Ultrasonic Attenuation Study of Critical Fluctuations in Rb₂CoF₄ near the Néel Temperature

Abstract

Ultrasonic attenuation in a two-dimensional Ising-like antiferromagnet Rb₂CoF₄ has been measured at frequencies from 10 to 150 MHz near the Néel temperature, T_N=103.20±0.02 K. Above T_N it is found that the critical attenuation α is described by a scaling function, ω^1+yf_F(ωτ), where ω is an angular frequency of sound wave and τ is a characteristic time of order-parameter fluctuations and is expressed as τ∝t^−x using reduced temperature, t=|T⁄T_N−1|, with results x=1.20±0.10 and y=0.31±0.03. The dynamic critical exponent is determined as z=x⁄ν=1.21±0.10 when the existing value of ν (=0.99±0.04) is used. The critical attenuation has a maximum below T_N which shifts to lower temperature as frequency increases. This behaviour can be explained in terms of the relaxation of order-parameter.