Comparison of magnetoresistance in various divalent doped LaMnO3

Abstract

Polycrystalline Lal_xAxMn03, where A=Ca, Sr, and Ba, x=0.20 and 0.15, were prepared by standard solid state reaction technique. High purity powders of LaZ03 (99.995%), CaC03 (99.9%), SrC03 (99.9%), BaC03 (99.9%) and MnC03 (99.9%). were mixed thoroughly in an appropriate amount. Mixing was performed in both dry and acetone media. The mixed powders were calcined at high temperature (l050°C) for 24 hours. After calcination pellets were prepared and sintered at 1300°C, 1400°C and 1500°C for 5h in air. X-ray diffraction was carried out to check the sample homogeneity and status of the samples. Microstructural properties and average grain size of the sample. were investigated from high-resolution optical microscopic picture. It was observed that the average grain size and density increases as the sintering temperature increases. The DC electrical resistivity of the samples were measured by the standard four-point probe and van der Paw technique from room temperature down to liquid nitrogen temperature in zero applied field and in presence of magnetic field. A liquid nitrogen cryostat and an electromagnet were used for this magnetoresistance measurement. Magnetoresistance as a function of applied magnetic field (0- 0.86 T) at 17K and at room temperature for all samples were compared. It was found that the resistivity of the sample decreases an order of magnitude as the sintering temperature increases. Most of the samples show semiconductor-metal transition at certain temperature, Tsm. It was fond that the Tsm increases as the average ionic radius of the A-site cation increases. For a fixed composition MR is higher for smaller grain size sample. A homemade ball-milling machine is designed and constructed.