Spontaneous ferromagnetism and finite surface energy gap in the topological insulator Bi₂Se₃ from surface Bi_Se antisite defects

We perform ab-initio calculations on Bi_Se antisite defects in the surface of Bi₂Se₃, finding strong low-energy defect resonances with spontaneous ferromagnetism, fixed to an out-of-plane orientation due to an exceptionally large magnetic anisotropy energy. For antisite defects in the surface layer, we find semi-itinerant ferromagnetism and strong hybridization with the Dirac surface state, generating a finite energy gap. For deeper-lying defects, such hybridization is largely absent, the magnetic moments become more localized, and no energy gap is present.