The project addressed the objectives by performing the experiments as envisioned in the work packages of the proposal and in addition, performing related measurements that complement the knowledge in cases where this was more relevant as the project progressed.
Namely, these measurements have been performed as envisioned in the work packages:
* Measurements of wavelength scaling of magnetization dynamics of the cobalt (transient metal material) sample. The measurements were performed in part by the applicant in collaboration at INRS-EMT in Canada.
* Implementation of a unique beamline for the small angle magnetic scattering at rare-earth composite materials harnessing the ytterbium laser technology developed at TU Vienna.
* In collaboration with ETH Zürich, a soft X-ray transient absorption beamline was implemented at the host institution of UniGE paving the way for studying dynamics in nano-magnets and their interaction with the environment.
One of the goals of the project is to study wavelength dependence of demagnetization dynamics in transition-metal samples. Up until now the vast majority of femtosecond magnetization studies have been performed using laser pulses in the NIR range, typically derived directly from a Ti:Sapphire laser amplifer centered at 800 nm. In our experiment the magnetization quenching was induced with pump pulses of three different wavelengths: 400 nm, 800 nm, and 1800 nm. This unprecedently large range of pump pulse wavelengths spanning from UV to mid-IR ensure high measurement precision and capture the underlying processes. We found that longer wavelength light quenches the demagnetization more efficiently in ferromagnets due to the higher electronic temperature and the wavelength scaling of the ponderomotive electron energy.
The other goal of the project is to truly exploit the element-sensitivity of the X-ray probing for studying composite-element materials. Time-resolved X-Ray techniques provide insights into spin changes with elemental-specificity on their natural spatial and time scales. The broad spectral bandwidth of the HHG spans the characteristic M- and N- absorption edges of the transition metal (TM) and rare-earth (RE) elements that exhibit magneto-optical activity. Despite the numerous proof-of-concept demonstrations using HHG sources, all of the previous studies were focused of the TM ferromagnets. The recent developed magnetic nano-structure materials are based on rare earth (RE) associate ferrimagnetic/multiferroics multicomponent systems, which are promising candidates for the future storage and information processing spintronic devises, owing to their nanometer size, fast speed, efficient driving dynamics, and topological protected stability. We demonstrate, for the first time, time-resolved X-ray resonant magnetic scattering (tr-XRMS) measurements on the inner-shell 4f-electron from a prototypical rare-earth (RE) composite ferrimagnetic system CoTb (Tb N-edge at 155eV), by using a bright HHG source. The tr-XRMS is enabled by direct driving HHG process with a power-scalable, high-energy Yb laser technology. The fully phase-matched broadband plateau (>200 eV) of the helium HHG with a record photon flux completely covers the desired resonant energy range of RE’s N-edges with <20 fs temporal duration.
Furthermore, project accomplished also a broader goal of utilizing the X-ray measurement techniques based on table-top laser-based sources for studying ultrafast processes in materials. We have demonstrated the first X-ray transient absorption (XAS) measurement in the water-window spectral range (284 - 538 eV) of a solution sample using a table-top HHG source by integrating it with a sub–micrometer–thin flat liquid jet. These measurements represent the first extension of table-top XAS to the oxygen edge of a chemical sample in the liquid phase. Using a soft X-ray beamline implemented at UniGE, we were able to temporally resolve the transient absorption features at the carbon K-edge of ethanol and methanol during strong–field ionization, which trace the valence-shell ionization dynamics of the liquid alcohols with a temporal resolution of ∼30 fs. These liquid-phase measurements will allow implementing studying magnetization dynamics in the magnetic nanoparticles and their interaction with the liquids in the future.
