A study of the magnetocaloric properties of La1-xCaxMnO3, La1-x-yYyCaxMnO3 and Pr1-xCaxMnO3 manganites with ferromagnetic and charge-ordered states is presented. The ferromagnetic Lanthanum manganites show a negative entropy change peak DS under the application of a magnetic field at the Curie temperature. However, the magneto-elastic coupling effects lead to a first-order paramagnetic-ferromagnetic transition and an increased DS compared to simple ferromagnets. This effect is analysed in the framework of the Landau theory of phase transitions which shows a comparable influence of the magnetoelastic couplings and the ordinary magnetic ordering effect.
In contrast, the Pr1-xCaxMnO3 system shows a richer electric and magnetic phase diagram. For 0.15<x<0.30 a ferromagnetic insulator phase is established. A more complex electric-magnetic phase diagram is found for 0.30<x<0.85, where the charge-ordering effect coexists with an antiferromagnetic insulator order. The results for x=0.25 and 0.30 show a simple ferromagnetic phase transition effect. For the samples above the onset concentration for the charge ordering (~0.30) an anomalous magnetic entropy change is observed below the charge ordering temperature (TCO). This effect is associated with a positive contribution from the magnetic entropy change due to charge-ordering, superimposed to the negative contribution due to spin-ordering. Such positive entropy contribution can be understood as the increase of accessible states due to the increase of electron mobility, under an applied magnetic field. Moreover, at low temperatures, when the magnetic field induced metal insulator transition becomes irreversible, we find extremely large values of the magnetic entropy change. For x=0.32, DS reaches -20.8 Jkg-1K-1, under a 4T magnetic field.

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