  The polarity-tunable anomalous Hall effect (AHE) is useful for electronic
device applications. Here in a magnetic topological insulator MnBi2Te4 grown by
molecular beam epitaxy, we report the polarity change of the AHE by increasing
the temperature or tuning the gate bias. This is possible because the anomalous
Hall response is composed of two competing contributions with opposite
polarities. The negative contribution is intrinsic to MnBi2Te4, follows an
ambipolar gate response and has a larger coercivity with increasing thickness.
Meanwhile, the positive one has a coercivity that is about one order of
magnitude greater than the negative one, dominates the Hall response at higher
temperatures, is more tunable by a gate bias and vanishes by increasing the
thickness of the thin film. One possible explanation for the additional
positive AHE is an extra surface ferromagnetism caused by the
surface-state-mediated RKKY interaction among magnetic impurities on the
surface. Our work provides the understanding of the AHE of MnBi2Te4, and paves
the way for many device applications, e.g. energy-efficient voltage-controlled
memory.
