  The topological insulator/ferromagnetic metal (TI/FMM) bilayer thin films
emerged as promising topological surface state-based spintronic devices, most
notably in their efficiency of current-induced spin torque. Using a cubic
lattice model, we reveal that the surface state Dirac cone of the TI can
gradually merge into or be highly intertwined with the FMM bulk bands, and the
surface states percolate into the FMM and eventually hybridize with the quantum
well states therein. The magnetization can distort the spin-momentum locking of
the surface states and yield an asymmetric band structure, which causes a
laminar flow of room temperature persistent charge current. Moreover, the
proximity to the FMM also promotes a persistent laminar spin current. Through a
linear response theory, we elaborate that both the surface state and the FMM
bulk bands contribute to the current-induced spin torque, and their real wave
functions render the spin torque predominantly field-like, with a magnitude
highly influenced by the degree of the percolation of the surface states. On
the other hand, impurities can change the spin polarization expected from the
Edelstein effect and generate a damping-like torque, and produce a torque even
when the magnetization points in-plane and orthogonal to the current direction.
