Generation of coherent magnons in NiO stimulated by EUV pulses from a seeded free-electron laser

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The recent development of seeded free-electron lasers made possible the implementation of a new class of time-resolved experiments, strictly requiring pulses characterized by quasi full coherence. Here, circularly polarized EUV pulses have been used to launch magnetic excitations (magnons) in a thin film of nickel oxide (NiO), subsequently revealed via optical techniques. The main novelty of this work is that these collective modes have been excited using EUV pulses resonant at core electrons energies. Switching the EUV polarization, a flip equal to Pi in the mode polarity has been observed, which can be ascribed to an optomagnetic effect, as well as a mode frequency shift, scaling as a function of the absorption cross section between core electrons and circularly polarized photons. A phenomenological model supports these findings, too.