Colossal magnetoresistance study in nanophasic La0.7Cao0.3MnO3 manganite

In this study we report the effect of sintering temperature on the low field magnetotransport properties of La0.7Ca0.3MnO3 manganite synthesized through the polymeric precursor route. The La0.7Ca0.3MnO3 has been sintered at 600 degrees C (S6), 700 degrees C (S7), 800 degrees C (S8), 900 degrees C (S9) and 1000 degrees C (S10). X-ray diffraction confirms that phase formation starts at 600 degrees C. All the samples are single phasic having an orthorhombic unit cell. The lattice parameters decrease on lowering the sintering temperature (T-S). The crystallite as well as the grain size also show strong dependence on the sintering temperature. All the samples possess characteristic insulator-metal (T-IM) as well as paramagnetic-ferromagnetic (PM-FM) (T-C) transitions. The TC varies in a small range [272-256 K] as a function of sintering temperature whereas the T-IM goes down from 267 K (S10) to 138 K (S6), a strong decrease of 129 K. This T-C-T-IM discrepancy is due to the fact that whereas the former is an intrinsic characteristic, the latter depends strongly on the extrinsic factors e.g. synthesis conditions, grain boundaries and associated disorders. For all the samples magnetoresistance (MR) shows strong dependence on T-S. The MR increases on lowering the temperature as well as on increasing the field with the occurrence of an intrinsic contribution around T-C. These variations of MR for all the samples have been explained in terms of the microstructural variations and spin-polarized tunnelling at low temperatures.