Hybridization-induced interface states in a topological-insulator-ferromagnetic-metal heterostructure

Recent experiments demonstrating large spin-transfer torques in topological-insulator (TI)-ferromagnetic-metal (FM) bilayers have generated a great deal of excitement due to their potential applications in spintronics. The source of the observed spin-transfer torque, however, remains unclear. This is because the large charge transfer from the FM to the TI layer would prevent the Dirac cone at the interface from being anywhere near the Fermi level to contribute to the observed spin-transfer torque. Moreover, there is still not much understanding of the impact on the Dirac cone at the interface from the metallic bands overlapping in energy and momentum, where strong hybridization could take place. Here, we build a simple microscopic model and perform first-principles-based simulations for such a TI-FM heterostructure, considering the strong hybridization and charge-transfer effects. We find that the original Dirac cone is destroyed by hybridization, as expected. Instead, we find an interface state that we dub a "descendent state" that forms near the Fermi level due to the strong hybridization with the FM states at the same momentum. Such a descendent state carries a sizable weight of the original Dirac interface state, and thus it inherits the localization at the interface and the same Rashba-type spin-momentum locking. We propose that the descendent state may be an important source of the experimentally observed large spin-transfer torque in the TI-FM heterostructure.