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Critical behaviour of the thermal properties of KMnF3

An ac photopyroelectric calorimeter has been used to simultaneously measure the specific heat, thermal conductivity, and thermal diffusivity around the antiferromagnetic to paramagnetic phase transition in KMnF3. 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 also structural as well. The thermal conductivity shows a broad peak at the Neel temperature that could be related to a reduction of the phonon scattering by the spin fluctuation as the transition temperature is approached.