Effect of process conditions and NiO on magnetoresistance of La-(Ca,Ba)-Mn-O composites

La_2/3(Ca_0.6Ba_0.4)_1/3MnO₃/XNiO heterogenous composites have been synthesized by modified sol-gel method. Resistivity versus temperature in difference magnetic fields for the samples, as well as magnetoresistance (MR) versus magnetic in room temperature, have been measured. It is indicated that the addition of NiO results to the increase in resistivity and the weakeness in magnetization. Enhanced MR has been obtained in NiO-doped composites. The temperature dependence of magenetoresistance for NiO-doped composites in low magnetic field may be explained by the model of spin-polarized tunneling. The vary trend of MR in MR-T curve is similar to the result obtained by Monte Carlo simulation. MR in high magnetic field may be explained by spin polarization inside ferromagnetic grains.