Critical sound propagation in magnetic metals and bottleneck phenomena

A semiphenomenological model of sound propagation in a critical magnetic sy stem is proposed, which takes into account a relaxation of energy between t he system of localised spins and the conduction electron spins as well as t he coupling of the longitudinal phonon to bilinear combination of spin fluc tuations and to the energy densities. A general expression for the acoustic self-energy is obtained within the one-loop approximation. The effect of " bottlenecking" and its influence on the critical singularities in the sound characteristics is discussed.