Magnetism at Topological Insulator-Ferromagnetic Insulator Interfaces

Recent experiments have shown a remarkable enhancement of the Curie temperature Tc of ferromagnetic insulators (FMI) proximitized with topological insulators (TI); examples include EuS/Bi₂Se₃ [1] and monolayer Cr₂Te₃/(Bi,Sb)₂Te₃ [2]. Motivated by these observations, we develop a theory of how the Dirac surface states of the TI influence magnetism near the interface, building on earlier work done in the context of the quantum anomalous Hall effect. We show that the Dirac electrons mediated exchange between magnetic ions have two contributions: an oscillatory Ruderman-Kittel-Kasuya-Yosida (RKKY) piece and a non-oscillatory Bloembergen-Rowland (BR) interaction. We find that the ferromagnetic BR interaction dominates in the regime of low carrier concentration and leads to an effective easy-axis magnetic anisotropy. It thus stabilizes out-of-plane ferromagnetic order in the FMI layer with an enhanced Curie temperature. We show that our theory provides a detailed microscopic understanding of the experiments [2] on monolayer Cr₂Te₃ films proximitized with (Bi,Sb)₂Te₃.