Critical behaviour of the thermal properties of KMnF3

TL;DR

This study investigates the critical behavior of thermal properties in KMnF3 around its phase transition, revealing that thermal diffusivity and specific heat share critical exponents consistent with the 3D Heisenberg model, despite structural influences.

Contribution

It provides experimental evidence linking thermal property critical exponents in KMnF3 to theoretical predictions for isotropic magnets, considering both magnetic and structural transitions.

Findings

Thermal diffusivity and specific heat share the same critical exponent.

Thermal diffusivity amplitude ratio matches that of specific heat.

Thermal conductivity peaks near Neel temperature, indicating phonon-spin interactions.

Abstract

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.