Current-induced order parameter dynamics: Microscopic theory applied to CoCCo 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.