  The 'magnetoelectric effect' arises from the coupling between magnetic and
electric properties in materials. The Z2 invariant of topological insulators
(TIs) leads to a quantized version of this phenomenon, known as the topological
magnetoelectric (TME) effect. This effect can be realized in a new topological
phase called an 'axion insulator' whose surface states are all gapped but the
interior still obeys time reversal symmetry. We demonstrate such a phase using
electrical transport measurements in a quantum anomalous Hall (QAH) sandwich
heterostructure, in which two compositionally different magnetic TI layers are
separated by an undoped TI layer. Magnetic force microscopy images of the same
sample reveal sequential magnetization reversals of the top and bottom layers
at different coercive fields, a consequence of the weak interlayer exchange
coupling due to the spacer. When the magnetization is antiparallel, both the
Hall resistance and Hall conductance show zero plateaus, accompanied by a large
longitudinal resistance and vanishing longitudinal conductance, indicating the
realization of an axion insulator state. Our findings thus show evidences for a
phase of matter distinct from the established QAH state and provide a promising
platform for the realization of the TME effect.
