Spin transport at interfaces with spin-orbit coupling

Spintronic devices typically contain multiple layers and utilize spin-orbit coupling through a variety of effects, such as anisotropy, damping, or novel transport processes like the spin Hall effect.~These devices are most simply analyzed when spin-orbit coupling plays a dominant role in the bulk layers rather than at the interfaces.~However, recent observations of novel phenomena suggest the importance of strong spin-orbit coupling at interfaces.~These phenomena can be desirable (e.g. perpendicular magnetic anisotropy, spin-to-charge conversion, topologically-protected magnetization textures) or parasitic (e.g. spin memory loss).~While the precise role of interfacial spin-orbit coupling on transport remains unclear, substantial evidence indicates that it can no longer be ignored.~We discuss the theory of spin transport at interfaces with spin-orbit coupling, focusing on phenomenological models and highlighting new effects.~In particular, we show that interfaces with spin-orbit coupling can generate spin currents with spin polarizations in unconventional directions (i.e not orthogonal to both the charge current and spin flow).~These spin currents are a direct consequence of interfacial spin-orbit coupling and could assist in switching magnetic layers with perpendicular magnetic anisotropy.~We present the boundary conditions needed for drift-diffusion models to capture interfacial spin-orbit effects, and discuss the interpretation of experiments in which interfacial spin-orbit coupling might play a significant role.