ACTA

Charge-Ordering and Magnetoelastic Coupling Effects on the Magnetocaloric Properties of Manganites

M.S. Reis^a,b, A.M. Gomes^b, J.P. Araujo^c, P.B. Tavares^d, J.S. Amaral^a, I.S. Oliveira^b and V.S. Amaral^a
^aDepartamento de Fisica and CICECO, Universidade de Aveiro, 3810-193 Aveiro, Portugal
^bCentro Brasileiro de Pesquisas Fisicas, R. Dr. Xavier Sigaud 150, 22290-180 Rio de Janeiro-RJ, Brasil
^cDepartamento de Fisica and IFIMUP, Universidade do Porto, 4169-007 Porto, Portugal
^dDepartamento e Centro de Quimica Universidade de Tras-os-Montes e Alto Douro, 5001-911 Vila Real, Portugal

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A study of the magnetocaloric properties of La_1-x-yY_yCa_xMnO₃, La_1-xCd_xMnO₃, and Pr_1-xCa_xMnO₃ manganites with ferromagnetic and charge-ordered states is presented. The ferromagnetic lanthanum manganites show a negative entropy change peak Δ S under the application of a magnetic field at the Curie temperature. However, the magnetoelastic coupling effects lead to a first-order paramagnetic-ferromagnetic transition and an increased Δ S compared to simple ferromagnets. This effect is analyzed 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. The Pr_1-xCa_xMnO₃ system shows a rich electric and magnetic phase diagram, with a ferromagnetic insulator phase for 0.15<x<0.30 and coexisting charge-ordering and antiferromagnetic insulator behavior for 0.30<x<0.85. The magnetocaloric effect 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 (T_co). 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.

DOI: 10.12693/APhysPolA.105.163
PACS numbers: 75.30.Sg, 75.30.Kz, 75.80.+q