Laser-Induced Ultrafast Electron- and Spin-Dynamics in the Electronic Band Structure of Co(001)

In the last two decades, ultrafast magnetization dynamics has been the subject of a large number of experimental and theoretical investigations, however, without establishing a consistent picture up to now. The objective of the work presented in this thesis is to provide a new and profound perspective on the underlying mechanisms by a direct observation of the spin-dynamics in the electronic band structure of 3d ferromagnets using femtosecond time- and spin-resolved photoelectron spectroscopy of the full energy range of the valence bands. For this purpose, a novel experimental setup has been developed and commissioned that combines a modern, highly ecient spin detector with a state-of-the-art, tabletop light source creating ultrashort extreme ultraviolet (XUV) pulses by laser-based high-order harmonic generation (HHG). The light source can be used in two operation modes, which provide XUV radiation with dierent wavelengths and photon
uxes. Static spin-resolved photoemission spectra of Co(001) lms have been measured in both operation modes to nd the optimum conditions for time-resolved experiments. Moreover, vacuum space-charge (VSC) eects within the dense electron clouds emitted by the femtosecond XUV pulses as well as the Coulomb interaction between electron clouds generated by XUV probe- and near-infrared (NIR) pumppulses in a pump-probe experiment have been investigated in detail. The ndings are used to determine and later minimize the in uence of VSC on photoemission results. The NIR pump-pulses have been employed in time-resolved measurements to trigger ultrafast demagnetization in Co samples. Using the capabilities of the new experiment, we monitored the time evolution of the electron-dynamics by measuring spin-resolved spectra over a broad energy range fully covering the valence bands. To our knowledge, such measurements have been performed for the rst time. The experiments lead to novel insights into the evolution of the spin system during an ultrafast demagnetization process, in particular by showing evidence of spin-mixing instead of a quenching of the exchange splitting as suggested by the Stoner-model.