First-principles Study on Spin-Dependent Surface State in MnBi₂Te₄ with Antisite Defects

MnBi₂Te₄ (MBT) has attracted attention as a promising material in the field of spintronics due to its distinctive magnetic and topological properties. While theoretical research has suggested the presence of a surface band gap within the (0001) plane, experimental measurements have observed an unexpected absence of such a gap in the surface states. The emergence of gapless surface states has been suggested to be related to in-plane spin alignment by density functional theory calculations. Here, we show that, in the presence of antisite defects, MBT has the potential to exhibit negative magnetic anisotropy energy, which promotes in-plane spin alignment. Furthermore, MBT with antisite defects can maintain mirror symmetry, protecting the Dirac state, even in the presence of defects. In addition, we present the estimation of defect concentrations by assessing Fermi level shifts due to electron doping, which can be correlated with measurements from ARPES. Moreover, we show that the application of compressive strain can induce a band gap and allow for manipulation of gap sizes, whereas tensile strain still allows the existence of a gapless surface state. Finally, a comprehensive analysis is carried out to explore the topological crystalline insulator properties of the system, including spin textures and the low-energy effective Hamiltonian, in order to unveil their inherent characteristics. This investigation provides valuable insights for further research on MBT and its potential applications in the field of spintronics.