Spin torque and waviness in magnetic multilayers: a bridge between Valet-Fert theory and quantum approaches

We develop a simple theoretical framework for transport in magnetic multilayers, based on the Landauer-Buttiker scattering formalism and random matrix theory. A simple transformation allows one to go from the scattering point of view to theories expressed in terms of local currents and the electromagnetic potential. In particular, our theory can be mapped onto the well-established classical Valet-Fert theory for collinear systems. For noncollinear systems, in the absence of spin-flip scattering, our theory can be mapped onto the generalized circuit theory. We apply our theory to the angular dependence of spin accumulation and spin torque in noncollinear spin valves.