Abstract:
The Lal.,.vCa,SryMnO) powders with various x and y values were synthesized
using a standard ceramic technique. Powders of LazO) (Aldrich 99.99%), CaC03 (BOH
999%), SrC01 (BOI-I 99.9%) and MnzO) (Aldrich 99.9%) were used to make the
required composition. The ingredients were weighed with appropriate proportions and
then mixed thoroughly in dry and acetone media. Four different compositions were
prepared for different x and y values keeping their sum 0.30. The mixed powders were
calcined at 950°C for 24 hours. After calcination powders were pressed in the form of
pellets. The pellets of four different compositions were sintered at 1200°C for various
dwell times (60h, 40h, 20h and 12h). X-ray diffraction was carried out in the powder
sample for structural characterization. Electrical resistance were measured in each
sample in zero field and in presence applied magnetic field (0.60 T) in the temperature
range 295-77K. It was observed that all samples show metal-insulator transition at
certain temperature (Tp). Metal-insulator transition temperature varies for different
compositions. It was observed that as the Sr concentration increases in the
polycrystalline LaJ.,.yCa,SryMnO).o samples the 1;' increases. Magnetoresistance (MR) as
a function field, MR(H), at constant temperatures (77 K and room temperature) were
measured. All samples show non-trivial MR at RT and at 77K. It was found that the MR
at 77K is higher for the sample sintered for lower dwell time. For all samples MR at 77K
showed two slopes. A sharp decrease of resistance is observed up to applied field H'.
Beyond H' resistance has a weak dependence in applied field. The sharp decrease of
resistance is due to the domain wall movement and the weak dependence is related to the
grain boundaries of the polycrystalline samples. The LU(17oCaoIOSrzoMn03-<i.sample
sintered at 1200°C for 40h shows 3% MR at RT in presence of 0.61' applied field. The
polycrystalline sample of nominal chemical composition Lao.7oCao.IOSr20MnO)-a<lsio has
highest 1;' (279K).
