Sound attenuation in a spin-1 Ising system near the critical temperature

Abstract

Sound attenuation coefficient is calculated for a spin-1 Ising system on the basis of Onsager theory of irreversible processes and its behaviour near the second-order phase transition point or the critical point is analyzed according to various values of Onsager or phenomenological rate coefficients ( γ ij ). For all γ S and γ Q values it is observed that the sound attenuation peaks occur below T C and depend on frequency ( ω) and on the value of the off-diagonal Onsager coefficient ( γ). On the other hand, in the hydrodynamic regime the attenuation converges to zero with a mean field exponent ( λ=1.0) just below the transition, while it vanishes above the critical temperature and then remains temperature-independent. Moreover, the behaviour of the sound attenuation as a function of frequency is also investigated and ω 2 dependence is observed for the attenuation coefficient. These results are in a good agreement with ultrasonic investigations of some magnetic systems, such as MnF 2, FeF 2, and RbMnF 3.