  Spin-orbit torques offer a promising mechanism for electrically controlling
magnetization dynamics in nanoscale heterostructures. While spin-orbit torques
occur predominately at interfaces, the physical mechanisms underlying these
torques can originate in both the bulk layers and at interfaces. Classifying
spin-orbit torques based on the region that they originate in provides clues as
to how to optimize the effect. While most bulk spin-orbit torque contributions
are well studied, many of the interfacial contributions allowed by symmetry
have yet to be fully explored theoretically and experimentally. To facilitate
progress, we review interfacial spin-orbit torques from a semiclassical
viewpoint and relate these contributions to recent experimental results. Within
the same model, we show the relationship between different interface transport
parameters. For charges and spins flowing perpendicular to the interface,
interfacial spin-orbit coupling both modifies the mixing conductance of
magnetoelectronic circuit theory and gives rise to spin memory loss. For
in-plane electric fields, interfacial spin-orbit coupling gives rise to torques
described by spin-orbit filtering, spin swapping and precession. In addition,
these same interfacial processes generate spin currents that flow into the
non-magnetic layer. For in-plane electric fields in trilayer structures, the
spin currents generated at the interface between one ferromagnetic layer and
the non-magnetic spacer layer can propagate through the non-magnetic layer to
produce novel torques on the other ferromagnetic layer.
