Spin-dependent electrical and thermal transport in magnetic tunnel junctions
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Date
2012-07, 2015-03, 2016-08
Authors
Zhang, Zhaohui
Journal Title
Journal ISSN
Volume Title
Publisher
APS
AIP
APS
AIP
APS
Abstract
Thermoelectricity can directly convert a temperature difference into a voltage or charge current. Recently, the development of spin caloritronics has introduced spin as another degree of freedom in traditional thermoelectrics. This discovery bodes a new generation of magnetic random access memories (MRAMs), where thermal spin-transfer torque (TSTT) rather than voltage driven spin-transfer torque (STT) is used to switch the magnetization in magnetic tunnel junctions (MTJs). To advance the rising trend of spin caloritronics, the coupling of charge, spin, and heat flow during electron transport in MTJs was systematically studied in this thesis.
To begin with, the static transport properties of MTJs were studied by observing current dependent tunnel magnetic resistance (TMR). The observed decrease of TMR with a biased current is attributed to the change in spin polarization of the free ferromagnetic layer. A phenomenological model has been built based on the current dependent polarization, which agrees with our experimental results. Next, the Seebeck rectification effect in MTJs was studied. By applying microwave currents to MTJs, an intrinsic thermoelectric coupling effect in the linear response regime of MTJs was discovered. This intrinsic thermoelectric coupling contributes a nonlinear correction to Ohm's law. In addition, this effect can be controlled magnetically since the Seebeck coefficient is related to magnetization configuration. Finally, TSTT in MTJs was systematically studied. A laser heating technique was employed to apply a temperature difference across the tunnel barrier and ferromagnetic resonance (FMR) spectra were measured electrically through spin rectification. By analyzing the FMR spectra, TSTT in MTJs was observed and the angular dependence of TSTT was found to be different from dc-biased STT. By solving the Landau-Lifshitz-Gilbert equation including STT, the experimental observations were well explained.
The discovery of Seebeck rectification refines the previous understanding of magneto-transport and microwave rectification in MTJs and provides a new possibility for utilizing spin caloritronics in high-frequency applications. The study of TSTT in MTJs shows clear experimental evidence of TSTT in MTJs. Further optimization of the design of MTJs may succeed in decreasing the necessary switching fields strength or even achieve a switching by only TSTT in MTJs.
Description
Keywords
spin-dependent, thermoelectricity, spintronics, magnetoresistance, spin-transfer torque, thermal spin-transfer torque, magnetic tunnel junction
Citation
Z.H. Zhang, Y.S. Gui, L. Fu, X.L. Fan, J.W. Cao, D.S. Xue, P.P. Freitas, D. Houssameddine, S. Hemour, K. Wu, and C.-M. Hu, “Seebeck Rectification Enabled by Intrinsic Thermoelectrical Coupling in Magnetic Tunneling Junctions” Phys. Rev. Lett. 109, 037206 (2012).
Z.H. Zhang, L. H. Bai, C.-M. Hu, S. Hemour, K. Wu, X.L. Fan, D.S. Xue, and D. Houssameddine, “The tunneling magnetoresistance current dependence on cross sectional area, angle and temperature” AIP Adv. 5, 037134 (2015).
Z.H. Zhang, L. H. Bai, X. B. Chen, H. Guo, X.L. Fan, D.S. Xue, D. Houssameddine, and C.-M. Hu, Observation of thermal spin-transfer torque via ferromagnetic resonance in magnetic tunnel junctions, Phys. Rev. B, 94, 064414 (2016).
Z.H. Zhang, L. H. Bai, C.-M. Hu, S. Hemour, K. Wu, X.L. Fan, D.S. Xue, and D. Houssameddine, “The tunneling magnetoresistance current dependence on cross sectional area, angle and temperature” AIP Adv. 5, 037134 (2015).
Z.H. Zhang, L. H. Bai, X. B. Chen, H. Guo, X.L. Fan, D.S. Xue, D. Houssameddine, and C.-M. Hu, Observation of thermal spin-transfer torque via ferromagnetic resonance in magnetic tunnel junctions, Phys. Rev. B, 94, 064414 (2016).