Determination of Spin-Orbit-Torque Efficiencies in Heterostructures with In-Plane Magnetic Anisotropy

Yan-Ting Liu, Tian-Yue Chen, Tzu-Hsiang Lo, Tsung-Yu Tsai, Shan-Yi Yang, Yao-Jen Chang, Jeng-Hua Wei, and Chi-Feng Pai

Phys. Rev. Applied 13, 044032 – Published 13 April 2020

Abstract

It has been shown that the spin Hall effect from heavy transition metals can generate sufficient spin-orbit torque and further produce current-induced magnetization switching in the adjacent ferromagnetic layer. However, if the ferromagnetic layer has in-plane magnetic anisotropy, probing such switching phenomenon typically relies on tunneling magnetoresistance measurement of nano-sized magnetic tunnel junctions, differential planar Hall voltage measurement, or Kerr-imaging approaches. We show that in magnetic heterostructures with spin Hall metals, there exist current-induced in-plane spin Hall effective fields and unidirectional magnetoresistance that will modify their anisotropic magnetoresistance behavior. We also demonstrate that by analyzing the response of anisotropic magnetoresistance under such influences, one can directly and electrically probe magnetization switching driven by the spin-orbit torque, even in micron-sized devices. This pump-probe method allows for efficient and direct determination of key parameters from spin-orbit torque switching events without lengthy device fabrication processes.

Received 13 November 2019
Revised 19 January 2020
Accepted 23 March 2020

DOI:https://doi.org/10.1103/PhysRevApplied.13.044032

Physics Subject Headings (PhySH)

Anisotropic magnetoresistance Magnetization switching Metrology Spin current Spin torque Spin-orbit coupling

Magnetic multilayers

Hall bar

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Yan-Ting Liu ¹, Tian-Yue Chen^1,†, Tzu-Hsiang Lo¹, Tsung-Yu Tsai¹, Shan-Yi Yang², Yao-Jen Chang², Jeng-Hua Wei², and Chi-Feng Pai ^1,3,*

¹Department of Materials Science and Engineering, National Taiwan University, Taipei 10617, Taiwan
²Electronic and Optoelectronic System Research Laboratories, Industrial Technology Research Institute, Hsinchu, Taiwan
³Center of Atomic Initiative for New Materials, National Taiwan University, Taipei 10617, Taiwan

^*cfpai@ntu.edu.tw
^†f05527067@ntu.edu.tw

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Vol. 13, Iss. 4 — April 2020

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Images

Figure 1
(a) Schematic illustration of $W$ / $Co$ - $Fe$ - $B$ Hall-bar device with lateral dimensions of 5 × 60 µm² and AMR measurement. The direction of the positive current defined along the -x direction. (b) Representative shifted AMR loops obtained from a $W$ (3)/ ${Co}_{40} {Fe}_{40} B_{20}$ (2.0) sample with dc current $I_{dc} = \pm 3 mA$ . Switching field $H_{sw}$ of (c) $W$ (3)/ ${Co}_{40} {Fe}_{40} B_{20}$ (1.8) and (d) $Pt$ (6)/ ${Co}_{40} {Fe}_{40} B_{20}$ (2.5) sample for left(-y)-to-right(+y) (L-to-R) and right(+y)-to-left(-y) (R-to-L) switching processes as functions of $I_{dc}$ . $H_{eff}^{y}$ represents the center of the AMR loop peaks. (e) $Δ R_{step}$ versus $I_{dc}$ for $W$ (3)/ ${Co}_{40} {Fe}_{40} B_{20}$ (1.8) and $Pt$ (6)/ ${Co}_{40} {Fe}_{40} B_{20}$ (2.5) samples. (f) $H_{eff}^{y} / I_{dc}$ and USMR/I_dc of $W$ (3)/ ${Co}_{40} {Fe}_{40} B_{20}$ ( $t_{Co − Fe − B}$ ) samples as functions of ${Co}_{40} {Fe}_{40} B_{20}$ thickness.
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Figure 2
(a), (c) Measurement sequence for the switching experiment with opposite sense currents. (b), (d) Current-induced DL SOT switching results of a $W$ (3)/ ${Co}_{40} {Fe}_{40} B_{20}$ (1.8) Hall-bar sample with opposite sense currents $I_{sense} = + 1 mA$ and $I_{sense} = - 1 mA$ . The black arrows indicate the switching direction. (e) Schematics of current-induced AMR loops shift (SHF-modified AMR). The perpendicular dashed line represents the center of two peaks of AMR by applying different sense currents.
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Figure 3
Measurement sequence of $Δ R$ by sweeping (a) in-plane magnetic field H_y and (b) write current $I_{write}$ . Representative $Δ R$ as a function of (c) in-plane magnetic field H_y and (d) write current $I_{write}$ for a $W$ (3)/ ${Co}_{40} {Fe}_{40} B_{20}$ (1.8) device. (e) The write-current pulse-width dependence of critical switching current I_c for a $W$ (3)/ ${Co}_{40} {Fe}_{40} B_{20}$ (1.8) device. Solid lines represent linear fits to the experimental data. (f) Spin-torque switching efficiency of $W$ (3)/ ${Co}_{40} {Fe}_{40} B_{20}$ ( $t_{Co − Fe − B}$ ) samples as a function of ${Co}_{40} {Fe}_{40} B_{20}$ thickness.
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Figure 4
(a) DL SOT efficiency and FL SOT efficiency of $W$ (3)/ ${Co}_{40} {Fe}_{40} B_{20}$ ( $t_{Co − Fe − B}$ ) samples as functions of ${Co}_{40} {Fe}_{40} B_{20}$ thickness. (b) DL SOT efficiency and FL SOT efficiency of $W$ (3)/ ${Co}_{40} {Fe}_{40} B_{20}$ (3.0)/ $Mg O$ ( $t_{Mg O}$ ) samples as functions of $Mg O$ thickness. Some uncertainties are smaller than the symbol size shown.
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Figure 5
Normalized AMR results from $W$ (4)/ ${Co}_{40} {Fe}_{40} B_{20}$ (1.4)/ $Mg O$ (2.1)/ $Ta$ (10) devices by applying currents $I_{sense} = \pm 4 mA$ (a) parallel to the easy axis (I‖EA) and (b) perpendicular to the easy axis (I⊥EA). $Δ R$ as functions of (c) in-plane magnetic field H_y and (d) write current $I_{write}$ for a $W$ (4)/ ${Co}_{40} {Fe}_{40} B_{20}$ (1.4)/ $Mg O$ (2.1)/ $Ta$ (10) device with I⊥EA.
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