Structural analysis, magnetic and transport properties of the (Ru_1−xCo_x)Sr₂GdCu₂O₈system

Abstract

The effects of Co substitution on the structural and superconducting properties of the RuSr 2 GdCu 2 O 8 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−x Co x )Sr 2 GdCu 2 O 8 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 RuO 6 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.