Unconventional spin-orbit torques from sputtered ${\mathrm{MoTe}}_{2}$ films (2024)

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  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 $1T^{\prime }{\mathrm{MoTe}}_2$ 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.

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  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 $\vec{\tau }{}_{\text{FL}}^z$ and $\vec{\tau }{}_{\text{DL}}^z$.

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  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).

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  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).

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  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.

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