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–δ, La0.7Sr0.3MnO3–δ and La0.7Pb0.3MnO3–δ) showing little lower metal‐insulator transition temperature (Tp) 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 TP 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‐grain 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*)n), 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 grain) 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. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)