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 } ^{ a }Departamento de Fisica and CICECO, Universidade de Aveiro, 3810-193 Aveiro, Portugal^{ b }Centro Brasileiro de Pesquisas Fisicas, R. Dr. Xavier Sigaud 150, 22290-180 Rio de Janeiro-RJ, Brasil^{ c }Departamento de Fisica and IFIMUP, Universidade do Porto, 4169-007 Porto, Portugal^{ d }Departamento 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-y }Y_{ y }Ca_{ x }MnO_{ 3 }, La_{ 1-x }Cd_{ x }MnO_{ 3 }, and Pr_{ 1-x }Ca_{ x }MnO_{ 3 } 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-x }Ca_{ x }MnO_{ 3 } 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 |