Observation of nontrivial spin-orbit torque in single-layer ferromagnetic metals

Qingwei Fu, Like Liang, Wenqiang Wang, Liupeng Yang, Kaiyuan Zhou, Zishuang Li, Chunjie Yan, Liyuan Li, Haotian Li, and Ronghua Liu

Phys. Rev. B 105, 224417 – Published 23 June 2022

Abstract

We report the experimental observation of net spin-orbit torques (SOT) in single conductive ferromagnets (FMs), e.g., ${Fe}_{20} {Ni}_{80} (Py)$ , Co, and Ni, despite the system symmetry constraint. Unlike the traditional FM/heavy metal (HM) bilayers, where only the in-plane (IP) damping-like SOT is presented, an unexpected out-of-plane (OOP) damping-like SOT is also unequivocally observed in the single FMs. Using the field angular-dependent, spin-torque ferromagnetic resonance technique, we quantify the IP and OOP SOT efficiencies of 0.018 ± 0.003 and 0.047 ± 0.002 in the pure Py, respectively. We argue that the IP SOT is primarily related to the Py bulk spin Hall effect, while the unconventional SOT is ascribed to the out-of-plane polarized spin current generated from the nonequilibrium spin swapping effect. Additionally, we confirm that such self-induced SOTs also generally exist in the most studied HM/FM multilayer systems, especially for the HMs with low conductivity. Hence, our findings provide a new perspective on understanding SOT control of magnetization reversal and dynamics by an in-plane current in thin, single FM and FM/HM bilayer systems.

Received 20 February 2022
Revised 13 June 2022
Accepted 13 June 2022

DOI:https://doi.org/10.1103/PhysRevB.105.224417

Physics Subject Headings (PhySH)

Magnetic interactions Magnetism Spintronics

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Qingwei Fu, Like Liang , Wenqiang Wang, Liupeng Yang, Kaiyuan Zhou, Zishuang Li, Chunjie Yan, Liyuan Li, Haotian Li, and Ronghua Liu ^*

National Laboratory of Solid State Microstructures, School of Physics and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China

^*rhliu@nju.edu.cn

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Vol. 105, Iss. 22 — 1 June 2022

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Images

Figure 1
(a, b) Schematic diagrams illustrating the directions for external magnetic field $H_{ext}$ , applied charge current ( $j_{c}$ ), spin current ( $j_{s}$ ), and current-induced out-of-plane torque $(τ_{⊥})$ and in-plane torque $(τ_{∥})$ in NM/FM bilayer (a) and single FM layer (b) systems, respectively. (c) Schematic of the experimental setup and the optical image of the active area of the ST-FMR device. (d) The waterfall plot of ST-FMR spectra of Py(10) obtained at room temperature. (e) The representative ST-FMR spectrum and its fitting curves of Py(10) at 9 GHz. The solid line is the Lorentzian fitting curve, including the symmetric (blue) and antisymmetric (red) parts. (f) Resonance frequency $f$ vs. magnetic field $H_{ext}$ data. Solid line: the Kittel fit.
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Figure 2
(a) The representative spectra obtained under three dc currents (–5, 0, and +5 mA) and the external magnetic field with in-plane angle $ϕ = 210^{\circ}$ . (b) Current-induced linewidth changes of Py(10) under positive $(ϕ = 30^{\circ})$ and negative $(ϕ = 210^{\circ})$ magnetic fields. (c) The in-plane, angle-dependent antisymmetric and symmetric components of the ST-FMR spectra for the Py(10) sample. The angular dependence of the antisymmetric parts are fitted by the sum of sin(2ϕ)cos(ϕ), sin(2ϕ)sin(ϕ), and sin (2ϕ); and the symmetric data are fitted by the sum of sin(2ϕ)cos(ϕ), sin(2ϕ)sin(ϕ), sin(2ϕ), and sin(ϕ)functions. (d) Same as (c) for the Pt(6)/Py(6) sample.
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Figure 3
The angle-dependent $V_{A}$ components of the ST-FMR spectra of the W(6)/ Py(6) sample (a), W(6)/CoFeB(6) sample (b), Pt(2)/Py(10)/Pt(2) sample (c). and Ta(2)/Py(10)/Ta(2) sample (d). The black solid lines are the best fits with the summation of sin(2ϕ)cos(ϕ) and sin(2ϕ) functions.
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Figure 4
Schematic diagrams of the generation of the unconventional spin currents in the single FM layer. (a) The spin Hall effect generates two opposite polarized spin currents ( $Q_{z y}$ ), where the broad arrows indicate the flow direction of spin currents and the small three-dimensional arrows denote the spin polarization. The region, as marked by the dotted red line, represents the spin swapping probabilities of the two opposite spin currents. (b) The schematic of spin swapping process. (c) The residual net spin currents with two spin polarization directions ( $Q_{z, y}$ and $Q_{y, z}$ ) after the spin swapping process.
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Figure 5
(a) SHE-like DL SOT efficiency ( $ξ_{y, DL}$ ) and unconventional DL SOT efficiencies ( $ξ_{z, DL}$ ) in single-layer ferromagnetic metals Py(10), Ni(11), and Co(11). (b) The $ξ_{y, DL}$ and $ξ_{z, DL}$ in our studied Py(6)/Ti(6), Py(6)/Ta(6), Py(6)/W(6), Py(6)/Cu(1)/W(6), and Ni(8)/Ta(5) multilayer structure.
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Figure 6
In-plane field angle (ϕ) dependence of second harmonic Hall voltage for the single-layer Py.
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