Transport properties and spin correlations of La_1.85-xSr_0.15+xCu_1-xFe_xO₄

A series of double-doped La1.85-xSr0.15+xCu1-xFexO4 (0less than or equal toxless than or equal to1) ceramic samples were prepared by the solid-state reaction method. The structure, transport properties, and spin correlations were studied by means of x-ray diffraction, resistivity, thermoelectric power (TEP) and electron spin resonance (ESR). Double doping causes an increase of the lattice parameter a and a decrease of c. A great discrepancy of the thermal activation energy is derived from the resistivity and TEP at high doping levels, which can be understood in terms of a polaron model. The ESR result indicates that the double doping influences the spin correlation of the CuO2 plane markedly, which suggests that the Fe3+ spins change from localization to delocalization as x increases from low doping to high doping. A close relation between localized spin scattering and the broad peak in the TEP versus T curve is proposed. A comparison of the transport mechanism between the Fe doped and the Co and Ni doped La214 systems is given in this paper.