Seminarium: Femtosecond time-resolved and element-specific x-ray absorption spectroscopy of Fe/MgO

  • Datum:
  • Plats: Ångströmlaboratoriet Å80101
  • Föreläsare: Prof. Heiko Wende of University of Duisburg-Essen, Germany
  • Kontaktperson: Biplab Sanyal
  • Föreläsning

A localized optical excitation of a metal/insulator heterostructure induces ultrafast dynamics through the interface in its individual compounds, which can involve charge and spin transfer processes as well as coupling to low energy excitations mediated by e.g. electron-electron and electron-phonon scattering. Femtosecond soft x-ray spectroscopy allows to separate and identify these electronic and lattice excitations directly in the time domain and, furthermore, is sensitive to the dynamics of the individual constituents itself due to its element-specific character. We have measured time- and element-resolved x-ray absorption spectroscopy of a [2 nm Fe/2 nm MgO]8 multilayer at the Fe L3- and O K-edge with a time resolution of 150 fs. After a local optical excitation of Fe with a UV laser pulse of 266 nm wavelength we see a clear pump-induced effect at both edges in fs time resolution. The Fe L3-edge shows an ultrafast 0.5 % dropdown of the signal in 240 fs, followed by an almost full recovery until 1.3 ps, while the O K-edge signal reaches its maximum only after 1 ps. At later delays the change at the Fe L3-edge reappears, but with a slower time constant. Furthermore, we measured the transient changes at the O K-pre-edge region. Comparison of the experimental XAS spectra with layer-resolved first principles calculations in the framework of density functional theory reveal that this pre-edge feature results from interface states near the Fermi level in MgO. Here, we observe an even slower pump-induced change than the ones at the both edges. The complex response at the Fe L3-edge represents local electronic excitations driven by the 266 nm pump, coupling to phonons in Fe, and their relaxation through coupling to the insulator constituent. The dynamics at the O K-edge is clearly faster than the lattice relaxation in Fe, mentioned above. We currently consider e-ph coupling at the interface to be responsible for such this ultrafast process. The slower response at the O pre-edge occurs on a similar time scale as lattice energy transfer of Fe to MgO and could be related to the specific interface nature of the probed state.

We acknowledge the financial support by the Deutsche Forschungsgemeinschaft (DFG) through the Collaborative Research Center 1242 (SFB 1242) for the subprojects A05, C01, C02.