Multi-Scale Study of All-Optically Induced Magnetization Dynamics in Co/Pt Multilayers utilizing TR-mSAXS at FEL Sources

Abstract

This thesis deals with the impact of ultrashort near-infrared (nIR) and extreme ultraviolet (XUV) laser pulses on the magnetic multi-domain states of particularly thin Co/Pt multilayers. The laser induced magnetization dynamics are investigated with femtosecond time and nanometer spatial resolution utilizing time-resolved magnetic small-angle X-ray scattering (TR-mSAXS) at the free-electron lasers (FEL) FLASH in Hamburg and FERMI@Elettra in Trieste. One part of the thesis deals with ultrafast demagnetization in three different Co/Pt-multilayer samples with total film thicknesses in the range of the attenuation length of nIR radiation in Co and Pt. For excitation of the magnetic states, nIR-laser pulses of different fluence, pulse duration and polarization are used, addressing important aspects of ultrafast demagnetization in such optically thin Co/Pt multilayers, for the first time, by resonant magnetic scattering. In particular, a model that accounts for both the low-temperature behavior of the remagnetization dynamics and its drastic slowing down at high temperatures is proposed, taking into account recent theoretical predictions. Within this model, the remagnetization dynamics are described via energy exchange between a strongly coupled electron–spin system and the phonon system. Another part of the thesis addresses the influence of nIR and XUV-laser (FEL) pulses on the lateral configuration of nanoscopic multi-domain states. Different multi-domain states are generated in a selected Co/Pt multilayer by using out-of-plane (OOP) magnetic fields. Aside from ultrafast demagnetization, that behaves similar for the different multidomain states, permanent lateral domain modifications forming on longer time scales are observed, that do depend on the underlying multi-domain state. Moreover, the permanent modifications only occur if nIR and XUV-laser pulses temporally overlap. Since the action of the combined (pump/probe) peak intensity alone cannot explain the observed effects, it is concluded that the permanent modifications also depend on the photon energies of the laser pulses. In particular, the permanent modifications in the close-to single-domain state point at laser induced nucleation processes and thus a novel all-optical switching (AOS) like mechanism that is based on the interplay of two different laser excitations.