Topological magneto-optical switching in even-layered MnBi ₂ Te ₄

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 a Wannier-based tight-binding Hamiltonian derived from 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 (SLs) provides the primary microscopic control of the total Chern number, optical conductivity, 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 an 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 the PT -symmetry and net magnetization. Increasing the thickness reveals an additional regime: while 8-SL MBT hosts only C=0 and C=1 states, a 12-SL MBT film supports a higher Chern number phase with C=2 , exhibiting a doubled low-frequency Faraday rotation owing to additional Berry-curvature contributions from adjacent near-surface SLs. Our results provide a thickness-dependent route to multilevel MO switching and establish MO spectroscopy as a direct probe of magnetism and topological order in MBT thin films.