Colossal magnetoresistance in La0.7Ca0.3MnO3-δ:: Comparative study of single-crystal and polycrystalline material

The magnetoresistance (MR) and transport properties of both single-crystal and polycrystalline samples of La0.7Ca0.3MnO3-delta (LCMO) have been investigated to elucidate their magneto transport behaviors near and below the Curie temperature T-C. We find that negative MR is heavily dependent upon the surface morphology in the low-temperature, ferromagnetic regime. The polycrystalline sample postannealed at 1400 degreesC under the oxygen atmosphere exhibits large low-field MR in the whole temperature range below T-C, which is about 20% at 0.5 T. On the other hand, the single-crystal sample shows the remarkable suppression of resistivity only at the narrow temperature region near T-C. We suggest that the MR found in the polycrystalline sample is dominated by the tunneling of spin-polarized electrons between grains below T-C while the MR in the single crystal is mainly ascribed to the suppression of spin fluctuation. The difference in the MR property between the single-crystal and polycrystalline samples is discussed in terms of oxygen inhomogeneity and granular effects affected by interfacial melting.