Spin accumulation in asymmetric topological insulator thin films in out of plane magnetic fields

TL;DR

This paper investigates how asymmetry in top and bottom surfaces of topological insulator thin films causes in-plane spin accumulation under magnetic fields, revealing effects absent in symmetric systems.

Contribution

It demonstrates that surface asymmetry in TI thin films leads to in-plane spin accumulation, a novel effect analyzed through Kubo formula calculations.

Findings

Asymmetry induces in-plane spin accumulation perpendicular to magnetization.

Breaking antisymmetry of energy contours causes spin accumulation.

Spin accumulation is absent in symmetric TI thin films.

Abstract

In this work we study the spin accumulation due to an in-plane electric field in an asymmetric topological insulator (TI) thin film system with an out of plane magnetic field and an in-plane magnetization. A TI thin film differs from the more typically studied thick TI system in that the former has both a top and a bottom surface where the states localized at both surfaces can couple to each other due to the finite thickness. In typical spin torque experiments on TI thin film systems, the top and bottom surfaces of the film are asymmetric as the former is in contact with a ferromagnetic layer while the latter is adjacent to a non magnetic substrate. This may lead to differing (i) potentials and (ii) magnetization strengths experienced by the top and bottom surface states. We show, via Kubo formula calculations, that each of these two effects can lead to in-plane spin accumulation perpendicular to the magnetization direction which are otherwise absent in a top-bottom symmetric TI thin film system. This spin accumulation results from the breaking of the antisymmetry of the spin accumulation around the zero magnetic field equal energy contours.