  Current-driven spin-orbit torques are investigated in a heterostructure
composed of a ferromagnet deposited on top of a three dimensional topological
insulator using the linear response formalism. We develop a tight-binding model
of the heterostructure adopting a minimal interfacial hybridization scheme that
promotes induced magnetic exchange on the topological surface states, as well
as induced Rashba-like spin-orbit coupling in the ferromagnet. Therefore, our
model accounts for spin Hall effect from bulk states together with inverse spin
galvanic and magnetoelectric effects at the interface on equal footing. By
varying the transport energy across the band structure, we uncover a crossover
from surface-dominated to bulk-dominated transport regimes. We show that the
spin density profile and the nature of the spin-orbit torques differ
substantially in both regimes. Our results, which compare favorably with
experimental observations, demonstrate that the large damping torque reported
recently is more likely attributed to interfacial magnetoelectric effect, while
spin Hall torque remains small even in the bulk-dominated regime.
