Marotzke, S.; Gupta, D.; Wang, R.P.; Pavelka, M.; Dziarzhytski, S.; von Korff Schmising, C.; Jana, S.; Thielemann-Kühn, N.; Amrhein, T.; Weinelt, M.; Vaskivskyi, I.; Knut, R.; Engel, D.; Braune, M.; Ilchen, M.; Savio, S.; Otto, T.; Tiedtke, K.; Scheppe, V.; Rönsch-Schulenberg, J.; Schneidmiller, E.; Schüßler-Langeheine, C.; Dürr, H.A.; Beye, M.; Brenner, G.; Pontius, N.: First experiments with ultrashort, circularly polarized soft x-ray pulses at FLASH2. Structural Dynamics 12 (2025), p. 034301/1-11
10.1063/4.0000298
Open Access Version
Abstract:
Time-resolved absorption spectroscopy and magnetic circular dichroism with circularly polarized soft x-rays (XAS and XMCD) are powerful tools to probe electronic and magnetic dynamics in magnetic materials element- and site-selectively. By employing these methods, groundbreaking results have been obtained, for instance, for magnetic alloys, which helped to fundamentally advance the field of ultrafast magnetization dynamics. At the free-electron laser facility FLASH, key capabilities for ultrafast XAS and XMCD experiments have recently improved. In an upgrade, an APPLE-III helical afterburner undulator was installed at FLASH2 in September 2023. This installation allows for the generation of circularly polarized soft x-ray pulses with a duration of a few tens of femtoseconds covering the L-3,L-2-edges of the important 3d transition metal elements with pulse energies of several mu J. Here, we present first experimental results with such ultrashort x-ray pulses from the FL23 beamline employing XMCD at the L-3,L-2-edges of the 3d metals, Co, Fe, and Ni. We obtain significant dichroic difference signals indicating a degree of circular polarization close to 100%. With the pulse-length preserving monochromator at beamline FL23 and an improved pump-laser setup, FLASH can offer important and efficient experimental instrumentation for ultrafast demagnetization studies and other investigations of ultrafast spin dynamics in 3d transition metals, multilayers, and alloys.