Magnetic entropy change ofPr1−xCaxMnO3manganites(0.2⩽x⩽0.95)

Abstract

In the present work we analyze the magnetic entropy change $\ensuremath{\Delta}{S}_{\mathrm{M}}$ of the ${\mathrm{Pr}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$ manganites, for a wide range of Ca concentrations $(0.20\ensuremath{\leqslant}x\ensuremath{\leqslant}0.95)$. The results for the samples with $0.20<x\ensuremath{\leqslant}0.30$ present the usual behavior expected for ferromagnetic systems, peaking at the Curie temperature ${T}_{\mathrm{C}}$. In contrast, for the charge-ordered antiferromagnetic samples $(0.30<x<0.90)$, an anomalous magnetic entropy change starts around the charge-ordering temperature ${T}_{\mathrm{CO}}$, persisting for lower values of temperature. This effect is associated to a positive contribution to the magnetic entropy change due to the charge-ordering $\ensuremath{\Delta}{S}_{\mathrm{CO}}$, which is superimposed to the negative contribution from the spin-ordering $\ensuremath{\Delta}{S}_{\text{spin}}$; that is described using a mean-field approximation. Supposing these contributions, we could also appraise $\ensuremath{\Delta}{S}_{\mathrm{CO}}^{\mathrm{max}}$ as a function of Ca content, which vanishes for the limits $x\ensuremath{\sim}0.30$ and 0.90 and presents a deep minimum around $x\ensuremath{\sim}0.50$, with two maxima at $x\ensuremath{\sim}0.35$ and 0.65. We conclude that for $x>0.65$ only the magnetic order governs the charge ordering, contrarily to $x<0.65$, where there are more than one mechanism ruling the charge ordering. Moreover, for the samples with phase coexistence $(0.30<x\ensuremath{\leqslant}0.40)$, we found extremely large values for the magnetic entropy change at low temperatures. Finally, for $x>0.90$, we found usual magnetic entropy change curves, peaking at the N\'eel temperature ${T}_{\mathrm{N}}$.