Antiferromagnetic quantum anomalous Hall effect under spin flips and flops

The interplay between nontrivial band topology and layered antiferromagnetism in MnBi₂Te₄ has opened a new avenue for exploring topological phases of matter^1-4. The quantum anomalous Hall effect⁵ and axion insulator state⁶ have been observed in odd and even number layers of MnBi₂Te₄, and the quantum metric nonlinear Hall effect^7,8 has been shown to exist in this topological antiferromagnet. The rich and complex antiferromagnetic spin dynamics in MnBi₂Te₄ is expected to generate new quantum anomalous Hall phenomena that are absent in conventional ferromagnetic topological insulators, but experimental observations are still unknown. Here we fabricate a device of 7-septuple-layer MnBi₂Te₄ covered with an AlO_x capping layer, which enables the investigation of antiferromagnetic quantum anomalous Hall effect over wide parameter spaces. By tuning the gate voltage and perpendicular magnetic field, we uncover a cascade of quantum phase transitions that can be attributed to the influence of complex spin configurations on edge state transport. Furthermore, we find that an in-plane magnetic field enhances both the coercive field and the exchange gap of the surface state, in contrast to that in the ferromagnetic quantum anomalous Hall state. Combined with numerical simulations, we propose that these peculiar features arise from the spin flip and flop transitions that are inherent to a van der Waals antiferromagnet. The versatile tunability of the quantum anomalous Hall effect in MnBi₂Te₄ paves the way for potential applications in topological antiferromagnetic spintronics^9,10.