Current-induced order parameter dynamics: Microscopic theory applied to Co ∕ Cu ∕ Co spin valves

Transport currents can alter order parameter dynamics and change steady states in superconductors, in ferromagnets, and in hybrid systems. In this paper, we present a scheme for fully microscopic evaluation of order parameter dynamics that is intended for application to nanoscale systems. The approach relies on time-dependent mean-field theory, on an adiabatic approximation, and on the use of nonequilibrium Green's function theory to calculate the influence of a bias voltage across a system on its steady-state density matrix. We apply this scheme to examine the spin-transfer torques which drive magnetization dynamics in CoCuCo spin-valve structures. Our microscopic torques are peaked near CoCu interfaces, in agreement with most previous pictures, but surprisingly act mainly on Co transition metal d orbitals rather than on s orbitals as generally supposed. © 2007 The American Physical Society.