Probing the magnetic band gap of the ferromagnetic topological insulator MnSb$_2$Te$_4$

TL;DR

This study investigates the magnetic gap in MnSb$_2$Te$_4$, revealing how magnetic fields and temperature fluctuations influence the gap, which is crucial for quantum anomalous Hall applications.

Contribution

It provides the first detailed analysis of the magnetic gap behavior under varying magnetic fields and temperatures in MnSb$_2$Te$_4$, highlighting the role of complex magnetic structures.

Findings

Magnetic gap shrinks under in-plane magnetic field but does not close.

Significant gap fluctuations occur at temperatures below T_C/2.

Magnetic gap behavior is influenced by competing magnetic orders.

Abstract

Mn-rich MnSb$_2$Te$_4$ is a ferromagnetic topological insulator with yet the highest Curie temperature T_C = 45-50 K. It exhibits a magnetic gap at the Dirac point of the topological surface state that disappears above T_C. By scanning tunneling spectroscopy, we probe this gap at different magnetic fields and temperatures. We firstly reveal that the gap size shrinks, when an in-plane magnetic field of up to B = 3 T is applied, but does not close completely as the magnetization is only partially rotated in-plane. This corroborates the magnetic origin of the gap and the complex magnetic structure. In addition, we demonstrate significant spatiotemporal fluctuations of the gap size at temperatures as low as T_C/2, above which the remanent magnetization indeed decays. This temperature is close to the antiferromagnetic transition temperature observed for bulk-type single crystals of MnSb$_2$Te$_4$, highlighting the important role of competing magnetic orders in the formation of the favorable ferromagnetic topological insulator. Our study, thus, provides crucial insights into the complex magnetic gap opening of topological insulators that is decisive for quantum anomalous Hall devices.