Ultrafast Acoustic-Strain Pulse-Driven Magnetization Dynamics and Spin-Strain Coupling in a Ferrimagnetic Garnet

Understanding how acoustic strain pulses drive magnetic dynamics in ferrimagnetic insulators is important for advancing magnonics and spincaloritronics. The magnetization dynamics occur over a wide range of temporal frequencies and wavevectors, posing a challenge for characterization techniques based on optical methods. We describe the results of an ultrafast optical pump/X-ray diffraction probe experiment in a heterostructure of Pt/GdIG/GGG, in which the laser-excited Pt thin film launches a longitudinal acoustic pulse that propagates through the GdIG layer. The acoustic strain pulse induces magnetic oscillations with a very broad THz-regime temporal frequency through the inverse magnetostriction effect. The magnetic oscillations occur at the picosecond time scale and are probed by ultrafast resonant magnetic X-ray scattering using the Gd L2 resonance. Magnetic oscillations appear at multiple combinations of temporal frequencies and wave vectors. A magnetic oscillation of frequency around 0.19 THz corresponds to repeated acoustic excitation at a mechanical resonance of the Pt thin film. Other magnetic oscillations follow the acoustic phonon dispersion, where the frequency of the magnetic oscillation and acoustic phonons overlap, indicating that the magnetism is coupled with the acoustic phonon. An additional magnetic oscillation with a period of approximately 2 ps, equivalent frequency of 0.5 THz, is observed at multiple wave vectors. Ultrafast acoustic strain pulses can thus coherently modulate the magnetization in ferrimagnetic garnets.