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European XFEL Science Seminar

Ultrafast magnetization dynamics probed with HHG time-resolved magneto-optics and spin-resolved ARPES

by Stefan Mathias (University of Göttingen)

E1.173 (European XFEL building)


European XFEL building

Ultrashort extreme-ultraviolet pulses from high-harmonic generation (HHG) provide a powerful tool for novel experiments in the area of ultrafast materials science. The short-wavelength nature of these sources provide important information related to all electronic, magnetic, structural, and chemical properties of a solid. In our experiments, we use HHG-based magneto-optical and photoemission experiments to probe ultrafast carrier dynamics and photo-induced phase transitions in materials. Using element-specific HHG magneto-optical techniques, we elucidate the role of photo-induced ultrafast spin currents in magnetic multilayer stacks and alloys, between the layers [1,2], and also between the different elemental subsystems as proposed by Sharma et al. [Dewhurst et al., Nano Letters 18, 1842 (2018)]. With the help of a HHG time-, spin-, and angle-resolved photoemission spectroscopy (spin-resolved trARPES), in the case of Co, we map the optically-induced transitions and the connected spin-dependent band structure response on femtosecond timescales [3]. Here, we observe two distinct processes at work. At energies near the Fermi-level, the spin dynamics are predominantly driven by a redistribution of spin-polarized carriers. At higher binding energies >1 eV, quenching of the spin polarization exhibits transient band dynamics that can be unambiguously traced back to rapid band mirroring of the electronic states. Because band-mirroring is a strong indication of spin fluctuations, we conclude that optically driven femtosecond spin currents induce collective spin excitations on extremely fast timescales [1] Turgut et al., PRL 110, 107201 (2013) [2] Hofherr et al., PRB 96, 100403(R), (2017) [3] Eich et al., Science Adv. 3, e1602094 (2017)