Effect of grain connectivity and density on the magnetoresistance in Ca or Li doped lanthanum manganites

The effect of grain connectivity on the magnetoresistance (MR) was studied on porous and highly dense polycrystalline bulk samples of La0.7Ca0.3MnO3 and La0.8Li0.2MnO3 synthesised by conventional solid state reaction and partial melting technique. SEM and XRD investigations demonstrated that the partial-melted samples were very dense with large grain sizes whereas the porous sample contained large numbers of small sized grains. The porous La0.7Ca0.3MnO3 and highly dense La0.8Li0.2MnO3 samples exhibited a very wide transition width in resistance vs. temperature (R−T) plots taken in magnetic fields of 0.3 or 1 T over a wide temperature range from 77 to 235 K. In contrast, the partially melted La0.7Ca0.3MnO3 sample only showed a very sharp transition around the ferromagnetic transition at 235 K in the R−T curve. It is proposed that it is the grain connectivity and not the density of the samples that is responsible for the transition width in R−T.