The influence of Gd on magnetic and electric transport properties in La0.67-xGdxSr0.33CoO3

The magnetic and transport properties of the polycrystalline samples La0.67-xGdxSr0.33CoO3 (0 ≤ x ≤ 0.67) are investigated. On incorporation of Gd, the Curie temperature TC progressively shifts to lower temperature, and the magnetization increases rapidly below 60 K with decreasing temperature. This increase of magnetization can be attributed to the paramagnetism of the Gd sublattice. However, in the system La0.7-xGdxSr0.3MnO3 the Gd sublattice forms a ferromagnetic structure at high-doping level, and the magnetic moments of the Gd sublattice are antiparallel to those of the Mn sublattice. The resistivity curves show a metal-semiconductor transition upon doping with Gd, and x = 0.40 is the critical doping point. The experimental resistivity can be fitted by the expression ρ = ρ0+ρ2T2 for metallic samples below TC. In the metallic samples (x < 0.4) the magnetoresistance (MR) peak is near the Curie temperature, and the MR occurs over a large temperature range. With decreasing temperature, the peak is reduced and the MR range is narrowed. For insulating compositions (x>0.4) the MR peak near TC disappears and the low-temperature MR is greatly enhanced