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 matter1–4. 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 effect7,8 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 AlOx 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 spintronics9,10.