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.
