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### Unconventional spin-orbit torques from sputtered ${\mathrm{MoTe}}_{2}$ films

##### Shuchen Li, Jonathan Gibbons, Stasiu Chyczewski, Zetai Liu, Hsu-Chih Ni, Jiangchao Qian, Jian-Min Zuo, Jun-Fei Zheng, Wenjuan Zhu, and Axel Hoffmann

##### Phys. Rev. B **110**, 024426 – Published 23 July 2024

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#### Abstract

Materials with strong spin-orbit coupling and low crystalline symmetry are promising for generating large unconventional spin-orbit torques (SOTs), such as in-plane fieldlike (FL) torques and out-of-plane dampinglike (DL) torques, which can effectively manipulate and deterministically switch an out-of-plane magnetization without the need for additional external in-plane magnetic fields. Here, we report SOTs generated by magnetron-sputtered $1{T}^{\prime}\phantom{\rule{0ex}{0ex}}{\mathrm{MoTe}}_{2}$/Permalloy (Py; ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$)/MgO heterostructures using both spin-torque ferromagnetic resonance (ST-FMR) and second harmonic Hall measurements. We observed unconventional FL and DL torques in our samples due to spins polarized normal to the interface of ${\mathrm{MoTe}}_{2}$ and Py layers, and studied the influence of crystallographic order and ${\mathrm{MoTe}}_{2}$ layer thickness on the SOTs. By comparing the Raman spectra of $1{T}^{\prime}\phantom{\rule{0ex}{0ex}}{\mathrm{MoTe}}_{2}$ samples prepared in different ways, we found a tensile strain in sputtered ${\mathrm{MoTe}}_{2}$ films, which might further enhance the generation of unconventional torques by reducing the symmetry of $1{T}^{\prime}\phantom{\rule{0ex}{0ex}}{\mathrm{MoTe}}_{2}$.

- Received 2 January 2024
- Revised 24 June 2024
- Accepted 10 July 2024

DOI:https://doi.org/10.1103/PhysRevB.110.024426

©2024 American Physical Society

#### Physics Subject Headings (PhySH)

- Research Areas

Spin injectionSpin-orbit torqueSpintronics

- Techniques

Ferromagnetic resonanceRaman spectroscopy

Condensed Matter, Materials & Applied Physics

#### Authors & Affiliations

Shuchen Li^{1,*}, Jonathan Gibbons^{1,2}, Stasiu Chyczewski^{3}, Zetai Liu^{3}, Hsu-Chih Ni^{1}, Jiangchao Qian^{1}, Jian-Min Zuo^{1}, Jun-Fei Zheng^{4}, Wenjuan Zhu^{3}, and Axel Hoffmann^{1,†}

^{1}Department of Materials Science and Engineering and Materials Research Laboratory, University of Illinois Urbana-Champaign, Urbana, Illinois 61801, USA^{2}Department of Physics, University of California–San Diego, La Jolla, California 92093, USA^{3}Department of Electrical and Computer Engineering, University of Illinois Urbana-Champaign, Urbana, Illinois 61801, USA^{4}Entegris Inc., Danbury, Connecticut 06810, USA

^{*}Contact author: sl117@illinois.edu^{†}Contact author: axelh@illinois.edu

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##### Issue

Vol. 110, Iss. 2 — 1 July 2024

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Article part of CHORUS

Accepted manuscript will be available starting23 July 2025.#### Images

###### Figure 1

(a)Raman spectra of magnetron-sputtered 40-nm (dark blue), 15-nm (blue), 7-nm (light blue), and exfoliated (gray) ${\mathrm{MoTe}}_{2}$ samples. The spectra are shifted by an offset of 10 with respect to each other. The green dashed line and the red arrows indicate the theoretical and the measured ${\mathrm{MoTe}}_{2}$ Raman shift position. (b)Crystal structure of $1{T}^{\prime}\phantom{\rule{0ex}{0ex}}{\mathrm{MoTe}}_{2}$ with the only mirror plane (red dashed line) along the $a$ axis. On the left shows the $A$-plane sapphire substrate orientations. (c)Polarized Raman spectra for 15-nm ${\mathrm{MoTe}}_{2}$/sapphire. A linearly polarized 633-nm light illuminates the sample with the polarization angle ${\varphi}_{R}$ with respect to the $\left[1\overline{1}00\right]$ direction of the $A$-plane sapphire substrate [shown in (b)]. ${\varphi}_{R}$ = ${0}^{\circ}$ means the polarization direction is parallel to the $\left[1\overline{1}00\right]$ direction. The green arrows indicate different Raman modes and different peak intensities. (d)The STEM image of the sputtered ${\mathrm{MoTe}}_{2}$ film and the fast Fourier transform of the blue circled area.

###### Figure 2

(a)Diagram of our measurement setup for ST-FMR. A signal generator injects a GHz rf current whose amplitude is modulated by the reference signal of a lock-in amplifier into the device through the rf port of a bias tee. The mixing rf voltage is measured by the lock-in amplifier through the rf port of the bias tee. The dimension of the device is 80–130 $\mu \mathrm{m}$ in length and 20–40 $\mu \mathrm{m}$ in width. (b)A schematic of the spin-torque ferromagnetic resonance measurements on ${\mathrm{MoTe}}_{2}$/Py/MgO devices. (c)The measured rf mixing voltages of sample 1 device 1 of ${\mathrm{MoTe}}_{2}$(15)/Py/MgO at ${\varphi}_{H}$ = ${45}^{\circ}$ for positive and negative field scans. The power and frequency of the current is 4 dBm and 6GHz, and the current direction is along $\left[1\overline{1}00\right]$. The fit for the mixing voltage is the green curve, which is the sum of ${V}_{S}$ (blue) and ${V}_{A}$ (red). (d)The mixing voltages ${V}_{\mathrm{mix}\_\mathrm{z}}$ with contributions solely from $z$-polarized spins, and we found ${S}_{z}=-0.313$ and ${A}_{z}=0.140$, which are proportional to the sizes of $\stackrel{\u20d7}{\tau}{}_{\text{FL}}^{z}$ and $\stackrel{\u20d7}{\tau}{}_{\text{DL}}^{z}$.

###### Figure 3

(a), (c), and (e) Antisymmetric components ${V}_{A}$ as a function of angle ${\varphi}_{H}$ for ${\varphi}_{I}$ = ${0}^{\circ},{30}^{\circ}$, and ${90}^{\circ}$. (b), (d), and (f) Symmetric components ${V}_{S}$ as a function of angle ${\varphi}_{H}$ for ${\varphi}_{I}$ = ${0}^{\circ},{30}^{\circ}$, and ${90}^{\circ}$. The red and blue dots are extracted from the measured ${V}_{\text{mix}}$, and the black lines are the fitted curves using Eqs.(3) and(2).

###### Figure 4

(a)$\xi {}_{\text{DL}}^{y}$ at different ${\varphi}_{I}$, (b)and (c)are absolute values of $\xi {}_{\text{FL}}^{z}$ and $\xi {}_{\text{DL}}^{z}$ for better study the trend with respect to ${\varphi}_{I}$. (d), (e), and (f) are $\xi {}_{\text{DL}}^{y},|\xi {}_{\text{FL}}^{z}|$, and $|\xi {}_{\text{DL}}^{z}|$ for devices with different ${\mathrm{MoTe}}_{2}$ thicknesses (7, 15, and 40nm).

###### Figure 5

(a)${V}^{2\omega}$ (dots) of ${\mathrm{MoTe}}_{2}$(15)/Py/MgO as a function of ${\varphi}_{H}$ for various fields and the fit curves (lines) using Eq.(5). (b)${V}^{\omega}$ as a function of ${\varphi}_{H}$ under ${H}_{\text{ext}}$ = 0.22T. (c)and (d)Components of ${V}^{2\omega},{V}_{\text{DL},z}^{2\omega}$, and ${V}_{\text{FL},y+\text{Oe}}^{2\omega}$, contributed by $H{}_{\text{DL}}^{z}$ and $H{}_{\text{FL}}^{y}+{H}_{\text{Oe}}$, with linear fit red lines.