  In bilayer nanowires consisting of a ferromagnetic layer and a non-magnetic
layer with strong spin-orbit coupling, currents create torques on the
magnetization beyond those found in simple ferromagnetic nanowires. The
resulting magnetic dynamics appear to require torques that can be separated
into two terms, damping-like and field-like. The damping-like torque is
typically derived from models describing the bulk spin Hall effect and the spin
transfer torque, and the field-like torque is typically derived from a Rashba
model describing interfacial spin-orbit coupling. We derive a model based on
the Boltzmann equation that unifies these approaches. We also consider an
approximation to the Boltzmann equation, the drift-diffusion model, that
qualitatively reproduces the behavior, but quantitatively fails to reproduce
the results. We show that the Boltzmann equation with physically reasonable
parameters can match the torques for any particular sample, but in some cases,
it fails to describe the experimentally observed thickness dependences.
