Coexistence of surface and bulk ferromagnetism mimics skyrmion Hall effect in a topological insulator

TL;DR

This study reveals that surface and bulk ferromagnetism coexist in a topological insulator, causing complex anomalous Hall effects that mimic skyrmion signatures, and highlights the importance of careful interpretation in magnetic topological materials.

Contribution

It demonstrates the coexistence of surface and bulk ferromagnetic orders in magnetic topological insulators and explains anomalous Hall effects without invoking skyrmions.

Findings

Sign inversion of anomalous Hall effect with gate voltage.

Different magnetization reversal fields indicate two ferromagnetic orders.

Hall response depends on film thickness and magnetic doping.

Abstract

Here we report the investigation of the anomalous Hall effect in the magnetically doped topological insulator (V,Bi,Sb)2Te3. We find it contains two contributions of opposite sign. Both components are found to depend differently on carrier density, leading to a sign inversion of the total anomalous Hall effect as a function of applied gate voltage. The two contributions are found to have different magnetization reversal fields, which in combination with a temperature dependent study points towards the coexistence of two ferromagnetic orders in the system. Moreover, we find that the sign of total anomalous Hall response of the system depends on the thickness and magnetic doping density of the magnetic layer. The thickness dependence suggests that the two ferromagnetic components originate from the surface and bulk of the magnetic topological insulator film. We believe that our observations provide insight on the magnetic behavior, and thus will contribute to an eventual understanding of the origin of magnetism in this material class. In addition, our data bears a striking resemblance to anomalous Hall signals often associated with skyrmion contributions. Our analysis provides a straightforward explanation for both the magnetic field dependence of the Hall signal and the observed change in sign without needing to invoke skyrmions, and thus suggest that caution is needed when making claims of effects from skyrmion phases.