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

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Reis M., Gomes A., Araújo J., Tavares P., Amaral J., Oliveira I., Amaral V.

Acta Physica Polonica A

2004

vol. 105

issue 1-2

163-171

A study of the magnetocaloric properties of La_{1-x-y}Y_yCa_xMnO_3, La_{1-x}Cd_xMnO_3, and Pr_{1-x}Ca_xMnO_3 manganites with ferromagnetic and charge-ordered states is presented. The ferromagnetic lanthanum manganites show a negative entropy change peakΔ S under the application of a magnetic field at the Curie temperature. However, the magnetoelastic coupling effects lead to a first-order paramagnetic-ferromagn etic transition and an increasedΔ 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_xMnO _3 system shows a rich electric and magnetic phase diagram, with a ferromagnetic insulator phase for 0.15

Phase and Structural Study of Self-Doped La_{1-x-y}Ca_xØ_yMnO_{3+δ}

100%

Figueiras F., Araújo J., Amaral V., Tavares P., Lopes A., Vieira J.

Acta Physica Polonica A

2004

vol. 105

issue 1-2

173-182

To understand the combined effect of divalent ion and A-site vacancy (Ø) self-doping, beyond the binary La_{1-x}MnO_3 system, polycrystalline La_{1-x-y}Ca_xØ_yMnO_{3+δ} samples were studied. Samples with Ca substitution and excess Mn in the range x<0.33 and y<0.45, respectively, were prepared by standard solid-state reactions. Structural and phase analysis of the samples were done by X-ray diffraction and transmission electron microscopy. The manganite structure in this composition range includes rhombic and orthorhombic phases. Most samples contain the hausmanite phase (Mn_3O_4) coexisting with the manganite phase. A small amount of perovskite related AMn_7O_{12} (A=La,Ca) structure near the hausmanite-manganite boundary is also found in samples treated in oxygen atmosphere. The calculated vacancy content accommodated in the manganite phase can be higher than 1/8, the reported limit for La_{1-x}MnO_3, which is confirmed in our x=0 sample. For the compositions studied, a ferromagnetic metallic behavior below T_C is found, confirming the stability of the Mn sublattice, and near the optimum stoichiometric ((x=0. 33, y=0) system, a net A-site vacancy content (up to y=0.2) does not degrade the magnetic and electric properties.

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2 Acta Physica Polonica A

2 2004

2 Amaral V.

2 Araújo J.

2 Tavares P.

1 Amaral J.

1 Figueiras F.

1 Gomes A.

1 Lopes A.

1 Oliveira I.

1 Reis M.

1 Vieira J.

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

Phase and Structural Study of Self-Doped La_{1-x-y}Ca_xØ_yMnO_{3+δ}