Theory of Enhanced T_C and Magnetic Anisotropy in Topological Insulator-Ferromagnetic Insulator Heterostructures

Recent experiments have revealed a significant increase in the Curie temperature T_C and perpendicular magnetic anisotropy (PMA) of ferromagnetic insulator (FMI) - topological insulator (TI) heterostructures, such as Cr₂Ge₂Te₆/ (Bi,Sb)₂Te₃ [1] and Cr₂Te₃ / (Bi,Sb)₂Te₃ [2]. Motivated by these experiments, we propose a theory where the short-range Bloembergen-Rowland (BR) interaction, mediated by TI surface states and existing even in the absence of a Fermi surface - unlike the oscillatory RKKY interaction- can lead to the observed enhancement in T_C and PMA [2]. Topological phase transitions with thickness variations in TI thin films are predicted to lead to systematic changes in these results, since the BR interaction is more pronounced in the Quantum Spin Hall insulator phase compared to the trivial insulator phase. Within mean-field theory, T_C enhancements for FMIs proximitized to TIs of different thicknesses scales with the Van Vleck susceptibilities of the surface state electrons in these regimes. Our predictions suggest that varying the thickness of the TI layer could help establish whether the TI surface states are indeed responsible for this T_C enhancement.

[1] L. D. Alegria et al., APL 105, 053512 (2014)

[2] Y. Ou, M. Mirzhalilov et al., arXiv:2312.15028.