Magnetoresistance of sol-gel derived manganite nanoparticles

We report the thermal variation of magnetoresistance of sol-gel derived nanocrystalline manganite samples (viz. La0.5Pb0.5MnO3-delta, 6, La0.7Sr0.3MnO3-delta,and La0.7Pb0.3MnO3-delta) showing little lower metal-insulator transition temperature (T-p) compared to those prepared by ceramic heating route. This is considered to be. due to the, grain boundaries and smaller grain size (15-20 nm) of the sol-gel derived samples. A maximum magnetoresistance (MR) of 13 - 17% at 0.5 T-1.15 T magnetic field was observed. The transport behavior in the low temperature metallic regime is primarily contributed by the electron-magnon scattering mechanism. The semiconducting property above T-P followed Mott's variable-range hopping mechanism at lower T and small polaron hopping mechanism at higher T. Unlike MR of many large grain samples, the MR vs. temperature curve of the present nano-gram samples did not show any peak and the corresponding MR data fitted well with an expression of the form MR = A + B/(1 + (T/T*)?), where A and B are constants, T* is a characteristic temperature of the system and n is a measure of the strength of competition between the intrinsic (arising from spin dependent part within the a-rain) and the extrinsic (due to spin polarized tunneling between the grain boundaries) MR components. For n = 1 the system follows spin polarized tunneling. Many other nanograin samples were found also to follow this relation. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.