Magnetocaloric and magnetoresistive properties of La_0.67Ca_0.33-xSr_xMnO₃

This thesis presents results of an experimental investigation of magneto-caloric and magnetoresistive properties of a series of polycrystalline Ca- and Sr-doped lanthanum manganites, La0.67Ca0.33-xSrxMnO3 (0=x=0.33 ), with the perovskite structure. The samples consisted of sintered oxide powders prepared the glycine-nitrate combustion technique.

The compounds were ferromagnetic and showed a Curie transition in the temperature range 267–370 K (TC increased with increasing x). An analysis of the structural properties was carried out by means of x-ray diffraction and the Rietveld technique. The variation of the Ca/Sr ratio was found to cause a transition from orthorhombic to rhombohedral symmetry in the composition range 0.110

The analysis suggested a strong correlation between structural properties and magnetism, for instance a relationship between the mean Mn–O–Mn bond angle and the Curie temperature. The Mn–O–Mn bonds mediate ferromagnetism and electrical transport in these materials via the double-exchange mechanism. The magnetocaloric effect of the La0.67Ca0.33-xSrxMnO3 samples was measured directly and indirectly (by means of magnetization measurements).

All the samples showed a magnetocaloric effect in the vicinity of TC. A model for the mag-netocaloric effect based on Weiss mean field theory and classical theories for heat capacities was developed. The model provided reasonable predictions of the magne-tocaloric properties of the samples. The compounds with low Sr content showed a magnetocaloric effect comparable to that of Gadolinium, the prototypical working material for magnetic refrigeration at room temperature.

A less comprehensive part of the investigation regarded the magnetoresistive properties of the La0.67Ca0.33-xSrxMnO3 system. It was found that the polycrystalline nature of the compounds played a decisive role for the magnetotransport properties. Characteristic grain boundary effects, such as a low-field magnetoresistance, which is absent in single-crystalline perovskites, were observed.

The low-field effect is usually ascribed to spin-dependent scattering in grain boundaries. Qualitatively the results obtained for the La0.67Ca0.33-xSrxMnO3 samples were consistent with this model. The resistivity contribution arising from the presence of grain boundaries increased with increasing Sr content.

Reducing the sintering temperature also enhanced the grain boundary effects. The samples with low Sr content showed colossal magnetoresistance (CMR) near room temperature (~20-45 % with µ0H = 0.8 T). The CMR effect was negligible for the samples with high Sr content. However, these samples exhibited a grain boundary-related magnetoresistance at room temperature.