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

Abstract

An ac photopyroelectric calorimeter has been used to simultaneously measure the specific heat ͑c p ͒, thermal conductivity ͑K͒, and thermal diffusivity ͑D͒ 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 ␣ in CoO, highly increase as crystal quality improves. For the best sample, a value ␣ = 0.81 has been obtained, which is not only higher than previously reported results, but far away from any universality class.

Critical behavior study of the spin ordering transition in RVO3 (R = Ce, Pr, Nd, Sm, Gd, Er) by means of ac photopyroelectric calorimetry

“…The magnetic transition is signaled as a strong dip, whose sharpness depends on the presence of one or other rare earth ion. This kind of dips is customarily found in magnetic transitions [30][31][32][33]. Again, we have carefully checked the character of the transitions using low temperature rates, finding a complete superposition of the curves in heating and cooling runs, confirming that they are all continuous.…”

Thermal properties and Ising critical behavior in EuFe 2 As 2

“…The antiferromagnetic transition is manifested as a structure superimposed on the thermal diffusivity curve. These structures, when associated with second order phase transitions, are observed as clear dips on the monotonous decaying thermal diffusivity curve at moderately low or medium temperatures (60-600 K) [15,20,21,23,[28][29][30]. When the critical temperatures are very low and there is a quick increase of thermal diffusivity as temperature is reduced, the shape of the dip is not so marked, as it happens here; it is worth noting that the Néel temperature corresponds to the minimum value of the dip.…”

“…The usefulness of this technique has been well demonstrated in the study of the critical behavior of second-order magnetic transitions in different materials, using the specific heat as well as the thermal diffusivity [20][21][22][23][24]. It is also an interesting technique to remove the ambiguity about the character of a phase transition, so it will also be applied in the present work to the study of the concomitant structural-magnetic phase transition that occurs near 190 K.…”

Invited Review Article: Photopyroelectric calorimeter for the simultaneous thermal, optical, and structural characterization of samples over phase transitions

“…The BPPE was recently used once again for renewed high temperature resolution studies of the critical behaviour of the thermal parameters over the same transition in several other compounds such as KMnF3, NiO, and CoO. [76][77][78] It was shown that KMnF3 and NiO presented a Heisenberg like singularity in c, and also a cusp-like critical behaviour in the thermal resistance 1/k, similar to that obtained in RbMnF. In CoO, an anomalously large critical exponent was found for c, not consistent with any universal critical behaviour.…”