  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 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 yet little 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 the hybridization as expected. Instead,
we find a new interface state which we dub 'descendent state' to form 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 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.
