Topological Magneto-Optical Switching in Even-Layered MnBi$_2$Te$_4$

TL;DR

This study demonstrates topological magneto-optical switching in even-layered MnBi₂Te₄ films, where layer thickness and spin alignment control the material's topological state and optical response, enabling multilevel MO switching.

Contribution

It introduces a thickness-dependent mechanism for topological MO switching in MnBi₂Te₄, revealing new Chern insulating phases with quantized MO responses.

Findings

Reversing outermost spin alignment switches the system from axion to Chern insulator.

6-layer MBT exhibits a switch from C=0 to C=1 with MO response change.

12-layer MBT supports a higher Chern number C=2 with doubled Faraday rotation.

Abstract

MnBi$_2$Te$_4$ (MBT) thin films provide a unique material platform in which magnetism, topology, and magneto-optical (MO) response can be tuned through layer-thickness and relative spin alignments. In this work, using a low-energy coupled Dirac cone model together with Wannier-based tight-binding Hamiltonian derived from \textit{ab-initio} calculations, we investigate topological MO switching in even-layered MBT films. We argue that the relative spin alignment of the outermost septuple-layers (SL) mainly controls the total Chern number, optical conductibility, and consequently, the MO response. For a 6-SL MBT thin film, we found that reversing the outermost-SL alignments from antiparallel to parallel switches the system from axion insulating state with $C=0$ and vanishing Faraday rotation to a Chern insulating state with $C=1$ and a quantized MO response, irrespective of $PT$-symmetry and net magnetization. Increasing thickness reveals an additional regime: while 8-SL MBT hosts only $C=0$ and $1$ states, a 12-SL MBT film supports a higher Chern number phase with $C=2$ with a doubled low-frequency Faraday rotation. Our results provide a thickness-dependent route to multilevel MO switching and establish MO spectroscopy as a direct probe of surface magnetism and topological order in MBT thin films.