Investigation of the magnetic, electronic and magnetocaloric properties of La0.7(Ca,Sr)0.3Mn1-xGdxO3 manganites

La0.7(Ca,Sr)0.3Mn1-xGdxO3 (with x=0; x=2% and x=6%) manganites samples have been studied by several techniques including X-ray diffraction (XRD), Scanning Electron Microscopy (SEM),55Mn and 139La nuclear magnetic resonance (NMR) spectroscopy and magnetometric measurements. The X-ray powder diffraction shows that all the prepared samples are single phase. La0.7Ca0.15Sr0.15MnO3 manganite is crystallized in the rhombohedral structure, whereas a structural transition towards orthorhombic system is observed for x≥ 2%. Magnetization as a function of temperature shows that all samples exhibit a ferromagnetic (FM)-paramagnetic (PM) phase transition. Gadolinium doping in Manganese site was found to induce a strong initial decrease of the Curie temperature TC at moderate Gd concentrations, followed by some levelling off at further doping, possibly due to the development of spatial correlations among the Gd impurities. The single double-exchange-narrowed 55Mn NMR line, though broadened by Gd, demonstrates a metallic state in all samples. The current manganites exhibit a second-order magnetic phase transition at TC, which is also confirmed by the criterion of Banerjee. In addition, the normalized entropy change curves of all the compounds collapse onto a universal master curve, a further signature of a second-order transition. The Gd-doped compounds undergo a large magnetocaloric effect, with an enhanced width TFWHM of the entropy change peak as compared to the Gd-free materials, and have consequently potential applications in magnetic refrigeration