Propagation of Laser‐Generated GHz Surface Acoustic Wavepackets in FeRh/MgO(001) Below and Above the Antiferromagnetic–Ferromagnetic Phase Transition

Magnetoacoustic devices that harness the strong coupling between acoustic waves and magnons have emerged as a promising platform for energy‐efficient spintronics. Laser‐generated pulsed surface acoustic waves (SAWs) are particularly attractive for such applications, offering broadband frequency content up to the gigahertz range, remote excitation without lithographic patterning, and surface localization for efficient on‐chip integration. This work presents a comprehensive experimental study of laser‐generated SAW pulses in the Fe ₄₉ Rh ₅₁ /MgO(001) system. A thin film of the near‐equiatomic FeRh alloy serves both as an opto‐acoustic transducer and as a mechanical load that modulates SAW propagation. The antiferromagnetic to ferromagnetic phase transition in FeRh, occurring slightly above room temperature, is accompanied by abrupt changes in its elastic properties, enabling controlled modification of the SAW excitation efficiency and dispersion characteristics by tuning the sample temperature and laser fluence. Using 160 fs laser pulses for excitation and time‐resolved Sagnac interferometry for detection, the key SAW parameters are evaluated, including amplitude, spectral content, phase and group velocities, and their in‐plane anisotropy. Particular emphasis is placed on the dispersion relation and its anisotropy, which govern the coherent interaction between phonons and magnons and are determined primarily by the FeRh film.