Investigation of the magnetic, electronic and magnetocaloric properties of La 0.7 (Ca,Sr) 0.3 Mn 1-x Gd x O 3 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. The single double-exchange-narowed 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.