Engineering Magnetoelastic Coupling in Boron Doped Fe₈₀Ga₂₀ Thin Films for Surface Acoustic Wave Devices

Magnetoelastic materials are critical for enabling phonons to serve as an intermediate medium that bridges the otherwise inefficient direct interaction between microwave photons and magnons. Here, we systematically studied boron doped Fe₈₀Ga₂₀ magnetoelastic thin films grown by magnetron co-sputtering. We observed an enhancement in magnetostriction and a reduction in Gilbert damping with addition of boron. Maximum magnetostriction was obtained for a boron concentration that led to a mixed-phase region between polycrystalline and amorphous structures. We subsequently integrated those films into surface acoustic wave devices and studied magnon-phonon interactions through acoustically driven ferromagnetic resonance. We observed a boron concentration dependence of acoustic absorption and amplitude nonreciprocity, consistent with the magnetostriction trend. To uncover the microscopic origin of the magnetostriction enhancement, we investigated the microscopic origin of boron induced magnetostriction enhancement by 4-dimensional scanning transmission electron microscopy (4D-STEM). Using 4D-STEM, we statistically quantified the nearest neighbor (NN) distances in FeGaB thin films. We revealed that the NN distances are inversely correlated with magnetostriction as a function of boron concentration, suggesting the NN distance has a strong impact on the magnetostriction, which can be tuned by boron doping.