Distinct Magnetic Gaps between Antiferromagnetic and Ferromagnetic Orders Driven by Surface Defects in the Topological Magnet MnBi2Te4

Many experiments observed a metallic behavior at zero magnetic fields (antiferromagnetic phase, AFM) in MnBi2Te4 thin film transport, which coincides with gapless surface states observed by angle-resolved photoemission spectroscopy, while it can become a Chern insulator at field larger than 6 T (ferromagnetic phase, FM). Thus, the zero-field surface magnetism was once speculated to be different from the bulk AFM phase. However, recent magnetic force microscopy refutes this assumption by detecting persistent AFM order on the surface. In this Letter, we propose a mechanism related to surface defects that can rationalize these contradicting observations in different experiments. We find that co-antisites (exchanging Mn and Bi atoms in the surface van der Waals layer) can strongly suppress the magnetic gap down to several meV in the AFM phase without violating the magnetic order but preserve the magnetic gap in the FM phase. The different gap sizes between AFM and FM phases are caused by the exchange interaction cancellation or collaboration of the top two van der Waals layers manifested by defect-induced surface charge redistribution among the top two van der Waals layers. This theory can be validated by the position -and field-dependent gap in future surface spectroscopy measurements. Our work suggests suppressing related defects in samples to realize the quantum anomalous Hall insulator or axion insulator at zero fields.

Automated summary

This letter investigates the electrical and magnetic properties of MnBi2Te4 thin films under different magnetic fields. A mechanism related to surface defects is proposed to explain the contradictory observations in previous experiments. It is found that co-antisites can strongly suppress the magnetic gap in the antiferromagnetic phase without violating the magnetic order, while preserving the magnetic gap in the ferromagnetic phase. This study is of great importance for future surface spectroscopy measurements.