The ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ material family has emerged as a key platform for exploring magnetic topological phases, most notably exemplified by the experimental realization of the axion insulator state. The interplay between spin dynamics and axion states in topological magnets is anticipated to lead to rich phenomena that yet remain largely unexplored. In this work, we employ an antiferromagnetic spin-chain model to demonstrate that an external magnetic field induces perpendicular magnetic anisotropy. We find that an in-plane field stabilizes the antiferromagnetic order, whereas an out-of-plane field destabilizes it and triggers spin-flop transitions. Remarkably, near the surface spin-flop transition in even-layer ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ films, the axion insulator state undergoes a sharp switching behavior accompanied by distinct magnetoelectric responses. Furthermore, we propose that this switchable axionic magnetoelectric effect can be utilized to convert alternating magnetic field signals into measurable square-wave magneto-optical outputs, thereby realizing an axionic analog of a zero-crossing detector.

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• Magneto-optical effect
• Magnetoelectric effect
• Quantum phase transitions
• Topological insulators
• Antiferromagnets
• Topological materials
• Density functional calculations
• Spin lattice models