Abstract
The magnetotransport properties at low temperature in La0.67Sr0.33MnO3 nanoparticles, La0.67Sr0.33MnO3/Al2O3 nanocomposites and ferromagnetic-manganite thick films have been studied. All the samples exhibit low-temperature upturn in resistivity and enhanced low-field magnetoresistance of varying magnitude below the resistivity minima. The voltage dependence of conductance at low temperature for all the samples can be described by a universal scaling function G=G0 exp (V/V0)1/2, where the scaling parameter V0 is sensitive to the magnetic field. The observed magnetotransport properties are explained in a unified treatment incorporating both inter-grain tunneling conduction and thermal activation over the inter-grain potential barrier lowered by the electric field.