Effect of chemical composition on superconductivity and magnetism in layeredruthenocuprates

Abstract

The ruthenium-based layered cupratesRuSr2GdCu2O8 (RuGd1212) can be considered naturally occurring magnetic and superconducting multilayersystems. We have concentrated on the preparation of RuGd1212-type compounds with nominallystoichiometric composition under ambient pressure conditions. For small rare earth ions R = Gd, Eu (and Sm), single phase compounds are obtained with the typical ordered layeredstructure and no significant changes of physical properties. With large rare earth ions (R = Nd, Pr), multiphase samples are obtained. In these cases, noordered layered structure was observed. The effect of substitutingSr2+ with thesmaller Ca2+ and larger Ba2+ is examined. A different number and different types of phasesin equilibrium are found with different alkaline earths (A = Ca, Sr, Ba) atthe nominal RuA2NdCu2O8 composition. The variation in the mismatch of the A/Nd size does not lead to the formation of an orderedlayered RuA2NdCu2O8 compound. Chemical transport in an open system was used to vary the Ru content in theRuGd1212 samples during the annealing step. With an increase of the Ru mass transportto the sample, the composition can be driven beyond the limit of the homogeneity range.Systematic changes in the phase composition of the resulting sample were observed.The magnetic and superconducting transition temperatures vary in a systematicway and are attributed to a variation of the Ru content in the RuR1212 phase.