Spin-Transfer Torque and Electron-Magnon Scattering

According to the spin-torque model, current-driven magnetic dynamics in ferromagnetic multilayers is determined by the transfer of electron spin perpendicular to the layers' magnetizations. By separating the largest contributions to the magnetic dynamics, we demonstrate that the dominant effect of spin-torque is rather due to the electron spin parallel to the field. We show that this effect can be equivalently described as stimulated current-driven excitation of spin-waves, and discuss four specifically quantum-mechanical aspects of spin-transfer, not described by the spin-torque.