Ultrasonic Attenuation Study of the Order-Parameter Relaxation Below the Néel Temperature in Rb₂CoF₄

Ultrasonic attenuation in Rb₂CoF₄has been measured at frequencies ranging from 10 to 150 MHz below and above the Néel temperature T_N. The critical attenuation has been separated into the relaxation and fluctuation contributions. The relaxation contribution α_Rtakes a maximum below T_Nand the temperature at which the maximum appears decreases as the frequency increases. Below T_Nit is found that α_Ris described by a scaling function, α_R∝ω ^1+y′f_R(ωτ′), where τ′=τ′_0mt^-x′ and t =1-T/T_Nwith the results x′=1.20±0.03 and y′=0.15±0.05. The scaling function f_R(ωτ′) takes such a form as 2ωτ′/[1+(ωτ′)²].