  In this study we report the effect of sintering temperature on the low field
magnetotransport properties of La_0.7Ca_0.3Mn_O3 manganite synthesized through
polymeric precursor route. The La_0.7Ca_0.3MnO_3 has been sintered at 600^oC
(S6), 700^oC (S7), 800^oC (S8), 900^oC (S9) and 1000^oC (S10). XRD confirms
that phase formation starts at 600^oC. All the samples are single phasic having
orthorhombic unit cell. The lattice parameters decrease on lowering the
sintering temperature (T_S). The crystallite as well as particle size also show
strong dependence on the sintering temperature. All the samples possess
insulator-metal (T_IM) as well as paramagnetic-ferromagnetic (T_C) transitions.
The T_C shows a small variation [274K (S10) to 256K (S6)] as a function of
sintering temperature whereas the T_IM goes down from 267K (S10) to 138K (S6),
a strong decrease of 129K. 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 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 tunneling at low temperatures.
