  Magnetic topological states refer to a class of exotic phases in magnetic
materials with their non-trivial topological property determined by magnetic
spin configurations. An example of such states is the quantum anomalous Hall
(QAH) state, which is a zero magnetic field manifestation of the quantum Hall
effect. Current research in this direction focuses on QAH insulators with a
thickness of less than 10nm. The thick QAH insulators in the
three-dimensional(3D) regime are limited, largely due to inevitable bulk
carriers being introduced in thick magnetic TI samples. Here, we employ
molecular beam epitaxy (MBE) to synthesize magnetic TI trilayers with a
thickness of up to ~106 nm. We find these samples exhibit well-quantized Hall
resistance and vanishing longitudinal resistance at zero magnetic field. By
varying magnetic dopants, gate voltages, temperature, and external magnetic
fields, we examine the properties of these thick QAH insulators and demonstrate
the robustness of the 3D QAH effect. The realization of the well-quantized 3D
QAH effect indicates that the nonchiral side surface states of our thick
magnetic TI trilayers are gapped and thus do not affect the QAH quantization.
The 3D QAH insulators of hundred-nanometer thickness provide a promising
platform for the exploration of fundamental physics, including axion physics
and image magnetic monopole, and the advancement of electronic and spintronic
devices to circumvent Moore's law.
