Room-Temperature Spin-Orbit Torque from Topological Surface States

TLDR

Spin‑momentum locked surface states in topological insulators offer a promising route to high spin‑orbit torque efficiency beyond bulk spin‑orbit coupling in heavy metals, yet reported SOT values vary widely across studies. The study aims to systematically evaluate spin‑orbit torque efficiency in TI(HM)/Ti/CoFeB/MgO stacks to compare topological insulators and heavy metals. The authors use a unified experimental method across these stacks to measure SOT. Results show that TIs exhibit more than an order‑of‑magnitude higher SOT efficiency than HMs at room temperature, achieving a switching current density of 5.2×10⁵ A cm⁻², and that efficiency peaks near the Dirac point where bulk is insulating and surface states conduct, confirming that topological surface states dominate SOT.

Abstract

Spin-momentum locked surface states in topological insulators (TIs) provide a promising route for achieving high spin-orbit torque (SOT) efficiency beyond the bulk spin-orbit coupling in heavy metals (HMs). However, in previous works, there is a huge discrepancy among the quantitative SOTs from TIs in various systems determined by different methods. Here, we systematically study the SOT in the TI(HM)/Ti/CoFeB/MgO systems by the same method, and make a conclusive assessment of SOT efficiency for TIs and HMs. Our results demonstrate that TIs show more than one order of magnitude higher SOT efficiency than HMs even at room temperature, at the same time the switching current density as low as 5.2×10^{5} A cm^{-2} is achieved with (Bi_{1-x}Sb_{x})_{2}Te_{3}. Furthermore, we investigate the relationship between SOT efficiency and the position of Fermi level in (Bi_{1-x}Sb_{x})_{2}Te_{3}, where the SOT efficiency is significantly enhanced near the Dirac point, with the most insulating bulk and conducting surface states, indicating the dominating SOT contribution from topological surface states. This work unambiguously demonstrates the ultrahigh SOT efficiency from topological surface states.