Spin-orbit torque in 3D topological insulator-ferromagnet heterostructure: crossover between bulk and surface transport

TL;DR

This paper investigates spin-orbit torques in a heterostructure of a ferromagnet and a 3D topological insulator, revealing a crossover from surface to bulk transport regimes and clarifying the origins of damping and spin Hall torques.

Contribution

It introduces a minimal tight-binding model capturing both surface and bulk effects, and analyzes the crossover between transport regimes in spin-orbit torque phenomena.

Findings

Surface-dominated and bulk-dominated transport regimes exhibit distinct spin density profiles.

Large damping torque is mainly due to interfacial magnetoelectric effects.

Spin Hall torque remains small even in the bulk-dominated regime.

Abstract

Current-driven spin-orbit torques are investigated in a heterostructure composed of a ferromagnet deposited on top of a three dimensional topological insulator using the linear response formalism. We develop a tight-binding model of the heterostructure adopting a minimal interfacial hybridization scheme that promotes induced magnetic exchange on the topological surface states, as well as induced Rashba-like spin-orbit coupling in the ferromagnet. Therefore, our model accounts for spin Hall effect from bulk states together with inverse spin galvanic and magnetoelectric effects at the interface on equal footing. By varying the transport energy across the band structure, we uncover a crossover from surface-dominated to bulk-dominated transport regimes. We show that the spin density profile and the nature of the spin-orbit torques differ substantially in both regimes. Our results, which compare favorably with experimental observations, demonstrate that the large damping torque reported recently is more likely attributed to interfacial magnetoelectric effect, while spin Hall torque remains small even in the bulk-dominated regime.