Topological Surface States Originated Spin-Orbit Torques in ${\mathrm{Bi}}_2{\mathrm{Se}}_3$

The three dimensional topological insulator bismuth selenide (${\mathrm{Bi}}_2{\mathrm{Se}}_3$) is expected to possess strong spin-orbit coupling and spin-textured topological surface states and, thus, exhibit a high charge to spin current conversion efficiency. We evaluate spin-orbit torques in ${\mathrm{Bi}}_2{\mathrm{Se}}_3/{\mathrm{Co}}_{40}{\mathrm{Fe}}_{40}{\mathrm{B}}_{20}$ devices at different temperatures by spin torque ferromagnetic resonance measurements. As the temperature decreases, the spin-orbit torque ratio increases from $\sim 0.047$ at 300 K to $\sim 0.42$ below 50 K. Moreover, we observe a significant out-of-plane torque at low temperatures. Detailed analysis indicates that the origin of the observed spin-orbit torques is topological surface states in ${\mathrm{Bi}}_2{\mathrm{Se}}_3$. Our results suggest that topological insulators with strong spin-orbit coupling could be promising candidates as highly efficient spin current sources for exploring the next generation of spintronic applications.

Figure 1

(a) Temperature-dependent sheet resistivity of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ films (20 QL). (b) The magnetization versus field ($H$) for ${\mathrm{Bi}}_2{\mathrm{Se}}_3(20\mathrm{QL})/\mathrm{CFB}$ ($t$) (nominal thickness $t=1.5$, 2, 3, 4, and 5 nm) at room temperature. The inset shows the magnetization per unit area versus CFB thickness.

Figure 2

(a) The schematic diagram of the ST-FMR measurement, illustrating a bias tee, lock-in amplifier, rf signal generator (SG), and ST-FMR device with a ${\mathrm{Bi}}_2{\mathrm{Se}}_3/\mathrm{CFB}$ (5 nm). The microstrip is denoted by a dashed blue rectangle. (b) The measured ST-FMR signals from a ${\mathrm{Bi}}_2{\mathrm{Se}}_3/\mathrm{CFB}$ (5 nm) device ($D1$) at different temperatures.

Figure 3

Temperature dependence of (a) ${\tau }_{\parallel }$, (b) ${\tau }_{\perp }$, (c) ${\theta }_{\parallel }$, and (d) $[{\theta }_{\parallel }(\text{by}{V}_s\text{\hspace{0.17em}only})-{\theta }_{\parallel }(\text{by}{V}_s/V_a)]/[{\theta }_{\parallel }(\text{by}{V}_s/V_a)]$ in ${\mathrm{Bi}}_2{\mathrm{Se}}_3/\mathrm{CFB}$ (5 nm) for $D1$, $D2$, and $D3$. The ${\theta }_{\parallel }$ is analyzed by two different methods: by $V_s$ only and by $V_s/V_a$.

Figure 4

(a) Temperature-dependent out-of-plane torque ($\mathrm{\Delta }\tau ={\tau }_{\perp }-{\tau }_{\mathrm{Oe}}$) and (b) out-of-plane torque ratio (${\theta }_{\perp }$) in ${\mathrm{Bi}}_2{\mathrm{Se}}_3/\mathrm{CFB}$ (5 nm) devices.