Odd-Even Layer-Number Effect and Layer-Dependent Magnetic Phase Diagrams in MnBi2Te4

Recently reported with nontrivial topological properties and magnetic orders, MnBi2Te4 is an intrinsic, magnetic topological insulator which holds promise for exploring exotic quantum phenomenasuch as the quantum anomalous Hall effect. However, the layer-dependent magnetism of MnBi2Te4, 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 MnBi2Te4 exhibits an evident odd-even layer-number effect, i.e., the oscillations of the coercivity of the hysteresis loop (at μ0Hc) and the spin-flop transition (at μ0H1), concerning the Zeeman energy and magnetic anisotropy energy. Noticeably, an anomalous magnetic hysteresis loop is observed in the even-number septuple-layered MnBi2Te4, which might be attributed to thethickness-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 MnBi2Te4 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 MnBi2Te4.