In this thesis, structural, magnetic and electrical properties of the La 0.8-x Y x Sr 0.2 MnO 3 (0?x?0.6) manganites prepared by a solid-state reaction technique was studied systematically. The X-ray diffraction (XRD) shows that at low Y concenteration no impurity peaks are observed (x?0.1), indicating the single-phase formation of the La 0.8-x Y x Sr 0.2 MnO 3 samples with doping limit of x=0.1. The segregation of Y 2 O 3 oxide was observed for x 0.1. The structural parametrs calculated from Rietveld refinements and it was found that the cell volume gradually decreases with increasing Y concentration. The observation by scanning electron microscopy (SEM) reveals a change in the morphology of grains and the porosity of the samples by doping. The ac magnetic susceptibility measurements show that all samples undergo a transition from paramagnetic (PM) to ferromagnetic (FM) phase at Curie temperature, (T c ). By increasing in Y concenteration Curie temperature shifts to lower temperature. This behavior indicates that the substitution of Y weakens the double exchange (DE) process, but an anomaly behavior in Curie temperature is observed in highly doped samples. The temperature dependence of resistivity curves show that all samples have an insulator-metal transition. This transition is in accordance with the evolution to T c values. By increasing the Y doping level, resistivity gradually increased. The metallic region of the (T) curve was fitted with the model of electron-electron and electron-magnon scattering, while the insulating region was fitted with the small polaron hopping (SPH). The application of 1 T magnetic field resistivity of the samples decrease. We conclud that the magnetoresistance in the polycrystalline sample is dominated by traort across grain boundaries that is extremely sensitive to an applied magnetic field.
