SPP 1133: Ultrafast magnetization processes - WP: Spin-polarized image-potential-state electrons as ultrafast magnetic sensors in front of ferromagnetic surfaces (3rd funding period)

Basic data for this project

Type of project: Subproject in DFG-joint project hosted outside University of Münster
Duration: 01/07/2006 - 30/06/2008 | 3rd Funding period

Description

Ultrafast electron dynamics at surfaces and in thin films can be unravelled by femtosecond laser experiments probing unoccupied electronic states in two-photon photoemission. On ferromagnetic surfaces, dynamics of photoexcited electrons include Stoner and electron-magnon excitations as well as phonon and defect scattering. The latter may provide direct channels for transfer of spin to orbital momentum. In case the processes occur on a femtosecond timescale and involve a spin flip, they are ingredients for an elementary understanding of ultrafast magnetization processes. In our spin-, time-, angle- and energy-resolved two-photon photoemission experiment, funded by the priority program, we have achieved an unprecedented resolution and 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. Measurements below and above the Curie point and the corresponding temperature dependence of exchange splitting, spin polarization, and linewidth demonstrate 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 quasielastic 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 femtosecond timescale and hints to quasielastic electron-magnon or spin-flip defect scattering. The spin-dependent lifetimes of optically excited valence-band electrons in iron and cobalt thin films have been studied. For cobalt we find a long-living component of majority electrons which is related to an occupied surface resonance with minority-spin character. These results suggest that exchange scattering dominates electron-electron interaction and adapts the measured lifetimes of hot electrons in 3d ferromagnets. In a collaboration with the groups of U. Bovensiepen / M. Wolf and H. Dürr / W. Eberhardt we have studied magnetization and spin dynamics in Gadolinium thin films. We have corroborated picosecond Gd 4f spin - lattice relaxation and established ultrafast magnetization dynamics and coupling of Gd valence-band electrons and 4f magnetic momenta.

Keywords: surface science; surface ferromagnetism; spin-resolved photoelectron spectroscopy; femtosecond lasers