Magnetoresistive properties of Ni-doped La0.7Sr0.3MnO3 manganites

La0.7Sr0.3Mn1-xNixO3（x = 0, 0.025, 0.050 and 0.075） ceramics were prepared by the conventional solid-state reaction method. The partial substitution of Mn by Ni2+ leads to a decrease in cell volume as well as a structural transition from the rhombohedral to the orthorhombic structure. Ni2+ doping increases the electrical resistivity, decreases the semiconductor–metal transition temperature（Tms） and relatively enhances the room temperature magnetoresistance（MR）, especially in x = 0.025 and around Tms. With respect to conduction mechanism, the small polaron hopping（SPH） and the variable range hopping（VRH） models were used to examine conduction in the semiconducting region.