Spin-Torque Generation in Topological-Insulator-Based Heterostructures

TL;DR

This paper investigates the origin of spin-torque in topological insulator heterostructures, focusing on surface state contributions and providing a unified theoretical framework for different bilayer configurations.

Contribution

It introduces a theoretical model describing spin-torque generation from topological surface states in various heterostructures, clarifying mechanisms and setting constraints.

Findings

Large spin-torque efficiencies in TI/FM bilayers

Dominance of spin-transfer-like torque in TI/mdTI bilayers

Model explains spin-torque generation but not magnitude enhancements

Abstract

Heterostructures utilizing topological insulators exhibit a remarkable spin-torque efficiency. However, the exact origin of the strong torque, in particular whether it stems from the spin-momentum locking of the topological surface states or rather from spin-Hall physics of the topological-insulator bulk remains unclear. Here, we explore a mechanism of spin-torque generation purely based on the topological surface states. We consider topological-insulator-based bilayers involving ferromagnetic metal (TI/FM) and magnetically doped topological insulators (TI/mdTI), respectively. By ascribing the key theoretical differences between the two setups to location and number of active surface states, we describe both setups within the same framework of spin diffusion of the non-equilibrium spin density of the topological surface states. For the TI/FM bilayer, we find large spin-torque efficiencies of roughly equal magnitude for both in-plane and out-of-plane spin torques. For the TI/mdTI bilayer, we elucidate the dominance of the spin-transfer-like torque. However, we cannot explain the orders of magnitude enhancement reported. Nevertheless, our model gives an intuitive picture of spin-torque generation in topological-insulator-based bilayers and provides theoretical constraints on spin-torque generation due to topological surface states.