Ultrafast spin, lattice, and electron dynamics in thin metallic films : in the terahertz, near-infrared, and extreme ultraviolet regimes

Abstract: In this thesis, we studied the ultrafast spin and electron dynamics triggered by electromagnetic radiation belonging to three different regions of the electromagnetic spectrum, namely terahertz, near-infrared and extreme ultraviolet. By performing pump–probe measurements, we explored the role of the lattice in ultrafast spin and electron dynamics in metallic thin films.Using femtosecond pulses in the near-infrared range, we investigated the role of the magneto-crystalline anisotropy in the ultrafast spin dynamics in epitaxial hcp-cobalt thin film. We observed an average 33 % slower dynamics along the easy magnetization axis compared to the hard axis one, which we could attribute to the magneto-crystalline anisotropy of the electron–phonon coupling, supported by ab initio calculations.Intense terahertz magnetic fields (of the order of 0.3 T, corresponding to 1 MV/cm electric fields) were implemented to trigger ultrafast spin dynamics in epitaxial cobalt films with different crystalline phase and magneto-crystalline anisotropy. We detected the appearance of nutation resonances and of a lagged magnetization response which we could describe with the formalism of magnetic inertia. We also observed a correlation between the strength of the magneto-crystalline anisotropy and the characteristic nutation frequency for each sample.Extreme ultraviolet (XUV) pulses were used to study ultrafast magnetization dynamics at nanometer length scales in a CoGd alloy via transient grating experiments, with the XUV radiation used to both pump and probe the material. We observed an ultrafast demagnetization response in the first 50 fs following the pump excitation, followed by a fast recovery within 500 fs, and subsequent slow recovery on the tens of picoseconds scale, which depended on the transient grating period. This work demonstrated the possibility of realizing transient magnetization gratings at the nanoscale, which will allow to study magnetism and its coupling to the lattice thermal bath combining ultrafast and nanoscale information.Finally, we investigated the electron and lattice dynamics of platinum and gold thin films looking at their transient reflectivity upon excitation with terahertz and near-infrared radiation. Platinum showed a qualitatively similar reflectivity loss and recovery at both wavelengths, which we could describe using a standard two-temperature model approach. On the other hand, for thin gold films which also showed the expected transient reflectivity behavior at the near-infrared pump wavelength, the terahertz-induced dynamics showed a much smaller reflectivity increase, which we could attribute to the field emission of electrons via Fowler-Nordheim tunneling.

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