Evolution of the crystal and electronic structures of magnetic RuSr _2-xla_xGdCu₂O₈ superconductors upon la substitution

We have investigated systematically the effects of La substitution on the chemical bonding nature and physical properties of magnetic RuSr _2-xLa_xGdCu₂O₈ superconductors. X-ray diffraction and energy dispersive spectroscopic microprobe analyses reveal that a fraction of Sr ions can be successfully replaced by La ions with the contraction of unit cell volume. According to electrical resistance and dc magnetization measurements, the La substitution gives rise to a significant reduction of superconducting transition temperature (T_c) but to an increase of magnetic ordering temperature with depressed remanent magnetization. Ru K- and Cu K-edge X-ray absorption spectroscopic results clarify that average Ru and Cu oxidation states decrease upon the La substitution. On the basis of the spectroscopic evidences presented here, we are able to attribute the T _c reduction upon the La substitution to the depletion of the hole density in CuO₂ layers and the accompanying variation of magnetic coupling behavior to the change of Ru oxidation state.