Thin Films*

A central theme in condensed matter physics is to create and understand the exotic states of matter by incorporating magnetism into topological materials. One prime example is the quantum anomalous Hall (QAH) state. Recently, MnBi₂Te₄ has been demonstrated to be an intrinsic magnetic topological insulator and the QAH effect was observed in exfoliated MnBi₂Te₄ flakes. In this work, we used molecular beam epitaxy (MBE) to grow MnBi₂Te₄ films with thickness down to 1 septuple layer (SL) and performed thickness-dependent transport measurements. We observed a non-square hysteresis loop in the antiferromagnetic state for films with thickness greater than 2 SL. The hysteresis loop can be separated into two AH components. Through careful analysis, we demonstrated that one AH component with the larger coercive field is from the dominant MnBi₂Te₄ phase, while the other AH component with the smaller coercive field is from a trace of Mn-doped Bi₂Te₃ phase in the samples. The extracted AH component of the MnBi₂Te₄ phase shows a clear even-odd layer-dependent behavior, a signature of antiferromagnetic thin films. Our studies reveal insights on how to optimize the MBE growth conditions to improve the quality of MnBi₂Te₄ films,  in which the QAH and other exotic states are predicted.