  An axion insulator is a three-dimensional (3D) topological insulator (TI), in
which the bulk maintains the time-reversal symmetry or inversion symmetry but
the surface states are gapped by surface magnetization. The axion insulator
state has been observed in molecular beam epitaxy (MBE)-grown magnetically
doped TI sandwiches and exfoliated intrinsic magnetic TI MnBi2Te4 flakes with
an even number layer. All these samples have a thickness of ~10 nm, near the
2D-to-3D boundary. The coupling between the top and bottom surface states in
thin samples may hinder the observation of quantized topological
magnetoelectric response. Here, we employ MBE to synthesize magnetic TI
sandwich heterostructures and find that the axion insulator state persists in a
3D sample with a thickness of ~106 nm. Our transport results show that the
axion insulator state starts to emerge when the thickness of the middle undoped
TI layer is greater than ~3 nm. The 3D hundred-nanometer-thick axion insulator
provides a promising platform for the exploration of the topological
magnetoelectric effect and other emergent magnetic topological states, such as
the high-order TI phase.
