  This paper presents the boundary conditions needed for drift-diffusion models
to treat interfaces with spin-orbit coupling. Using these boundary conditions
for heavy metal/ferromagnet bilayers, solutions of the drift-diffusion
equations agree with solutions of the spin-dependent Boltzmann equation and
allow for a much simpler interpretation of the results. A key feature of these
boundary conditions is their ability to capture the role that in-plane electric
fields have on the generation of spin currents that flow perpendicularly to the
interface. The generation of these spin currents is a direct consequence of the
effect of interfacial spin-orbit coupling on interfacial scattering. In heavy
metal/ferromagnet bilayers, these spin currents provide an important mechanism
for the creation of damping-like and field-like torques; they also lead to
possible reinterpretations of experiments in which interfacial contributions to
spin torques are thought to be suppressed.
