Crystallinity Control of the Topological-Insulator Surface <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><mml:msub><mml:mi>Bi</mml:mi><mml:mn>85</mml:mn></mml:msub><mml:msub><mml:mi>Sb</mml:mi><mml:mn>15</mml:mn></mml:msub><mml:mo stretchy="false">(</mml:mo><mml:mn>012</mml:mn><mml:mo stretchy="false">)</mml:mo></mml:math> via Interfacial Engineering for Enhanced Spin-Orbit Torque
Han Yin Poh, Calvin Ching Ian Ang, Gerard Joseph Lim, Tianli Jin, S. H. Lee, Eng Kang Koh, F. Poh, Wen Siang Lew
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.