Abstract
In the present work we analyze the magnetic entropy change of the manganites, for a wide range of Ca concentrations . The results for the samples with present the usual behavior expected for ferromagnetic systems, peaking at the Curie temperature . In contrast, for the charge-ordered antiferromagnetic samples , an anomalous magnetic entropy change starts around the charge-ordering temperature , persisting for lower values of temperature. This effect is associated to a positive contribution to the magnetic entropy change due to the charge-ordering , which is superimposed to the negative contribution from the spin-ordering ; that is described using a mean-field approximation. Supposing these contributions, we could also appraise as a function of Ca content, which vanishes for the limits and 0.90 and presents a deep minimum around , with two maxima at and 0.65. We conclude that for only the magnetic order governs the charge ordering, contrarily to , where there are more than one mechanism ruling the charge ordering. Moreover, for the samples with phase coexistence , we found extremely large values for the magnetic entropy change at low temperatures. Finally, for , we found usual magnetic entropy change curves, peaking at the Néel temperature .
3 More- Received 1 September 2004
DOI:https://doi.org/10.1103/PhysRevB.71.144413
©2005 American Physical Society