 CIP thickness-averaged two-current model

In a model of carriers sampling alternating magnetic and nonmagnetic layers, assume spin conservation, comparable carrier transport weights and negligible extra interface scattering. Averaging relaxation rates by the time spent in each material then gives a channel electrical resistivity $(t_1{\rho }_{\mathrm{FM},\mathrm{s}}+t_2{\rho }_{\mathrm{NM}})/(t_1+t_2)$. Here all constituent resistivities are per spin channel. A long mean free path compared with a period motivates interlayer sampling; it does not by itself prove the other assumptions. This is a homogenized current-in-plane model, rather than an independent parallel-layer limit or a current-perpendicular-to-plane resistor chain.