Microstructure-magnetotransport correlation in La0.7Ca0.3MnO3

Microstructure-magnetotransport correlation has been studied in polycrystalline La0.7Ca0.3MnO3 having varying grain size. Single phase samples were synthesized through the polymeric precursor route and grain size has been varied by heat treatment of the precursor in the temperature range of 500-1300 °C. Larger grain size has resulted in a reduction of the orthorhombicity caused possibly by decrease in intragranular strain. PM-FM phase transition sets on around ∼270 K; the mid point TC being ∼256, 260, 262 and 265 K, respectively, for samples having grain size ∼25, 40, 100 and 700 nm. The IM transition temperature, despite being suppressed in small grain samples equals to TC in the largest grain samples. Grain size variation from ∼25 to ∼700 nm causes decrease in resistivity by more than two orders of magnitude. The low field magnetoresistance (LFMR) is observed to be strongly correlated with the microstructural/transport parameters such as the grain size and the resistivity. In contrast to the generally observed results, the LFMR increases with grain size and the maximum LFMR is obtained when the grain size is ∼100 nm. We propose that for every composition there exist an optimum grain size at which the LFMR reaches a maximum. This optimal grain size may vary from sample to sample and may also depend on the synthesis technique.