Charge-ordering and magnetoelastic coupling effects on the magnetocaloric properties of manganites

Vitor S. Amaral 2Mario S. Reis 1,2A. M. Gomes 1Joao P. Araujo 3P. B. Tavares 4J. S. Amaral 2I. S. Oliveira 1

1. Centro Brasileiro de Pesquisas Físicas (CBPF), R. Dr. Xavier Sigaud 150 Urca, Rio de Janeiro 2229-180, Brazil
2. Universidade de Aveiro, Departamento de Fisica and CICECO (UA), Campus de Santiago, Aveiro 3810-193, Portugal
3. Departamento de Fisica and IFIMUP, Universidade do Porto (UP), Rua do Campo Alegre, 687, Porto 4169-007, Portugal
4. Departamento de Química, Universidade de Trás-os-Montes e Alto Douro (UTAD), Vila Real 5001-911, Portugal


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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Presentation: oral at E-MRS Fall Meeting 2003, Symposium D, by Vitor S. Amaral
See On-line Journal of E-MRS Fall Meeting 2003

Submitted: 2003-05-27 18:31
Revised:   2009-06-08 12:55
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