Extreme Ultraviolet Transient Grating Spectroscopy of High-Wavevector Coherent Magnons

Magnons, or spin waves, are a subject of intense research aimed at both uncovering fundamental physics and paving the way towards applications such as spintronics [1]. Traditionally, magnons have been studied using scattering spectroscopy techniques such as Brillouin light scattering (BLS), inelastic neutron scattering (INS), and resonant inelastic x-ray scattering (RIXS). Combined, these techniques cover wavevector ranges from &lt;0.03 nm^-1 (BLS) and &gt;0.5 nm^-1 (INS/RIXS), thereby leaving the wavevector gap from 0.03 to 0.5 nm^-1 inaccessible. Furthermore, these techniques typically rely on scattering of thermal magnons, limiting the study of coherent magnons to microwave or femtosecond laser techniques. The wavevector of coherent magnons is limited by the need to fabricate nanostructures in the former case and by the optical wavelength in the latter case. We present an ultrafast coherent magnon spectroscopy technique based on the extreme ultraviolet transient grating (EUV-TG) approach developed at the free-electron laser facility FERMI [2], that bridges the wavevector gap between BLS and INS/RIXS while providing a way to excite coherent magnons with wavelengths in the nanometer range without resorting to extensive fabrication. Furthermore, as a time-resolved method, it is not limited by spectrometer resolution, and allows for the direct measure of damping parameters. Upon excitation, transient magnetic gratings [3] where excited in Fe-Gd ferrimagnetic multilayer samples by crossed femtosecond EUV pulses, and detected by diffraction of a time-delayed EUV probe pulse whose wavelength was tuned to the N-edge of Gd. The rapid change in the magnetic anisotropy following the excitation drove spin precession and launched coherent magnons at the wavelength dictated by the transient grating period. By varying the pump wavelength from 25 – 42 nm, the magnon wavevector was varied from 0.07 – 0.12 nm^-1. In combination with optical measurements at zero wavevector, magnon dispersion curves were constructed. We also report a drastic change in the magnon response between samples with compensated and perpendicular magnetic anisotropy, arising from slight changes in the Fe-Gd ratio. In addition, we have produced transient magnetization gratings with a period as small as 17 nm, which opens the prospect for ultrafast control of magnetism on the sub-10 nm spatial scale, which is necessary for applications such as magnetic memory.<br/><br/>[1] Anjan Barman et al., The 2021 Magnonics Roadmap, J. Phys.: Condens. Matter 33, 413001 (2021).; [2] F. Bencivenga et al., Nanoscale transient gratings excited and probed by extreme ultraviolet femtosecond pulses, Sci. Adv. 5, eaaw5805 (2019).; [3] D. Ksenzov et al., Nanoscale Transient Magnetization Gratings Excited and Probed by Femtosecond Extreme Ultraviolet Pulses, Nano Lett. 21, 2905 (2021).