Abstract:
The Lal_x_y_,CaxSryBa,MnO, for various x, y and z (x=0.20,y=0.05, z=0.05; x=0.15,
y=0.10, z=0.05; x=0_15, y=0.05, z=O.IO and x=O.IO, y=0.1O, z=O.IO) powders were synthesised
using a standard solid state reaction technique. Powders of La,O, (Aldrich 99.99%), CaCO,
(BDH 99.9%), srCo, (BDH 99.9%), BaCO, (BDH 99.9%) and MnO, (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. The mixed powders were
calcined at 950°C for 6 h. This step was repeated tJu:eemore times with intermediate grindings to
obtain a homogeneous and phase pure composition. fJler a total period of 24 h of calcination,
powders were pressed in the fonn of pellets. The pellets were sintered at 1200°C. X-ray
diffraction analysis was carried out in the powder santple for structural characterisation.
Electrical resistance was measured in each sample in zero field and in presence applied magnetic
field (0.6 nin the temperature range 295-77K. This experiment was carried out using a locally
fabricated lisuid nitrogen cryostat and a electromagnet. Magnetoresistance (MR) as a function of
field, MR(H), at constant temperatures (77 K and 295 K) were reported.
The Lao.,Cao.,Sro.o,Baoo,MnO, powders were also synthesised using a standard ,solid state
reaction technique. Powders of La,O, (PI-Kern 99.99%), CaCO, (PI-Kem 99.9%), srCo, (PI-
'i"" ---S" Kern 99.9%), BaCO, (PI-Kern 99.9%) and MnCO, (pI-Kern 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. The mixed powders were calcined at 1050°C for 24 .
h. After calcination, powders were pressed in the fonn of pellets. The pellets were sintered at
1500°C, 1400°C and BOOoe in air. X-ray diffiaction analysis was carried out in the powder
sample for structural characterisation. Microstructures of the samples were investigated using a
high resolving optical microscope. It was obserVed that grain size and density of the sample
mcreases as the sintering temperature increase, On the other hand porosity of the sample
decreases as the sintering temperature increases. Electrical resistance was measured in each
sample in zero field and in presence applied magnetic field (0.7 T) in the temperature range
295-77 K with a cooling rate at about I K. All samples show a metal-Insulator transition at
certain temperature. Resistivity of the santple decreases as grain sIze increases.
Magnetoresistance (MR) as a function of magnetic field, MR(H), at constant temperatures
(77 K and room temperature) were compared for all samples. MR sensitivity is highest for
smaller grain size sample.
