Defect-driven ferrimagnetism and hidden magnetization in MnBi$_2$Te$_4$

TL;DR

This study reveals that MnBi$_2$Te$_4$ exhibits defect-driven ferrimagnetism with hidden magnetization components, requiring high magnetic fields to reach saturation, which impacts understanding of its magnetic and topological properties.

Contribution

It uncovers the complex magnetic behavior of MnBi$_2$Te$_4$ due to defects, showing ferrimagnetic stages and hidden magnetization not previously characterized.

Findings

Magnetization of MnBi$_2$Te$_4$ occurs in stages, saturating near 60 Tesla.

Low-field magnetization plateau involves ferrimagnetic blocks with mixed magnetization.

Defects significantly influence the magnetic properties and states of MnBi$_2$Te$_4$.

Abstract

MnBi$_2$Te$_4$ (MBT) materials are promising antiferromagnetic topological insulators where field driven ferromagnetism is predicted to cause a transition between axion insulator and Weyl semimetallic states. However, the presence of antiferromagnetic coupling between Mn/Bi antisite defects and the main Mn layer can reduce the low-field magnetization, and it has been shown that such defects are more prevalent in the structurally identical trivial magnetic insulator MnSb$_2$Te$_4$ (MST). We use high-field magnetization measurements to show that the magnetization of MBT and MST occur in stages and full saturation requires fields of~$\sim$~60 Tesla. As a consequence, the low-field magnetization plateau state in MBT, where many determinations of quantum anomalous Hall state are studied, actually consists of ferrimagnetic septuple blocks containing both a uniform and staggered magnetization component.