  Using a semiclassical approach that simultaneously incorporates the spin Hall
effect (SHE), spin diffusion, quantum well states, and interface spin-orbit
coupling (SOC), we address the interplay of these mechanisms as the origin of
the in-plane current induced spin torque observed in the normal
metal/ferromagnetic metal bilayer thin films. Focusing on the bilayers with a
ferromagnet much thinner than its spin diffusion length, such as Pt/Co with
$\sim 10$nm thickness, our approach addresses simultaneously the two
contributions to the spin torque, namely the spin-transfer torque (SHE-STT) due
to SHE induced spin injection, and the spin-orbit torque (SOT) due to SOC
induced spin accumulation. The SOC produces an effective magnetic field at the
interface, hence it modifies the angular momentum conservation expected for the
SHE-STT. The SHE induced spin voltage and the interface spin current are
mutually dependent, hence are solved in a self-consistent manner. In addition,
the spin transport mediated by the quantum well states may be responsible for
the experimentally observed rapid variation of the spin torque with respect to
the thickness of the ferromagnet.
