resumo
A study of the magnetocaloric properties of La1-x-yYyCaxMnO3, La1-xCdxMnO3, and Pr1-xCaxMnO3 manganites with ferromagnetic and charge-ordered states is presented. The ferromagnetic lanthanum manganites show a negative entropy change peak DeltaS 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 DeltaS 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 Pr1-xCaxMnO3 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 (approximate to 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.
palavras-chave
BEHAVIOR; PHASE
categoria
Physics
autores
Reis, MS; Gomes, AM; Araujo, JP; Tavares, PB; Amaral, JS; Oliveira, IS; Amaral, VS
nossos autores
João Cunha de Sequeira Amaral
Investigador Auxiliar