Structural analysis, magnetic and transport properties of the (Ru1-xCox)Sr2GdCu2O8 system

The effects of Co substitution on the structural and superconducting properties of the RuSr₂GdCu₂O₈ compound have been studied. Rietveld refinements of the x-ray diffraction patterns indicate that the cobalt ion progressively replaces ruthenium sites. This replacement induces significant changes on the crystal structure and on the magnetic and superconducting properties. The effects of Co substitution on the superconducting behaviour, and more particularly on the changes induced by the hole doping mechanism, were investigated in (Ru_1-xCo_x)Sr₂GdCu₂O₈ by a 'bond valence sum' analysis with Co content from x = 0.0 to 0.2. The weak ferromagnetic transition at T_M = 138.2 K is shifted to lower temperature, and suppressed at higher Co content. From the crystallographic point of view the Ru-O(1)-Cu bond angle, associated to the rotation of the RuO₆ octahedra around the c-axis, remains essentially constant when Ru is substituted by Co. Furthermore, increasing Co content has the effect of increasing the weak ferromagnetic moment, which may be interpreted as the main feature responsible for breaking the delicate balance between magnetic and superconducting ordering.