• Dai, X. F., Liu, G. D., Liu, Z. H., Wu, G. H., Chen, J. L., Meng, F. B., Liu, H. Y., Yan, L. Q., Qu, J. P., Li, Y. X., Wang, W. G., & Xiao, J. Q. (2005). Superelasticity of CoNiGa:Fe single crystals. Applied Physics Letters, 87(11).
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  Abstract: We have fabricated CoNiFeGa single crystals with excellent superelasticity. The superelastic strains of 4% and 6.7% in compression have been obtained along the [001] and [110] directions, respectively. These single crystals show strong anisotropy in strains, superelastic parameters, and even transformation path related to the different crystalline directions. A large superelastic strain up to 11% has been obtained in tension test. The perfect superelasticities have also displayed in bending and torsion tests. © 2005 American Institute of Physics.
• Moriyama, T., Xiang, X. H., Wang, W., Li, F., Zhu, T., Du, J., Xiao, M., Li, Z., An, H. u., & Xiao, J. Q. (2005). Negative TMR in magnetic tunneling junctions with Zr oxide barrier. INTERMAG ASIA 2005: Digests of the IEEE International Magnetics Conference, 995-.
• Pan, L., Zhu, H., Fan, C., Wang, W., Zhang, Y., & Xiao, J. Q. (2005). Mn-doped Cu ₂O thin films grown by rf magnetron sputtering. Journal of Applied Physics, 97(10).
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  Abstract: Cu2 O has excellent optical properties and could be a good host semiconductor for dilute magnetic semiconductors because doped Cu2 O is a p type and direct wide-band-gap semiconductor. In this work, Mn-doped Cu2 O thin films were deposited on thermally oxidized silicon substrates by radio-frequency magnetron sputtering. X-ray diffraction results show that highly (200) oriented (Mn0.06 Cu0.94) 2 O films can be achieved at elevated substrate temperature. X-ray photoelectronic spectroscopy confirms that the valence of copper in the films is only +1, and no other valence states. The Mn-doped Cu2 O films show primary paramagnetic behaviors above 25 K. Very weak ferromagnetic property interspersed with paramagnetic phase appears near 5 K. The high magnetic moment of 5.3 μB per Mn ion and high resistivity suggest the valence state of manganese is mainly +1. © 2005 American Institute of Physics.
• Zhang, Z., Zhu, T., Shen, F., Sheng, W., Wang, W., Xiao, J. Q., & Zhang, Z. (2005). Electron holography of barrier structures in Co/ZrAlO _x/Co magnetic tunnel junctions. Chinese Physics Letters, 22(7), 1732-1735.
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  Abstract: We investigate the potential profiles and elemental distribution of barriers in Co/ZrAlO x/Co magnetic tunnel junctions (MTJs) using electron holography (EH) and scanning transmission electron microscopy. The MTJ barriers are introduced by oxidizing a bilayer consisting with a uniform 0.45-nm Al layer and a wedge-shaped Zr layer (0-2 nm). From the scanning transmission electron microscopy, AlO x and ZrO x layers are mixed together, indicating that compact AlO x layer cannot be formed in such a bilayer structure of barriers. The EH results reveal that there are no sharp interfaces between the barrier and magnetic electrodes, which may be responsible for a smaller tunnelling magnetoresistance compared with the MTJs of Co/AlO x/Co. © 2005 Chinese Physical Society and IOP Publishing Ltd.
• Sheng, W., Wang, W. G., Xiang, X. H., Shen, F., Fei-Fei, L., Zhu, T., Zhang, Z., Li, Z., Jun, D. U., Hu, A. N., & Xiao, J. Q. (2004). Probing tunnel barrier shape and its effects on inversed tunneling magnetoresistance at high bias. Journal of Electronic Materials, 33(11), 1274-1279.
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  Abstract: We have used an electron holography (EH) technique to directly probe the potential profile of tunnel barriers in magnetic tunnel junctions (MTJs). Barriers with under-, optimum-, or over-oxidized condition have been investigated. One important finding is that there is always slight oxidation of the top electrode because of film morphology. Sharp interfaces can be achieved in the bottom interface of optimally oxidized barrier or both interfaces in MTJs with under-oxidized barriers. We also demonstrate, theoretically and experimentally, how barrier shape affects the bias dependence and, in low barrier height case, result in inversed tunneling magnetoresistance (TMR) at high bias. The mechanism is very different from that responsible for inversed TMR in all biases. The finding leads to the possibilities of achieving better signals at high bias in real applications.

Presentations

• Wang, W. (2015, June). Voltage-controlled Magnetic Anisotropy and Magnetization in 3d Transitional Metals. The 1st【ImPACT】International Symposium: Toward the Ultimate Green IT Devices and System,. Tokyo.
• Wang, W. (2015, March). “Voltage controlled magnetism in 3d transitional metals",. The American Physical Society March Meeting.
• Wang, W. (2015, May). Electrical control of magnetization switching in spintronic devices. School of Materials Science & Engineering Seminar, Tsinghua University.
• Wang, W. (2015, May). Electrical control of magnetization switching in spintronic devices. Seminar State Key Laboratory of Superlattices and Microstructures, Institute of Semiconductors. Beijing.
• Wang, W. (2015, May). Voltage controlled magnetism in 3d transitional metals. 2015 International Magnetics Conference (INTERMAG).
• Wang, W. (2014, January). Toward Ultra-low Energy Switching in Magnetic Nanostructures for Next Generation Transistors. Department of Aerospace and Mechanical Engineering Seminar. Tucson, AZ: University of Arizona.
• Wang, W. (2014, Oct). Voltage-controlled magnetic anisotropy and magnetism in perpendicular tunnel junctions. 2014 Materials Science & Technology Conference.

Poster Presentations

• Wang, W. (2013, August). Unipolar Switching in Magnetic Tunnel Junctions with Tunable Energy Barrier. 13th Non-Volatile Memory Technology Symposium. Minneapolis, MN.
• Wang, W. (2013, March). Spintronic Phenomena in Magnetic Nanostructures. International Workshop on Policy and Ethics in Science and Technology. Islamabad, Pakistan.
• Wang, W. (2013, March). Voltage-controlled magnetic anisotropy in perpendicular magnetic tunnel junctions. The American Physical Society March Meeting. Baltimore, Maryland.
• Wang, W. (2012, August). Voltage-controlled magnetic anisotropy in magnetic tunnel junction. The NanoScience/Engineering conference of the International Society for Optics and Photonics (SPIE). San Diego, California.
• Wang, W. (2012, July). Electric Field Induced Switching in Soft Magnetic CoFeB Thin Films with Interfacial Anisotropy. International Conference of Young Researchers on Advanced Materials. Singapore.
• Wang, W. (2012, July). Toward Ultra-low Energy Switching in Spintronic Nanostructures. Department of Physics seminar. Beijing, China: University of Science and Technology Beijing.
• Wang, W. (2012, July). Ultra-low current switching in magnetic tunnel junctions with perpendicular magnetic anisotropy. Data Storage Institute, Agency for Science, Technology and Research. Singapore.
• Wang, W. (2012, May). Voltage induced switching in CoFeB/MgO/CoFeB magnetic tunnel junctions. International Magnetics Conference (INTERMAG). Vancouver, Canada.