  The intricate interplay between nontrivial topology and magnetism in
two-dimensional (2D) materials has led to the emergence of many novel phenomena
and functionalities. An outstanding example is the quantum anomalous Hall (QAH)
effect, which was realized in magnetically doped topological insulators (TIs)
in the absence of magnetic field. Recently, the layered van der Waals compound
MnBi2Te4 has been theoretically predicted and experimentally verified to be a
TI with interlayer antiferromagnetic (AFM) order. It is a rare stoichiometric
material with coexisting topology and magnetism, thus represents a perfect
building block for complex topological-magnetic structures. Here we investigate
the quantum transport behaviors of both bulk crystal and exfoliated MnBi2Te4
flakes in a field effect transistor geometry. In the 6 septuple layers (SLs)
device tuned into the insulating regime, we observe a large longitudinal
resistance and zero Hall plateau, which are characteristic of the axion
insulator state. The robust axion insulator state occurs in zero magnetic
field, over a wide magnetic field range, and at relatively high temperatures.
Moreover, a moderate magnetic field drives a quantum phase transition from the
axion insulator phase to a Chern insulator phase with zero longitudinal
resistance and quantized Hall resistance h/e2 (h is the Plank constant and e is
the elemental charge). These results pave the road for using even-number-SL
MnBi2Te4 to realize the quantized topological magnetoelectric effect and axion
electrodynamics in condensed matter systems.
