  Femtosecond laser-pulse excitation provides an energy efficient and fast way
to control magnetization at the nanoscale, providing great potential for
ultrafast next-generation data manipulation and nonvolatile storage devices.
Ferromagnetic van der Waals materials have garnered much attention over the
past few years due to their low dimensionality, excellent magnetic properties,
and large response to external stimuli. Nonetheless, their behaviour upon fs
laser-pulse excitation remains largely unexplored. Here, we investigate the
ultrafast magnetization dynamics of a thin flake of Fe$_3$GeTe$_2$ (FGT) and
extract its intrinsic magnetic properties using a microscopic framework. We
find that our data is well described by our modelling, with FGT undergoing a
slow two-step demagnetization, and we experimentally extract the
spin-relaxation timescale as a function of temperature, magnetic field and
excitation fluence. Our observations indicate a large spin-flip probability in
agreement with a theoretically expected large spin-orbit coupling, as well as a
weak interlayer exchange coupling. The spin-flip probability is found to
increase when the magnetization is pulled away from its quantization axis,
opening doors to an external control over the spins in this material. Our
results provide a deeper understanding of the dynamics van der Waals materials
upon fs laser-pulse excitation, paving the way towards two-dimensional
materials-based ultrafast spintronics.
