An ac photopyroelectric calorimeter has been used to measure simultaneously the specific heat, thermal conductivity and thermal diffusivity around the antiferromagnetic to paramagnetic phase transition in KMnF 3 . It has been found that the critical exponent and the amplitude ratio of the thermal diffusivity are the same as those of the specific heat. Both values agree with the predictions of the three-dimensional Heisenberg model for isotropic magnets, although this transition is not only magnetic but structural as well. The thermal conductivity shows a broad peak at the Néel temperature that could be related to a reduction of the phonon scattering by the spin fluctuation as the transition temperature is approached.

A general theory of critical sound propagation, including phonon-spin-energy coupling, is studied in anisotropic magnets above their transition temperature. The Kawasaki weak singularity in the ultrasonic attenuation is found as a nonasymptotic effect. A new nonasymptotic regime similar to the one in the binary mixture is also determined. The role of coupling constants and the bare relaxation times in establishing the dominance region of particular terms, is discussed.PACS. 05.70.Jk Critical point phenomena -62.65.+k Acoustical properties of solids

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