Twofold-Symmetric Magnetoelasticity Induced by Dominant Vertical Shear Strain

We report an unconventional twofold-symmetric magnetoelastic coupling in Ni films, mediated by Rayleigh surface acoustic waves (SAWs). This unique magnetoelastic symmetry originates from a dominant vertical shear strain ϵ_{yz}, which becomes prominent due to the low effective elastic modulus of Ni film. As the film thickness increases, ϵ_{yz} surpasses the conventional Rayleigh SAW strain ϵ_{xx}, a consequence originated from the elastic modulus mismatch at the film-substrate interface. This finding highlights the dominance of ϵ_{yz} in soft thin film under SAW excitation, offering a new platform to excite strong nonreciprocal and topological magnon-phonon hybridization.