Odd-Even Layer-Number Effect and Layer-Dependent Magnetic Phase Diagrams in MnBi 2 Te 4

摘要

Recently reported with nontrivial topological properties and magnetic orders, MnBi 2 Te 4 is an intrinsic, magnetic topological insulator which holds promise for exploring exotic quantum phenomena such as the quantum anomalous Hall effect. However, the layer-dependent magnetism of MnBi 2 Te 4 , which is fundamental and crucial for further exploration of related quantum phenomena in this system, remains elusive. Here, by using polar reflective magnetic circular dichroism spectroscopy, we show that few-layered MnBi 2 Te 4 exhibits an evident odd-even layer-number effect, i.e., the oscillations of the coercivity of the hysteresis loop (at μ 0 H c ) and the spin-flop transition (at μ 0 H 1 ), concerning the Zeeman energy and magnetic anisotropy energy. Noticeably, an anomalous magnetic hysteresis loop is observed in the even-number septuple-layered MnBi 2 Te 4 , which might be attributed to the thickness-independent surface-related magnetization. A linear-chain model is applied to elucidate this odd-even layer-number effect of the spin-flop field and to determine the evolution of the magnetic states when subjected to an external magnetic field. A mean-field method further allows us to fully map the MnBi 2 Te 4 flake’s magnetic phase diagrams in the parameter space of the magnetic field, layer number, and, especially, temperature. By harnessing the unusual layer-dependent magnetic properties, our work paves the way for further study of quantum phenomena of MnBi 2 Te 4 .