Spin-orbit torques in asymmetric Pt/Co/Pt structures

We have studied the generation of spin-orbit torques in Pt/Co/Pt trilayers fabricated on ${\mathrm{Gd}}_3{\mathrm{Ga}}_5{\mathrm{O}}_{12}$ (GGG) and commonly used ${\mathrm{SiO}}_2$ substrates. By fabricating Pt/Co/Pt trilayers with symmetric thicknesses of the top and bottom Pt layers, it has been confirmed that complete cancellation of both fieldlike and dampinglike torques can be obtained on the GGG substrate. However, the spin-orbit torques cannot be completely canceled out on the ${\mathrm{SiO}}_2$ substrate, which suggests that a GGG substrate is more suitable for precise measurements of the spin-orbit torques. We have also fabricated asymmetric Pt/Co/Pt trilayers on a GGG substrate to generate nonzero spin-orbit torques, and a relatively strong dampinglike torque has been measured accompanied with a fieldlike torque which is about four times smaller. Furthermore, by conducting the vector measurements of the spin-orbit torques from 300 to 10 K, it has been found that the dampinglike torque only slightly varies with temperatures.

Figure 1

(a) Schematic illustration of the device structure. The current is injected along the $x$ axis. The external magnetic field is along the $x$ axis for the magnetization switching and $H_{\text{D}}$ measurement, and along the $y$ axis for the $H_{\text{F}}$ measurement. (b) The x-ray reflectivity profile for the Pt(5 nm)/Co(1.1 nm)/Pt(2 nm)/GGG trilayers. The solid circles are the experimental data and the red curve is the fitting result.

Figure 2

The first $V_{\omega }$ and second $V_{2\omega }$ harmonic Hall voltage signals as a function of the in-plane magnetic field applied along (a) the $x$ axis and (b) the $y$ axis for the symmetric Pt(3.5 nm)/Co(1.1 nm)/Pt(3.5 nm)/GGG trilayers. The applied current density was $J=0.42\times {10}^6{\mathrm{A}/\mathrm{cm}}^2$. The solid circles are the experimental data. The solid lines are the fitting results using parabolic and linear functions.

Figure 3

The in-plane magnetic field dependence of $V_{2\omega }$ for the symmetric Pt/Co/Pt structure on the (a) ${\mathrm{SiO}}_2$ and (b) GGG substrates. The in-plane magnetic field was applied along the $x$ axis. The thickness $d_{\text{N}}$ dependence of the resistivity ${\rho }_{\text{N}}$ of Pt films on (c) ${\mathrm{SiO}}_2$ and (d) GGG substrates.

Figure 4

(a) and (b) Current-induced magnetization switching curves for the asymmetric ${\mathrm{Pt}/\mathrm{Co}/\mathrm{Pt}/\mathrm{SiO}}_2$ and Pt/Co/Pt/GGG with different in-plane external magnetic field $H_x$, respectively. (c) and (d) Switching phase diagrams for ${\mathrm{Pt}/\mathrm{Co}/\mathrm{Pt}/\mathrm{SiO}}_2$ and Pt/Co/Pt/GGG, respectively. (e) Anomalous Hall resistance $R_{\text{H}}$ dependence on the perpendicular magnetic field.

Figure 5

The temperature $T$ dependence of the (a) $V_{\omega }$ and (b) $V_{2\omega }$ for the asymmetric Pt/Co/Pt/GGG device measured with the in-plane magnetic field along the $x$ axis. (c) The $T$ dependence $H_{\text{D}}/J$, where $H_{\text{D}}$ is the dampinglike effective field. (d) $H_z$ dependence of $V_{2\omega }$ for the asymmetric Pt/Co/Pt/GGG trilayers. (e) The $T$ dependence of the saturation magnetization measured with a vibrating sample magnetometer.