Laser-generated GHz surface acoustic waves with tunable amplitude during the magnetostructural phase transition in FeRh thin films

TL;DR

This study demonstrates tunable GHz surface acoustic waves in FeRh thin films during a phase transition, enabling control over SAW amplitude via laser-induced magnetic and structural changes.

Contribution

It introduces a method to generate and control GHz SAWs in FeRh films by exploiting the material's phase transition induced by femtosecond laser pulses.

Findings

SAWs generated via laser pulses during FeRh phase transition.

SAW amplitude controlled by temperature proximity to transition.

Lattice transformation is the main strain mechanism for SAW generation.

Abstract

Laser-generated surface acoustic waves (SAW) facilitate efficient information processing in modern spintronics and magnonics. The ability to tune the SAW parameters such as amplitude is crucial to achieve acoustic control over magnonic properties. Such tunability can be achieved in phasechanging magnetic materials that accommodate both spin waves and SAWs. A promising material is the FeRh 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 Fe49Rh51 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 AFM-FM transition temperature and 'switches off' when heated above it, allowing for control of the SAW amplitude. A model based on thermodynamic parameters of Fe49Rh51 shows that the lattice transformation occurring within 95 ps effectively contributes to SAW generation happening on a comparable timescale, while non-equilibrium fast kinetics of the phase transition does not.