Magnetic and intergranular transport properties in manganite/alumina composites

We report the magnetic and transport properties of La2/3Ca1/3MnO3/Al2O3 crystal composites at low insulator component ratio. Phase purity and microstructure are guaranteed by X-ray and microanalysis studies. DC electric conduction is progressive blocked by scattering in alumina regions. With an insulator percentage around 10% in volume, percolation occurs. Merging the spin polarization of the manganite and the extra disorder contribution to electric transport induced by alumina we have been able to increase the low temperature magnetoresistance (MR) nearly three times with respect to the undoped manganese perovskite. Experimental results are analyzed on the basis of an earlier theory developed for a granular ferromagnetic material in an insulator matrix.