Evaluation of the magnetocaloric effect in nano-crystalline La0.7Ca0.3Mn1-xAlxO3 manganites synthesized via wet-chemistry methods
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Abstract
Nanosized La0.7Ca0.3Mn1-xAlxO3 (x=0, 0.02, 0.07, 0.1) manganites were synthesized via two different wet-chemistry methods, namely Pechini auto-combustion. The samples were carefully studied after their structural, chemical, and magnetic properties by means of standard characterization methods. Analysis of the X-ray diffraction patterns indicated that the samples were single phase with orthorhombic perovskite-type structure, within the accuracy limit of the technique. The lattice parameters were obtained by means of Rietveld refinement of the X-ray diffraction patterns. The targeted Al3+ doping of the pristine La0.7Ca0.3MnO3 sample was verified by the systematic shifting of the diffraction peaks towards higher values of the 2-angle. The elemental composition of the samples as well as the chemical environment of Mn-ion were investigated by means of X-ray photoelectron spectroscopy. The magnetic properties of the samples were carefully studied using vibrating sample magnetometry. The dependence of the magnetization on the temperature recorded, in field as high as 200 Oe, showed a clear paramagnetic-ferromagnetic transition for all studied samples. In turn, well-defined hysteresis was observed at several temperatures below the Curie temperature for all the measured samples. The magnetocaloric function of the samples was evaluated from the isothermal magnetization versus field curves, recorded around the Curie temperature. From the static magnetic measurement results, near the respective transition temperatures, the critical behavior of the different samples was carefully investigated. The analysis of the experimental data was carried out by means of various techniques, such as modified Arrott plots, the Kouvel-Fisher method, and critical isotherm analysis. The application of these models allowed determining the values of the critical exponents , , and .