Semi-metallic bulk generated spin-orbit torques in disordered topological insulator

Abstract

Spin-orbit torque (SOT) induced by the transfer of orbital angular momentum from a lattice to a spin system offers an efficient route for manipulating spin-based devices. Among various potential candidates, three-dimensional topological insulators (TIs) with inherently strong spin-orbit coupling promise to be a powerful source of SOTs. While the huge SOTs observed in ferromagnet (FM)/TI bilayers are generally claimed to be of topological surface states (TSS) nature, the contributions from the surface and bulk states in realistic systems are undistinguishable, rendering the underlying physics elusive. Here, we provide direct evidence that the bulk spin-Hall effect dominates the SOTs generated by disordered TIs. We show that sizable SOTs with clear bulk feature are generated by bismuth antimonides, in which the semi-metallic bulk state intermediately couples to the surface states. From our analysis based on a drift diffusion approach, the lower limit of spin Hall conductivity turns out to be $0.66 \times 10 ^{5} (\hbar/2e)$ $\Omega$$^{-1}$m$^{-1}$, which is comparable to the reported values against the general belief in TSS origin. Furthermore, the complementary results of SOT generation and Gilbert damping enhancement suggest an essential role of band bending near the FM/TI interface upon modifying the relative magnitude of the real and imaginary parts of spin mixing conductance. Together with the bulk spin Hall effect, our finding may alter the landscape of the field of spin-orbitronics in TI based systems and develop new applications such as SOT transistors