In this thesis, We report on the fabrication and characterization of La 0.5 Ca 0.5 MnO 3 (LCMO) with different grain size from nanometer up to micrometer. The samples have been prepared by new solid state method and their structure, magnetic and electric properties have been investigated. In the second part of this project, the effect of cation size on the structure, magnetic and electrical properties of La 0.5 Ca 1-x Sr x MnO 3 (LCSMO) (0?x?0.5) which is prepared at temperature 1350 o C have been studied. X-ray diffraction of LCMO samples, hows that grain size increases with increase of sintering temperature without any variation in structure. The structural parameters and space groups have been obtained by FULLPROF soft ware, Rietveld refinement. It shows that all the LCMO samples have orthorhombic structure with pnma space group and by increasing of sintering temperature unit cell volume decreased. SEM images show that, grain sizes have been converted from anometer to micrometer with increase of sintering temperature. The ac magnetic susceptibility measurements showed that all LCMO samples have a PM-FM transition and with increase in sintering temperature, the charge ordering and AFM phases were observed. Curie temperature T c , decreases by growing of crystallite size and suppressed FM phase and induced robust CO and AFM phases at the low temperature. Samples have thermal hysteresis because of coexistence of the FM and AFM phases and phase separation, which is a characteristic of first order transition. All LCMO samples have a insulator behavior. Rietveld refinement for LCSMO samples shows that all compounds with x?0.3 crystallize in the orthorhombic structure, with space group of pnma and x=0.4 and x=0.5 compounds have tetragonal structure with space group of 14/mcm. This structural variation is due to substitution of Ca by Sr and unit cell volume increase with Sr 2+ doping. All the LCSMO samples have a PM-FM and FM-AFM transition that with increase of Sr 2+ doping AFM phase decreased and FM phase increased and consequently thermal hysteresis has been decreased. All compounds with x?0.4 have insulator behavior and by increase of Sr 2+ doping, resistance decreased. Sample x=0.5 shows metal- insulator transition. Kay words: Manganite, Thermal hysteresis, ac susceptibility, Sr doping, Charge ordering, FM, AFM.
