Title : Synthesis, structure, morphology, magnetic and magnetocaloric-effect studies of (La1-xPrx)0.7Sr0.3MnO3 nanoparticles perovskites
Single-Phase (La1-xPrx)0.7Sr0.3MnO3 (x= 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) nanoparticles perovskites were synthesized by the sol-gel method followed by sintering at 700 oC for 5 h. The X-Ray diffraction rietveld refinement revealed that samples with x=0.0, 0.2 and 0.4 formed a single-phase rhombohedral structure with R-3C space group while the ones with x=0.6, 0.8 and 1.0 crystallized into orthorhombic structure with Pbnm space group. The TEM and SEM images show that all the samples are nanoparticles with nearly spherical and irregular shapes with different sizes and they tend to aggregate. The average particle size of the series is in the range of 21 to 44 nm. The EDXS spectra confirm the purity of the samples prepared and the estimated ratios of the elements in the samples are very closed to their nominal values within the experimental limits. The XPS measurement confirmed the co-existence of mixed valence state of Mn4+ and Mn3+ in our compounds. Magnetization versus temperatures measurements shows that all samples undergo a second order magnetic phase transition near the Curie temperature (TC). It was observed that the nanoparticles exhibit a ferromagnetic (FM) to paramagnetic (PM) transition at TC which is in an agreement with our results obtained from the Arrott plots. We found that as the Pr concentration increases there is a linear decrease in the Curie temperature from TC = 356 K for x = 0 to 275 K for x = 1.0. For each sample below TC the magnetic coercivity (Hc) is found to be small (~ -10 Oe) while the blocking temperature (TB) decreases with increasing the Pr concentration. Using the isothermal magnetization curves, the magnetic entropy change was obtained using the thermodynamic Maxwell relation. It was found that the magnitude of the magnetic entropy change (-DSM) and the maximum relative cooling power (RCP) for the samples having orthorhombic structure (x = 0.6, 0.8 and 1.0) increases with Pr content reaching a maximum value of 4.67 J/kg.K and 560 J/kg at DµoH = 9 T for x = 1.0. On the other hand the ones with rhombohedral structure (x = 0.0, 0.2 and 0.4) have a maximum (-DSM) and RCP of 4.62 J/kg.K and 485 J/kg at DµoH = 9 T for x = 0.2. Using the Curie-Weiss law, the experimental effective paramagnetic moment (µeff) have been found to increase with increasing the Pr concentration from 3.99 µB for x = 0.0 to 5.05 µB for x = 1.0 which is in an agreement with the trend of the calculated effective paramagnetic moment (4.62 µB for x = 0.0 to 5.50 µB for x = 1.0) using the free ions moments for Pr3+, Mn3+ and Mn4+.