Critical behavior of CoO and NiO from specific heat, thermal conductivity, and thermal diffusivity measurements

An ac photopyroelectric calorimeter has been used to simultaneously measure the specific heat (c(p)), thermal conductivity (K), and thermal diffusivity (D) around the antiferromagnetic to paramagnetic phase transition in CoO and NiO single crystals. Up to now, no agreement on the critical behavior of both oxides has been obtained. The results for seven samples grown in different laboratories have been compared. We have found that, irrespective of the origin of the samples, the critical exponent and amplitude ratio of c(p), D, and K in NiO agree with the predictions of the three-dimensional Heisenberg model for isotropic antiferromagnets, which contradicts previous results. On the other hand, the sharpness of the peak of specific heat and of the dip of thermal diffusivity, together with the value of the critical exponent alpha in CoO, highly increase as crystal quality improves. For the best sample, a value alpha=0.81 has been obtained, which is not only higher than previously reported results, but far away from any universality class.