Effect of chemical composition on superconductivity and magnetism in layered ruthenocuprates

The ruthenium-based layered cuprates RuSr2GdCu2O8 (RuGd1212) can be considered naturally occurring magnetic and superconducting multilayer systems. We have concentrated on the preparation of RuGd1212-type compounds with nominally stoichiometric composition under ambient pressure conditions. For small rare earth ions R = Gd, Eu (and Sm), single phase compounds are obtained with the typical ordered layered structure and no significant changes of physical properties. With large rare earth ions (R = Nd, Pr), multiphase samples are obtained. In these cases, no ordered layered structure was observed. The effect of substituting Sr2+ with the smaller Ca2+ and larger Ba2+ is examined. A different number and different types of phases in equilibrium are found with different alkaline earths (A = Ca, Sr, Ba) at the nominal RuA(2)NdCu(2)O(8) composition. The variation in the mismatch of the A/Nd size does not lead to the formation of an ordered layered RuA(2)NdCu(2)O(8) compound. Chemical transport in an open system was used to vary the Ru content in the RuGd1212 samples during the annealing step. With an increase of the Ru mass transport to 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 systematic way and are attributed to a variation of the Ru content in the RuR1212 phase.