  Surface acoustic waves (SAWs) traveling on the surface of a piezoelectric
crystal can, through the magnetoelastic interaction, excite traveling spin-wave
resonance in a magnetic film deposited on the substrate. This spin-wave
resonance in the magnetic film creates a time dynamic surface stress of
magnetoelastic origin that acts back on the surface of the piezoelectric and
modifies the SAW propagation. Unlike previous analyses that treat the
excitation as a magnon-phonon polariton, here the magnetoelastic film is
treated as a perturbation modifying boundary conditions on the SAW. We use
acoustical perturbation theory to find closed form expressions for the
back-action surface stress and strain fields and the resultant SAW velocity
shifts and attenuation. We demonstrate that the shear stress fields associated
with this spin-wave back-action also generate effective surface currents on the
piezoelectric both in-phase and out-of-phase with the driving SAW potential.
Characterization of these surface currents and their applications in
determination of the magnetoelastic coupling are discussed. The perturbative
calculation is carried out explicitly to first order (a regime corresponding to
many experimental situations of current interest) and we provide a sketch of
the implications of the theory at higher order.
