Abstract
The ultrasonic attenuation in MnP has been measured at frequencies omega /2 pi from 30 to 210 MHz near the Curie temperature, Tc=290.50+or-0.01K. The frequency and temperature dependences of the critical attenuation alpha are determined and the dynamic scaling hypothesis is examined. It is found that the critical attenuation alpha is described by a scaling function alpha = omega 1+y Psi ( omega epsilon -x) which differs above and below Tc, where x=0.92+or-0.05, y=-0.18+or-0.04 and epsilon = mod T/Tc-1 mod . The critical attenuation near Tc arises from a relaxation process occurring only below Tc, and a fluctuation process occurring on both sides of Tc. The critical relaxation times are the same above and below Tc apart from a constant factor; tau = tau 0 epsilon -x (T>Tc) and tau '= tau 'o epsilon -x' (T<Tc) with x'=x, tau 0=(9.0+or-0.5)*10-12 s and tau '0=(1.8+or-0.5)*10-12 s. The critical attenuation below Tc is separated into the relaxation and fluctuation parts. The scaling form of the relaxation part is discussed in the light of the time-dependent behaviour of the spin correlation function.