Colossal magnetoresistance in La_0.7Ca_0.3MnO_3-δ: Comparative study of single-crystal and polycrystalline material

The magnetoresistance (MR) and transport properties of both single-crystal and polycrystalline samples of La_0.7Ca_0.3MnO_3-δ (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 °C 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.