Nature and entropy content of the ordering transitions inRCo2

Abstract

Differential scanning calorimetry experiments have been performed under magnetic field in RCo2 Laves phases compounds R = Pr, Nd, Tb, Dy, Ho, and Er. The thermodynamical nature of the ferromagnetic transitions in PrCo2 and NdCo2 is clarified, after the controversy present in the literature about the character of their ordering transitions. The magnetocaloric effect in the compounds showing a first-order magnetostructural transition R = Dy, Ho, and Er is characterized. The latent heat, L, and the entropy change at the transition, S, have been studied by inducing the transition sweeping the temperature at a constant field and sweeping the field at a constant temperature. Results from calorimetric data show that L is essentially temperature independent, suggesting that the first order transitions in RCo2 are dominated by the structural effects. The magnetocaloric effect has been also characterized from magnetization data and the results are in excellent agreement with those from calorimetric data. However, the corresponding values of S for isothermal and isofield experiments differ significantly. We interpret the differences in terms of the broadness of the transitions and prove that the various S values reported in literature for first-order transitions are not always directly comparable.