Spin-polarized image-potential-state electrons as ultrafast magnetic sensors in front of ferromagnetic surfaces

摘要

We report on a spin-, time-, angle- and energy-resolved two-photon photoemission experiment with unprecedented resolution and adequate sensitivity, which allows us to study spin-dependent electron dynamics. Image-potential-state electrons on iron and cobalt thin films serve as well-defined model systems. The observed exchange splitting of these states reflects the exchange-split boundaries of the bulk-band gap. The temperature dependence of the spin polarization demonstrates that image-potential states are true sensors of the near surface magnetization. We have gained insight into quasielastic, i.e. resonant intra- and interband scattering processes and their inelastic counterparts. Lifetimes of minority and majority image-potential states differ primarily due to the spin-dependent density of states. In the minority channel of iron thin films quasi-elastic scattering processes become significant and are interpreted in terms of interband scattering between spin-up and spin-down image-potential-state bands. The latter process involves a spin flip on a sub-hundred femtosecond timescale and hints at quasielastic electron-magnon scattering.