Three regimes of a picosecond magnetoacoustics in ferromagnetic structures

TL;DR

This paper identifies and characterizes three distinct regimes of picosecond magnetoacoustic interactions in ferromagnetic structures, using direct optical measurements to explore their signatures and transition criteria.

Contribution

It introduces a direct measurement approach to distinguish and analyze the three regimes of magnetoacoustic interaction in ferromagnetic thin films.

Findings

Transition from coupled magneto-elastic wave to Cherenkov radiation observed.

Detuning between magnon and phonon velocities controls the regimes.

Acoustic pulse influence is limited to the magneto-elastic regime.

Abstract

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.