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

摘要

In the last two decades, ultrafast magnetization dynamics has been the subject of alarge number of experimental and theoretical investigations, however, without establishinga consistent picture up to now. The objective of the work presented in thisthesis is to provide a new and profound perspective on the underlying mechanismsby a direct observation of the spin-dynamics in the electronic band structure of 3dferromagnets using femtosecond time- and spin-resolved photoelectron spectroscopyof the full energy range of the valence bands.For this purpose, a novel experimental setup has been developed and commissionedthat combines a modern, highly effcient spin detector with a state-of-the-art, tabletoplight source creating ultrashort extreme ultraviolet (XUV) pulses by laser-basedhigh-order harmonic generation (HHG). The light source can be used in two operationmodes, which provide XUV radiation with different wavelengths and photonuxes. Static spin-resolved photoemission spectra of Co(001) films have been measuredin both operation modes to find the optimum conditions for time-resolved experiments.Moreover, vacuum space-charge (VSC) effects within the dense electronclouds emitted by the femtosecond XUV pulses as well as the Coulomb interactionbetween electron clouds generated by XUV probe- and near-infrared (NIR) pumppulsesin a pump-probe experiment have been investigated in detail. The findingsare used to determine and later minimize the inuence of VSC on photoemissionresults.The NIR pump-pulses have been employed in time-resolved measurements to triggerultrafast demagnetization in Co samples. Using the capabilities of the newexperiment, we monitored the time evolution of the electron-dynamics by measuringspin-resolved spectra over a broad energy range fully covering the valence bands.To our knowledge, such measurements have been performed for the first time. Theexperiments lead to novel insights into the evolution of the spin system during anultrafast demagnetization process, in particular by showing evidence of spin-mixinginstead of a quenching of the exchange splitting as suggested by the Stoner-model.