Ultrafast Spindynamics in Ferromagnetic Thin Films

Ultrafast magnetization dynamics describe the physics of a magnetic system on the perturbation by a strong laser pulse. This pulse excites the electrons close to the Fermi energy. It is followed by a thermalization due to a variety of scattering processes. The plenitude of these processes is of fundamental interest for solid states physics, but isnot yet understood properly. In the field of ultrafast magnetization dynamics most progress was achieved by all optical measurements. However, the interpretation of all optical experiments is still under debate. In order to gain direct insight into the time evolution of the spin polarization a time and spin-resolved pump-probe experiment is applied. This allows one to measure the quenching of the spin polarization without any optical contributions. In our experiment the spin polarization of the photoexcited electrons of in situ prepared thin Fe and Co films is measured directly with a Mott-detector. The overall signal shape matches the one of the all optical measurements. Thus, with the help of our measurements it could thus be shown, that the all optical techniques indeed contain a contribution of magnetic origin.