  The magnetoresistance of a three-dimensional Rashba material placed on top of
a ferromagnetic insulator is theoretically investigated. In addition to the
intrinsic Rashba spin-orbit interaction, we also consider extrinsic spin-orbit
coupling via side-jump and skew scattering, and the Elliott-Yafet spin
relaxation mechanism. The latter is anisotropic due to the mass anisotropy
which reflects the noncentrosymmetric crystal structure of three-dimensional
Rashba metals. A quasiclassical approach is employed to derive a set of coupled
spin-diffusion equations, which are supplemented by boundary conditions that
account for the spin-transfer torque at the interface of the bilayer. The
magnetoresistance is fully determined by the current-induced spin polarization,
i.e., it cannot in general be ascribed to a single (bulk) spin Hall angle. Our
theoretical results reproduce several features of the experiments, at least
qualitatively, and contain established phenomenological results in the relevant
limiting cases. In particular, the anisotropy of the Elliott-Yafet spin
relaxation mechanism plays a major role for the interpretation of the observed
magnetoresistance.
