Sign Change of Spin-Orbit Torque in $\mathrm{Pt}/\mathrm{NiO}/\mathrm{CoFeB}$ Structures

Antiferromagnetic insulators have recently been proved to support spin current efficiently. Here, we report the dampinglike spin-orbit torque (SOT) in $\mathrm{Pt}/\mathrm{NiO}/\mathrm{CoFeB}$ has a strong temperature dependence and reverses the sign below certain temperatures, which is different from the slight variation with temperature in the $\mathrm{Pt}/\mathrm{CoFeB}$ bilayer. The negative dampinglike SOT at low temperatures is proposed to be mediated by the magnetic interactions that tie with the “exchange bias” in $\mathrm{Pt}/\mathrm{NiO}/\mathrm{CoFeB}$, in contrast to the thermal-magnon-mediated scenario at high temperatures. Our results highlight the promise to control the SOT through tuning the magnetic structure in multilayers.

Figure 1

(a) The stack layer structure of the samples, with a $\mathrm{MgO}(1)/{\mathrm{Al}}_2{\mathrm{O}}_3(3)$ capping layer to protect the underneath layers from degradation. (b) The x-ray diffraction patterns for the sample with NiO of 50 nm. (c) The schematic of measurement configuration for the electrical transport experiments. (d) The perpendicular magnetic field ($H_z$) dependence of the Hall resistance ($R_H$) for the sample with 3 nm NiO at 300 K.

Figure 2

(a),(b) The first ($R_{\omega }$) and the second ($R_{2\omega }$) harmonic Hall resistance as a function of in-plane rotation angle $\varphi$ for the sample with 3 nm NiO at 300 K, under external magnetic field of 30 mT. The decomposed $\cos \varphi$ and $2{\cos }^3\varphi -\cos \varphi$ terms of the $R_{2\omega }$ are also displayed in (b). (c) The obtained ${\xi }_{\mathrm{DL}}$ for the samples with NiO of 0, 2, and 3 nm, and the $\mathrm{NiO}/\mathrm{CoFeB}$ sample at different temperatures. (d) The $\xi$ of $H_{\mathrm{FL}}+H_{\mathrm{Oe}}$ ($\mathrm{FL}+\mathrm{Oe}$), $H_{\mathrm{Oe}}$ (Oe), and $H_{\mathrm{FL}}$ (FL) for the sample with 3 nm NiO at different temperatures.

Figure 3

(a) The obtained SMR ratio for the samples with NiO of 0, 2, and 3 nm at different temperatures. (b) Comparison of the temperature dependence of the dampinglike SOT efficiency and the SMR ratio for the sample with 3 nm NiO.

Figure 4

(a) The schematic of the dampinglike spin-orbit effective fields ($H_{\mathrm{DL}}^{\mathrm{FM}}$) for the $\mathrm{Pt}/\mathrm{NiO}/\mathrm{CoFeB}$ at high temperature. (b) The schematic “spin-flop” configuration between the magnetic moments of the CoFeB (${\mathbf{m}}_{\mathrm{FM}}$) and NiO (${\mathbf{m}}_{\mathrm{AF}}^1$, ${\mathbf{m}}_{\mathrm{AF}}^2$) at the $\mathrm{NiO}/\mathrm{CoFeB}$ interface at low temperature. (c) The schematic of the dampinglike spin-orbit effective field for the $\mathrm{Pt}/\mathrm{NiO}/\mathrm{CoFeB}$ at low temperature. (d),(e) The schematic dampinglike SOTs for the $\mathrm{Pt}/\mathrm{NiO}/\mathrm{CoFeB}$ at high and low temperatures, respectively.