  Axion, first postulated as a hypothetical particle in high-energy physics, is
now extended to describe a novel topological magnetoelectric effect derived
from the Chern-Simons theory in condensed matter systems. The recent discovered
intrinsic magnetic topological insulators MnBi2Te4 and its derivatives have
attracted great attention because of their potential as a material platform to
realize such a quantized axion field. Since the magnetic exchange gap can bring
the half-quantized anomalous Hall effect at the surface, an axion insulator
manifests as quantum anomalous Hall and zero Hall plateau effects in the thin
films. However, many puzzles about this material family remain elusive yet,
such as the gapless surface state and the direct experimental evidence of the
axion insulator. In this Perspective, we discuss the preconditions,
manifestations and signatures of the axion-insulator phase, in the context of
the development of the natural magnetic topological heterostructure
MnBi2Te4(Bi2Te3)n family with various intriguing quantum phenomena. Recent
theoretical and experimental efforts regarding the intrinsic magnetic
topological insulators are summarized here to pave the way for this
phenomenally developing field.
