Spin-torque resonance due to diffusive dynamics at a surface of topological insulator

TL;DR

This paper explores how diffusive electron dynamics at a topological insulator surface induce a novel anisotropic anti-damping spin-orbit torque, affecting spin-torque resonance in adjacent ferromagnetic layers.

Contribution

It identifies a new diffusive spin-orbit torque mechanism at TI surfaces that influences magnetization dynamics, especially under specific electric field configurations.

Findings

Discovery of an anisotropic anti-damping-like spin-orbit torque.

The diffusive torque affects spin-torque resonance at finite frequencies.

The torque depends on the electric field orientation relative to the TI surface.

Abstract

We investigate spin-orbit torques on magnetization in an insulating ferromagnetic (FM) layer that is brought into a close proximity to a topological insulator (TI). In addition to the well-known field-like spin-orbit torque, we identify an anisotropic anti-damping-like spin-orbit torque that originates in a diffusive motion of conduction electrons. This diffusive torque is vanishing in the limit of zero momentum (i. e. for spatially homogeneous electric field or current), but may, nevertheless, have a strong effect on spin-torque resonance at finite frequency provided external field is neither parallel nor perpendicular to the TI surface. The required electric field configuration can be created by a grated top gate.