Unconventional Planar Hall Effect in Exchange-Coupled Topological Insulator-Ferromagnetic Insulator Heterostructures

TL;DR

This study reports the first experimental observation of an unconventional planar Hall effect and gate-tunable magnetoresistance in EuS/TI heterostructures, highlighting the role of topological surface states and exchange coupling.

Contribution

It provides the first experimental evidence of an unconventional PHE in exchange-coupled topological insulator-ferromagnetic insulator heterostructures and explores its dependence on Dirac surface states.

Findings

Significant PHE when magnetic field is parallel to current

Suppressed PHE when magnetic field is perpendicular

PHE suppression in thin TI films with hybridization gap

Abstract

The Dirac electrons occupying the surface states (SSs) of topological insulators (TIs) have been predicted to exhibit many exciting magneto-transport phenomena. Here we report on the first experimental observation of an unconventional planar Hall effect (PHE) and an electrically gate-tunable hysteretic planar magnetoresistance (PMR) in EuS/TI heterostructures, in which EuS is a ferromagnetic insulator (FMI) with an in-plane magnetization. In such exchange-coupled FMI/TI heterostructures, we find a significant (suppressed) PHE when the in-plane magnetic field is parallel (perpendicular) to the electric current. This behavior differs from previous observations of the PHE in ferromagnets and semiconductors. Furthermore, as the thickness of the 3D TI films is reduced into the 2D limit, in which the Dirac SSs develop a hybridization gap, we find a suppression of the PHE around the charge neutral point indicating the vital role of Dirac SSs in this phenomenon. To explain our findings, we outline a symmetry argument that excludes linear-Hall mechanisms and suggest two possible non-linear Hall mechanisms that can account for all the essential qualitative features in our observations.