PhD Thesis Defenses

Thesis defense: Ultrafast spin dynamics at the nanoscale using coherent X-ray and terahertz radiation

The field of ultrafast magnetism is driven by the growing need for faster and more efficient magnetic data storage, which comprises the vast majority of the digital information worldwide. However, after more than two decades of intense research, the understanding of the fundamental physical processes governing the transfer of angular momentum necessary for magnetic switching, is still lacking, partially hampered by the appropriate experimental tools. This situation is rapidly changing with the advent of X-ray free electron lasers (XFEL), which combine high temporal and spatial resolutions, necessary for a complete view of the physics at play.

In the first work presented in this thesis, we demonstrate the capabilities of the recently built Spectroscopy and Coherent Scattering (SCS) instrument at the European XFEL. We perform ultrafast time-resolved small angle X-ray scattering (SAXS) on nanometre magnetic domains, combining ultrafast temporal resolution with high spatial resolution. We also demonstrate X-ray holographic reconstruction of similar magnetic domains. Our results show that the efficient data acquisition for holographic imaging is possible thanks to the MHz-operation of the European XFEL, paving the way for new studies and ultimately to create femtosecond movies of magnetism at the nanoscale.

In the second work of this thesis, we describe a subsequent experiment at the SCS instrument, where we focus on the impact of symmetry breaking on the ultrafast dynamics of magnetic domains by looking at the diffracted SAXS data. Surprisingly, we observe a different ultrafast response depending on the anisotropy of the domains. We observe a clear contraction of the isotropic scattering ring in the reciprocal wavevector space (characteristic of randomly oriented domains), while no such contraction is observed in the anisotropic scattering pattern (distinctive of stripe-ordered domains). While the fundamental physical reason for the occurrence of the shift in wavevector space remains unexplained, we find that they correlate well with the domain symmetry. Our observation underlines the importance of symmetry as a critical variable for far-from-equilibrium dynamics.

Finally, in the last work of the thesis, we look at the possibility of triggering ultrafast spin dynamics using intense THz magnetic field pulses. Typically, ultrafast spin dynamics is triggered using femtosecond lasers in the visible range. While readily available, these pulses cause highly non-equilibrium processes to take place because of the excitation energies in the eV range, comparable to the width of a typical electronic band. The potential excitation of all possible states within a band makes it difficult to disentangle which are the fundamental physical processes responsible for ultrafast demagnetization. On the other hand, radiation in the THz frequency range (meV energy range) directly couples to the magnetization without the risk of masking key processes. However, intense THz radiation is not easily generated because the relatively long wavelengths hamper the focusing capabilities due to the diffraction limit. To address this issue, we propose a metamaterial structure that enhances the THz magnetic field component of a free-space coupled THz field by more than one order of magnitude and exceeding the 1 T value. A table-top ultrafast time-resolved Faraday microscope setup with sub- micrometer resolution was built in order to investigate this experimentally.