Transport properties and magnetoresistance effect of nano-polycrystalline La_(0.7)Sr_(0.3)MnO_(3-δ)

The resistivity of bulk polycrystalline La_(0.7)Sr_(0.3)MnO_(3-δ)(LSM),deliberately fabricated to produce nanoscale grains sintered at different temperatures,was investigated as a function of temperature.With temperature decreasing below room temperature the resistivity (ρ) exhibited a maximum near 250 K below which it displayed"metallic"behaviour,i.e.ρ∝T~2 at 50～170 K.However,ρsubsequently exhibited a minimum near 50 K,below which it well fit the predictions for the tunneling of conduction electrons through insulating interfacial layers viz.inρ∝T~(-1/2),a result interpreted as conduction-electron tunneling between adjacent LSM granules.The ~(55)Mn zero field spin-echo nuclear magnetic resonance spectra indicated that the insulating barrier existed in LSM.The minimum ofρoccuring in these samples may have resulted from the competition of the tunneling effect and the intrinsic metallic transport mechanism of LSM.