  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.
