Detection of Magnetization Dynamics in Metal/Ferrimagnetic Insulator Heterostructure using Ultrafast Resonant Hard X-ray Magnetic Diffraction*

Magnetization dynamics underpin spin transport in ferrimagnetic insulators for spincaloritronics and spintronics. We present a hard X-ray resonant magnetic diffraction study conducted in a pump-probe setup with 100 fs time resolution probing the structural and magnetic responses of a Pt/Gd₃Fe₅O₁₂ (GdIG)/Gd₃Ga₅O₁₂ heterostructure to an acoustic pulse.  The acoustic impulse had a strain amplitude of 0.1% generated by a 400 nm-wavelength optical pulse absorbed in the Pt layer. The strain due to the expansion of the Pt propagates into GdIG and induces magnetization dynamics apparent in x-ray intensities recorded near the GdIG 444 reflection. The time-domain Fourier transform (FT) has a wavevector dependence that was used to determine the longitudinal acoustic phonon dispersion. Magnetic dynamics were probed using circularly polarized incident X-rays and analyzed using the circular intensity flipping ratio (f_c). The time-domain FT of f_c was used to probe the wavevector dependence of the magnetization dynamics. The measurement probed a wavevector range spanning a region of the magnon and phonon dispersions where a hybrid magnon-polaron excitation has been predicted. The results provide a way to probe the coupling of structural and magnon dynamics, particularly magnon-phonon interactions.