Structural, magnetic and transport properties of Ca and Sr doped Lanthanum manganites

This article presents a comparative study for the effect of average A-site cation size on the structural, transport and magnetic properties of Lanthanum manganites. Three polycrystalline colossal magneto-resistive compounds were synthesized using standard solid state reaction method. The electrical resistivity data were analyzed employing standard two-phase model to understand the conduction mechanism. The resistance of polycrystalline ceramic depends on the intragrain resistance (intrinsic resistance) and the intergrain or grain-boundary resistance (extrinsic resistance). The substitution of Sr ions at La-site provides higher magnetic and metal-insulator transitions as compared to Ca ions. The combined substitution of Ca and Sr ions at La-site offers nearby room temperature magnetic and metal-insulator transitions. Irreversibility in the temperature dependent DC magnetization is observed in the zerofield-cooled and field-cooled measurements. It is noticed that the larger average radius of the A-site cations possesses higher magnetic and metal-insulator transition temperatures. Temperature dependent thermoelectric power curves show a hump like behavior, which indicates a smooth transition from the low-temperature metallic behavior to high-temperature semiconductor-like behavior.