Spin-flop transition and Zeeman effect of defect-localized bound states in the antiferromagnetic topological insulator MnBi₂Te₄

The correlation of surface impurity states with the antiferromagnetic ground states is crucial for understanding the formation of the topological surface state in the antiferromagnetic topological insulators MnBi₂Te₄. By using low-temperature scanning tunneling microscopy and spectroscopy, we observed a localized bound state around the Mn-Bi antisite defect at the Te-terminated surface of the antiferromagnetic topological insulator MnBi₂Te₄. When applying a magnetic field perpendicular to the surface (B_z) from –1.5 to 3.0 T, the bound state shifts linearly to a lower energy with increasing B_z, which is attributed to the Zeeman effect. Remarkably, when applying a large range of B_z from –8.0 to 8.0 T, the magnetic field induced reorientation of surface magnetic moments results in an abrupt jump in the local density of states (LDOS), which is characterized by LDOS-change-ratio $\text{d}\tilde{\sigma }\text{/d}B$ quantitatively. Interestingly, two asymmetric critical field, –2.0 and 4.0 T determined by the two peaks in $\text{d}\tilde{\sigma }\text{/d}B$ are observed, which is consistent with simulated results according to a Mills-model, describing a surface spin flop transition (SSF). Our results provide a new flatform for studying the interplay between magnetic order and topological phases in magnetic topological materials.