Spin-orbit torque from a ferromagnetic metal

Hao Wu, Seyed Armin Razavi, Qiming Shao, Xiang Li, Kin L. Wong, Yuxiang Liu, Gen Yin, and Kang L. Wang

Phys. Rev. B 99, 184403 – Published 6 May 2019

Abstract

The switching of magnetization by current-induced spin-orbit torque (SOT) has potential applications for energy-efficient spintronic devices. In the past, most conventional works have been focused on SOT in heavy metals. Here the SOT from a ferromagnetic metal is investigated, and two mechanisms of the field-free SOT induced magnetization switching are demonstrated to be from the interlayer exchange coupling and the tilted perpendicular magnetic anisotropy. We exclude the spin torque contribution from the anomalous Hall effect and the interfacial Rashba effect combined with spin precession. A spin Hall angle $θ_{SH} = - 0.022$ of CoFeB is obtained by the current-induced hysteresis loop shift method, and the obtained $θ_{SH}$ is comparable with heavy metals. This work demonstrates that a considerable SOT can come from a ferromagnetic metal, and indicates the unconventional origin of spin-orbit coupling.

Received 16 July 2018
Revised 10 March 2019

DOI:https://doi.org/10.1103/PhysRevB.99.184403

Physics Subject Headings (PhySH)

Exchange interaction Magnetization switching Spin torque Spin-orbit coupling Spintronics

Ferromagnets

Hall bar Magnetization measurements

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Hao Wu^*, Seyed Armin Razavi, Qiming Shao, Xiang Li, Kin L. Wong, Yuxiang Liu, Gen Yin, and Kang L. Wang^†

Department of Electrical and Computer Engineering, and Department of Physics and Astronomy, University of California, Los Angeles, California 90095, USA

^*Corresponding author: wuhaophysics@ucla.edu
^†Corresponding author: wang@ee.ucla.edu

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Issue

Vol. 99, Iss. 18 — 1 May 2019

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Images

Figure 1
(a) Schematic of the Mo(2)/ $I$ -CoFeB(3)/Mo(2-wedge)/ $P$ -CoFeB(1.1)/MgO(2) stack (thickness in nanometers), where the $I$ -CoFeB and $P$ -CoFeB layers have in-plane and perpendicular magnetic anisotropy (IMA and PMA), respectively, and the interlayer Mo is wedged along the $y$ axis. The microscope image in the bottom shows the patterned Hall-bar device, where the current is applied along the $x$ axis. (b) $M - H$ curves under magnetic fields along $x, y$ , and $z$ axes, and the inset shows the results at a larger magnetic field range $(\pm 2 T)$ , which clearly shows the IMA of the $I$ -CoFeB layer and the PMA of the $P$ -CoFeB layer, respectively.
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Figure 2
(a) Schematic of the measurement setup, where the magnetic field is initially applied along the $x$ axis to align the $I$ -CoFeB magnetization; and the cross-sectional view of the film stacks in the $x - z$ plane. (b) The planar Hall effect shows the coercivity of $I$ -CoFeB is around 5.5 Oe. (c) SOT induced magnetization switching for varying $H_{x}$ ; a 1-ms writing current pulse $I_{x}$ is applied to provide SOT to switch the top $P$ -CoFeB layer first, and then another 1-ms reading current pulse $I_{R}$ (1 mA) is applied to read the magnetization of $P$ -CoFeB by AHE resistance, where $H_{x}$ is varying from $+ 100 Oe$ to $- 100 Oe$ . (d) Field-free SOT switching for $\pm x I$ -CoFeB magnetization directions, respectively, where $\pm 100 Oe H_{x}$ are applied to initialize the $I$ -CoFeB and then removed.
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Figure 3
(a) Schematic of the measurement setup, where the magnetic field is applied along the $y$ axis, and the cross-sectional view of the film stacks in the $y - z$ plane, which shows a wedged Mo interlayer thickness and an induced tilted PMA of the $P$ -CoFeB. (b) As $H_{y}$ is scanned to switch the $I$ -CoFeB, due to the interlayer exchange coupling, the $P$ -CoFeB also switches between the up and down magnetization states. As a result, the AHE signals can be switched by $H_{y}$ . (c) SOT induced magnetization switching for a series of $H_{y}$ , where $H_{y}$ is varying from $+ 100 Oe$ to $- 100 Oe$ . (d) Field-free SOT switching for the $\pm y$ initial $I$ -CoFeB magnetization directions, respectively.
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Figure 4
(a) SOT induced magnetization switching of the control sample [Mo(2)/ $P$ -CoFeB(1.1)/MgO(2)] and the test sample [Mo(2)/ $I$ -CoFeB(3)/Mo(2-wedge)/ $P$ -CoFeB(1.1)/MgO(2)], where a $+ 100 Oe H_{x}$ is applied to break the inversion symmetry during the measurement. (b) The $R_{xy} - H_{z}$ curve of the test sample with a dc current of 1 mA.
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Figure 5
Current-induced hysteresis loop shift measurement. (a) Schematic of the current-driven domain wall motion, when the interlayer exchange coupling (IEC) and the in-plane field $H_{x}$ pin the magnetization of the down-up and up-down DWs in the same direction, and thus the domain could expend or shrink. (b) The $R_{xy} - H_{z}$ curves for $\pm 20 mA$ dc current at zero field. The shift of the center field of the hysteresis loop corresponds to the SOT-induced out-of-plane effective field ( $H_{z}^{eff}$ ). (c) The extracted $χ_{SOT}$ as a function of $H_{x}$ , which shows the saturated value of $χ_{SOT}$ .
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