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- ItemUltrafast Demagnetization Dominates Fluence Dependence of Magnetic Scattering at Co M Edges(College Park, Md. : APS, 2020) Schneider, Michael; Pfau, Bastian; Günther, Christian M.; von Korff Schmising, Clemens; Weder, David; Geilhufe, Jan; Perron, Jonathan; Capotondi, Flavio; Pedersoli, Emanuele; Manfredda, Michele; Hennecke, Martin; Vodungbo, Boris; Lüning, Jan; Eisebitt, StefanWe systematically study the fluence dependence of the resonant scattering cross-section from magnetic domains in Co/Pd-based multilayers. Samples are probed with single extreme ultraviolet (XUV) pulses of femtosecond duration tuned to the Co M3,2 absorption resonances using the FERMI@Elettra free-electron laser. We report quantitative data over 3 orders of magnitude in fluence, covering 16 mJ/cm2/pulse to 10 000 mJ/cm2/pulse with pulse lengths of 70 fs and 120 fs. A progressive quenching of the diffraction cross-section with fluence is observed. Compression of the same pulse energy into a shorter pulse—implying an increased XUV peak electric field—results in a reduced quenching of the resonant diffraction at the Co M3,2 edge. We conclude that the quenching effect observed for resonant scattering involving the short-lived Co 3p core vacancies is noncoherent in nature. This finding is in contrast to previous reports investigating resonant scattering involving the longer-lived Co 2p states, where stimulated emission has been found to be important. A phenomenological model based on XUV-induced ultrafast demagnetization is able to reproduce our entire set of experimental data and is found to be consistent with independent magneto-optical measurements of the demagnetization dynamics on the same samples.
- ItemUltrafast Optically Induced Ferromagnetic State in an Elemental Antiferromagnet(College Park, Md. : APS, 2021) Golias, E.; Kumberg, I.; Gelen, I.; Thakur, S.; Gördes, J.; Hosseinifar, R.; Guillet, Q.; Dewhurst, J.K.; Sharma, S.; Schüßler-Langeheine, C.; Pontius, N.; Kuch, W.We present evidence for an ultrafast optically induced ferromagnetic alignment of antiferromagnetic Mn in Co/Mn multilayers. We observe the transient ferromagnetic signal at the arrival of the pump pulse at the Mn L3 resonance using x-ray magnetic circular dichroism in reflectivity. The timescale of the effect is comparable to the duration of the excitation and occurs before the magnetization in Co is quenched. Theoretical calculations point to the imbalanced population of Mn unoccupied states caused by the Co interface for the emergence of this transient ferromagnetic state.
- ItemEvolution of a molecular shape resonance along a stretching chemical bond(College Park, Md. : APS, 2020) Brausse, Felix; Bach, Florian; Krečinić, Faruk; Vrakking, Marc J.J.; Rouzée, ArnaudWe report experiments on laser-assisted electron recollisions that result from strong-field ionization of photoexcited I2 molecules in the regime of low-energy electron scattering (<25 eV impact energy). By comparing differential scattering cross sections extracted from the angle-resolved photoelectron spectra to differential scattering cross sections from quantum-scattering calculations, we demonstrate that the electron-scattering dynamics is dominated by a shape resonance. When the molecular bond stretches during the evolution of a vibrational wave packet this shape resonance shifts to lower energies, both in experiment and theory. We explain this behavior by the nature of the resonance wave function, which closely resembles an antibonding molecular orbital of I2.