Studies of structural and the giant magneto - caloric effect of some manganities perovskite

Abstract

This thesis presents the results of an experimental investigation of the structural, magnetic properties and the magneto-caloric effect of three series of polycrystalline La0.7(Ca1-xMx)0.3MnO3 (M = Pb, Sr, Ba) with x = 0.00, 0.05, 0.10, 0.15, 0.20, 0.25), samples. Scanning Electron Microscope analysis of the samples indicates that the crystallites of the samples are fairly homogeneous with varying grain sizes. The X-ray diffraction pattern and Rietveld refinement confirmed that all compounds crystallized in the orthorhombic structure with Pbnm space group. The lattice parameters and unit cell volume increased with increasing doping level for all the samples. The samples were paramagnetic at the room temperature and have shown a paramagnetic to ferromagnetic transition temperature (Tc) in the temperature range 252 K - 291 K, 252 K - 308 K, and 157 K - 205 K for very low substitution of Pb, Sr and Ba respectively. The transition temperature changed with increasing x content. The variation of the Ca/Pb, Ca/Sr and Ca/Ba ratios were found to cause a transition from cubic to orthorhombic symmetry in all compositions for very low substitution of doping level. The analysis suggested a strong correlation between the structural and the magnetic properties. A good correlation between the ionic radius and the Curie temperature, the mean Mn–O–Mn bond angle and the Curie temperature has been observed for all the studied samples. The Mn–O–Mn bonds mediate ferromagnetism in these materials via the double-exchange mechanism. The magneto-caloric effects of the La0.7(Ca1-xMx)0.3MnO3 samples were measured indirectly from magnetization measurements. The samples with Pb=0.075 has shown the highest value (5.98 J/kgK) and (3.47 J/kgK) of magnetic entropy change around 291 K with applied magnetic field 5 T and 2 T respectively. The samples with Sr=0.075 has shown the highest value (5.00 J/kgK) of magnetic entropy change around 293 K with applied magnetic field 1.35 T. The samples with Ba=0.015 has shown the highest value (5.99 J/kgK) and (3.92 J/kgK) of magnetic entropy change around 205 K with applied magnetic field 5 T and 2 T respectively. The grain sizes of the manganites seem to have played a role in the observed magnetic properties. Qualitatively, the results obtained for the La0.7(Ca1-xMx)0.3MnO3 A significant large magnetic entropy change, |Δ S samples are consistent with the similar studies by other authors. m|max has been obtained for the studied samples, which can be considered as the potential candidates for giant magneto-caloric effect (GMCE) in the research on manganites perovskites. Specially, the compounds with low Strontium (Sr) contents have shown a GMCE comparable to that of Gadolinium, which is considered as a prototype and relatively expensive working material for magnetic refrigeration near room temperature.