Magneto-optical Response of Spin-Flipped Five-Layer MnBi₂Te₄

Magneto-optical effects (such as the Kerr and Faraday rotation) arise due to light-matter interactions and provide a direct probe to observe topological order in thin-films of magnetic topological insulator MnBi₂Te₄ (MBT). Motivated by recent experiments on spin-flipped odd-layered MBTs (Nature 641, 70–75 (2025)), we investigate the interplay between different interlayer spin-flipped arrangements, topological order, and magneto-optical response in five septuple-layer (5SL) MBT systems using first-principles calculations and a simplified coupled-Dirac-cone model. Interestingly, our results reveal that a 5SL MBT, despite possessing a net out-of-plane magnetization, can exhibit both non-vanishing (C=+1) and vanishing (C=0) Chern insulating phases depending on the relative spin orientations of the top and bottom SLs, which can be controlled by an external magnetic field. Furthermore, by evaluating the quantized Kerr and Faraday rotation angles from tight-binding models derived from ab-initio calculations, we uncover the limitations of simple Dirac-cone model in capturing the magneto-optical response of spin-flipped configurations. These theoretical findings highlight the tunability of topological properties by manipulating relative SL spin configurations in MBT thin films, which can guide future magneto-optical experiments. Our work provides microscopic insight into the emergence of complex topological order in layered antiferromagnetic topological systems.