Laser-generated surface acoustic waves (SAWs) facilitate efficient information processing in modern spintronics and magnonics. The ability to tune the SAW parameters such as the amplitude is crucial to achieve acoustic control over magnonic properties. Such tunability can be achieved in phase-changing magnetic materials that accommodate both spin waves and SAWs. A promising material is the $\mathrm{Fe}$-$\mathrm{Rh}$ alloy, a metallic antiferromagnet at room temperature that undergoes a phase transition to the ferromagnetic state accompanied by a crystal lattice expansion at 370 K. This transition can also be induced by femtosecond laser pulses. In this paper, we use the phase transition in a 60-nm $\mathrm{Fe}$${}_{49}$$\mathrm{Rh}$${}_{51}$ film to optically generate pulses of Gigahertz quasi-Rayleigh SAWs. We detect them via the photoelastic effect and show that the lattice transformation during the phase transition is a dominant strain-generation mechanism for above-threshold excitation. The weight of this contribution rises as the sample is heated closer to the antiferromagnetic-ferromagnetic transition temperature and “switches off” when heated above it, allowing for control of the SAW amplitude. A model based on thermodynamic parameters of $\mathrm{Fe}$${}_{49}$$\mathrm{Rh}$${}_{51}$ shows that the lattice transformation occurring within 95 ps effectively contributes to SAW generation happening on a comparable timescale, while nonequilibrium fast kinetics of the phase transition does not.

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• Light-matter interaction
• Magnetic phase transitions
• Magnonics
• Structural phase transition
• Antiferromagnets
• Magnetic thin films
• Surface acoustic wave
• Ultrafast femtosecond pump probe