Symmetry of the dissipation of surface acoustic waves by ferromagnetic resonance

We study the symmetry of the coupling between surface acoustic waves (SAWs) and ferromagnetic resonance in a thin magnetic film of CoFeB deposited on top of a piezoelectric Z-cut LiNbO3 substrate. We vary the orientation of the applied magnetic field with respect to the wavevector of the SAW. Experiments indicate an unexpected twofold symmetry of the absorption of the SAW energy by the magnetic film. We discuss whether this symmetry can arise from the magnetoelastic torque of the longitudinal strain and the magnetic susceptibility of ferromagnetic resonance. We find that one origin of the twofold symmetry can be the weak in-plane uniaxial anisotropy present within the magnetic film. This phenomenon adds to the previously identified other source of twofold symmetry but shall persist for ultrathin films when the dipolar interactions cease to contribute to the anisotropy of the slope of the spin wave dispersion relation.