Ultrafast heterogeneous magnetization dynamics in granular FePt media

This thesis presents research on the microscopic mechanisms behind ultrafast demagnetization dynamics. In particular it highlights the importance of the heterogeneity of magnetic materials and its influence on optical excitation and magnetization processes.
MOKE laser systems, x-ray diffraction at synchrotrons and pump-probe x-ray diffraction at a free-electron laser were used to study this topic. The utilized measurement techniques, methods and light sources are described. This thesis investigated granular FePt in the L1₀ phase. This material has extraordinary magnetic properties and is a key material for future generations of magnetic storage devices. The growth process of specifically for this experiment designed samples is described and the samples structural and magnetic properties are characterized.
For data interpretation, a diffraction model specifically designed for granular samples is derived. Furthermore, a method to model the magnetization distribution within single nano particles is demonstrated. The thesis shows that the energy absorbed per nanoparticle from the pump pulse strongly depends on the interaction with its surrounding nanoparticles. Illumination with fs laser pulses leads to heat-assisted magnetic switching of the majority of the FePt nanoparticles but leaves a significant fraction of nanoparticles unreversed. Following heterogeneous excitation, we observed a heterogeneous demagnetization process. We find that the magnetization dynamics differ between the core region of the nanoparticle and shell region at and near the surface.
Revealing these heterogeneous effects contributes to a better understanding of ultrafast magnetization dynamics. Furthermore, being able to utilize these effects will be of importance for achieving controllable spin dynamics in the nanometer-femtosecond domain.