Spin-transfer torque and magnetoresistance in superconducting spin valves

We study the spin-transfer torque and magnetoresistance of a ferromagnet vertical bar superconductor vertical bar ferromagnet spin valve, allowing for an arbitrary magnetization misorientation and treating both s-wave and d-wave symmetries of the superconductor. We take fully into account Andreev reflection and also the spin-triplet correlations that are generated when the magnetizations are noncollinear. It is found that the torque and magnetoresistance are both strongly enhanced when topological zero-energy states are present at the interfaces, which is the case for d-wave superconductors with a crystallographic orientation of [110] relative to the interface (d(xy)-wave symmetry). Moreover, we find that the magnetoresistance displays a strong oscillatory and nonmonotonous behavior as a function of d(S)/xi where d(S) and xi are the interlayer width of the superconducting region and the superconducting coherence length, respectively. This feature is also attributed to the crossover from layers of size d(S)similar to 2 xi to layers of size d(S)2 xi, where the contribution to transport from zero-energy states gradually vanishes.