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Since its discovery, great work has been done to uncover the nature of the ultrafast demagnetization process. However, the key question of how the angular momentum is transferred away from the spin system remains unanswered. This thesis advances a small piece of the puzzle by uncovering ultrafast phenomena in magnetic FePt nanoparticles.

This work uses ultrafast electron diffraction to demonstrate that energy is transferred from the electronic system to the two atomic sub-lattices inhomogeneously. Further investigation proves a preferred transfer of energy to high-energy modes in the Brillouin zone boundary. To this date, this is the first ultrafast pump-probe study that decouples the atomic motion of different elemental species inside a crystal. This opens the door for new avenues of investigation for diatomic materials by taking advantage of all the available reciprocal space in a diffraction experiment.

A complementary view on the magnetization dynamics from experiments in free electron laser sources shows the emergence of a magnetic soliton generated after completely quenching the magnetization in FePt nanoparticles. This magnetic soliton is exceptionally small, under 10 nm, and has a high frequency near the THz range. This discovery makes it a potential starting point for developing new devices for information processing technology. 

In addition, the magnetization of the ground state of FePt nanoparticles was imaged using coherent diffraction imaging along with circularly polarized X-rays. This experiment opens the path to new methods for probing the magnetization within nanoparticles, potentially allowing for a better understanding of the internal fields that govern the magnetization dynamics.