Theory of critical sound attenuation in Ising-type magnets

The critical behaviour of sound attenuation has been studied in an elastica lly isotropic Ising system above the critical point on the basis of a compl ete stochastic model including both spin-energy and lattice-energy modes li nearly coupled to the longitudinal sound mode. The effect of spin-lattice r elaxation on the ultrasonic attenuation is investigated. The crossover betw een Kawasaki behaviour and Murata-Iro-Schwabl behaviour is studied as depen dent on the values of ultrasonic frequency, reduced temperature, relaxation times, etc. A new high-frequency regime is discussed in the magnetic syste ms. This new regime corresponds to an adiabatic sound propagation and is ve ry similar to the ones in binary mixture and liquid helium. A new frequency -dependent specific heat being the harmonic average of the bare lattice and critical spin specific heats is introduced. It was shown that such specifi c heat describes the process of equilibration between spin and lattice subs ystems and includes the most important features of critical sound attenuati on.