Exchange Biased Anomalous Hall Effect in Chern Insulators

A central theme in condensed matter physics is the interplay between magnetism and topology, which gives rise to many exciting quantum phenomena, including the quantum anomalous Hall (QAH) state and axion insulator state. Over the past five years, the intrinsic antiferromagnetic (AFM) topological insulator (TI) MnBi₂Te₄ has attracted much attention, primarily due to the realization of the QAH and axion insulator states in odd and even number layers, respectively. However, the existence of native defects in MnBi₂Te₄, such as Mn/Bi antisites and Te vacancies, has greatly hindered the realization of these two phenomena. In this work, based on the manually exfoliated MnBi₂Te₄ thin flakes, we fabricated a series of MnBi₂Te₄ devices with the thickness of 5 septuple layers (SLs) to 8SLs. We first demonstrated the presence of the Chern insulator state under high magnetic field (> 6T). We observed a large negative exchange bias effect in odd-number layer devices, while this effect is absent in the even-layer samples. Our theoretical calculations suggest that the layer-dependent exchange biased anomalous Hall effect is a result of the formation of the surface reconstruction in MnBi₂Te₄, particularly near zero magnetic field.