Three regimes of a picosecond magnetoacoustics in ferromagnetic structures

The development of magnonics requires energy-efficient methods for generating spin waves and controlling their parameters. Acoustic waves are known to resonantly excite spin waves through magneto-elastic wave formation or to induce non-resonant forced magnetization oscillations. Short acoustic wavepackets enable another unexplored resonant interaction regime -- the Cherenkov radiation of spin waves. This raises a question regarding the criteria and signatures of these three regimes of picosecond magnetoacoustics.Here, we use a scanning magneto-optical pump-probe technique to directly observe all three regimes of interaction between laser-driven acoustic and magnetostatic wavepackets in a thin permalloy film and a waveguide fabricated on Si substrate. Direct measurements of the phase velocities reveal the transition from a coupled magneto-elastic wavepacket to Cherenkov-like radiation and the non-resonant regime, controlled by the detuning between magnon and phonon group velocities. The acoustic pulse is found to be affected by the excited magnetization dynamics only in the magneto-elastic regime.