Inelastic electron-magnon interaction and spin transfer torque

A combined experimental and theoretical study on the inelastic transfer of spin momentum between a spin-polarized tunneling current and a ferromagnetic electrode is presented. Using inelastic tunneling spectroscopy across a vacuum gap at 4 K we show that high-energy magnons are efficiently excited in inelastic-scattering events and that the asymmetry of magnon excitation for tunneling into and out of the ferromagnet is proportional to the spin polarization of the tunneling current. We discuss the size of the resulting spin torque and explain the energy distribution of the excited magnons on basis of spin scattering mediated by the itinerant exchange interaction.