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Spin transport currents and the spin-transfer torques in voltage-biasedsuperconducting-ferromagnetic nanopillars (SFNFS point contacts) are computed.We develop and implement an algorithm based on the Ricatti formulation of thequasiclassical theory of superconductivity to solve the time-dependent boundaryconditions for the nonequilibrium Green's functions for spin transport throughthe ferromagnetic interfaces. A signature of the nonequilibrium torque is acomponent perpendicular to the plane spanned by the two ferromagnetic moments.The perpendicular component is absent in normal-metal-ferromagnetic nanopillars(NFNFN) contacts, but is shown to have the same order of magnitude as thein-plane torque for non-equilibrium SFNFS contacts. The out-of-plane torque isdue to the rotation of quasiparticle spin by the exchange fields of theferromagnetic layers. In the ballistic limit the equilibrium torque is relatedto the spectrum of spin-polarized Andreev bound states, while the {\sl ac}component, for small bias voltages, is determined by the nearly adiabaticdynamics of the Andreev bound states. The nonlinear voltage dependence of thenon-equilibrium torque, including the subharmonic gap structure and thehigh-voltage asymptotics, is attributed to the interplay between multipleAndreev reflections, spin filtering and spin mixing. These properties of spinangular momentum transport may be exploited to control the state ofnanomagnets.Comment: 15 pages, 14 figure

Theory of nonequilibrium spin transport and spin-transfer torque in superconducting-ferromagnetic nanostructures