Crystallinity Control of the Topological-Insulator Surface Bi85Sb15(012) via Interfacial Engineering for Enhanced Spin-Orbit Torque

Abstract

Topological insulators demonstrate high charge-spin conversion efficiency due to their spin-momentum locking at the Dirac surface states. However, the surface states are sensitive to disruption caused by exchange coupling when interfaced with a ferromagnet. Here, we demonstrate the use of various nonmagnetic insertion layer materials, $\mathrm{Ti},\phantom{\rule{0.2em}{0ex}}\mathrm{Cu},$ and $\mathrm{Pt}$, at the $\mathrm{Co}/\mathrm{Bi}$-$\mathrm{Sb}(012)$ interface to preserve the topological surface state and promote spin-orbit-torque efficiency through the crystallinity control of $\mathrm{Bi}$-$\mathrm{Sb}(012)$. For 20-nm-thick $\mathrm{Bi}$-$\mathrm{Sb}$, a spin Hall angle of up to 8.93 is observed with the use of a $\mathrm{Pt}$ insertion layer, while it is otherwise negligible for $\mathrm{Co}/\mathrm{Bi}$-$\mathrm{Sb}(012)$ interfaces. We further explore the enhancement of $\mathrm{Bi}$-$\mathrm{Sb}(012)$ crystallinity with increasing $\mathrm{Bi}$-$\mathrm{Sb}$ thickness, revealing a rapidly increasing spin-orbit-torque efficiency that gradually saturates above 30 nm. A clear correlation between spin-orbit-torque efficiency and $\mathrm{Bi}$-$\mathrm{Sb}(012)$ crystalline size is identified using x-ray diffractometry, establishing the origin of the high spin-orbit efficiency to be the $\mathrm{Bi}$-$\mathrm{Sb}(012)$ crystalline orientation. Our work demonstrates the spin-orbit-torque origin in $\mathrm{Bi}$-$\mathrm{Sb}$ experimentally and paves the way for the adaptation of topological insulators as a class of low-energy spin source material for spintronics applications.