Magneto-optical response of five-septuple-layer MnBi 2 Te 4 in spin-flip states

This is the authors' abstract. We don't add key points for this paper.

Magneto-optical (MO) effects like Kerr and Faraday rotations provide a direct probe of topological order in thin films of the magnetic topological insulator (TI) MnBi 2 Te 4 (MBT). Motivated by recent experimental studies of spin-flip/flop transitions in MBT thin films, we investigate the interplay between interlayer spin configurations, topological order, and MO response in five septuple-layer (5-SL) MBT using first-principles calculations and a simplified coupled Dirac cone model. Our results reveal that, despite possessing a nonzero out-of-plane magnetization, 5-SL MBT thin films can be either C = + 1 TIs or C = 0 topologically trivial insulators depending on the relative spin orientations of the top and bottom SLs. We evaluate the Faraday and Kerr rotation angles using tight-binding models derived from calculations and by comparing our results with those of a simplified coupled Dirac cone model to clarify the macroscopic mechanisms underlying the MO response of spin-flip states. These theoretical findings highlight the tunability of topological and MO properties in MBT thin films and provide microscopic insight into the emergence of complex topological order in layered antiferromagnetic materials.