  The interplay between nontrivial band topology and layered antiferromagnetism
in MnBi2Te4 has opened up a new avenue for exploring topological phases of
matter. Representative examples include the quantum anomalous Hall effect and
axion insulator state observed in odd and even number layers of MnBi2Te4, when
the top and bottom surfaces have parallel and antiparallel spin alignments
respectively. The rich and complex spin dynamics associated with the van der
Waals antiferromagnetic order is expected to generate novel topological phases
and phase transitions that are unique to MnBi2Te4. Here we fabricate a device
of 7-septuple-layer MnBi2Te4 covered with 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 spin configurations on charge transport. Furthermore, we find that
an in-plane magnetic field enhances both the coercive field and exchange gap of
the surface state, in sharp contrast to that in ferromagnetic quantum anomalous
Hall state. We propose that these peculiar features arise from the spin flip
and flop transitions inherent to 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.
