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Transient magnetic gratings on the nanometer scale

Weder, David; Korff Schmising, Clemens von; Günther, Christian Michael; Schneider, Michael; Engel, Dieter; Hessing, P.; Struber, Christian; Weigand, M.; Vodungbo, B.; Jan, Emmanuelle; Liu, X.; Merhe, Alaa el dine; Pedersoli, Emanuele; Capotondi, Flavio; Lüning, J.; Pfau, Bastian; Eisebitt, Stefan

FG Röntgenoptik und Nanometer-Optik

Laser-driven non-local electron dynamics in ultrathin magnetic samples on a sub-10 nm length scale is a key process in ultrafast magnetism. However, the experimental access has been challenging due to the nanoscopic and femtosecond nature of such transport processes. Here, we present a scattering-based experiment relying on a laser-induced electro- and magneto-optical grating in a Co/Pd ferromagnetic multilayer as a new technique to investigate non-local magnetization dynamics on nanometer length and femtosecond timescales. We induce a spatially modulated excitation pattern using tailored Al near-field masks with varying periodicities on a nanometer length scale and measure the first four diffraction orders in an x-ray scattering experiment with magnetic circular dichroism contrast at the free-electron laser facility FERMI, Trieste. The design of the periodic excitation mask leads to a strongly enhanced and characteristic transient scattering response allowing for sub-wavelength in-plane sensitivity for magnetic structures. In conjunction with scattering simulations, the experiment allows us to infer that a potential ultrafast lateral expansion of the initially excited regions of the magnetic film mediated by hot-electron transport and spin transport remains confined to below three nanometers.