Dynamical scaling and ultrasonic attenuation in KMnF₃ at the structural phase transition

Ultrasonic attenuation in KMnF₃ has been measured at frequencies ranging from 10-170 MHz near 186-2K. The frequency and temperature dependences of the critical attenuation are determined and the dynamic scaling hypothesis is examined. Above 186-2K, it is found that the critical attenuation alpha is described by a scaling function, alpha = omega ^yF( omega / Omega _kappa), where y=0.13+or-0.05 and Omega _kappa is the characteristic frequency of the order parameter fluctuation with the wave number equal to the inverse correlation length. The temperature dependence of Omega _kappa is discussed in the light of the Murata's (1976) theory for the sound attenuation of the perovskite crystal near the structural phase transition point.