Effect of sintering temperature and grain size on low field magnetoresistance of La0.70 Ca0.20 Sr0.05 Ba0.05 MnO3

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.