Substitutions at the gadolinium and the strontium sites in the RuSr2GdCu2O8 magnetic superconductor

The coexistence of superconductivity and magnetic order seems to take place in the so called ruthenate-cuprates. Previous work has mainly focused on RuSr₂GdCu₂O₈ where a magnetic transition is observed at 133 K and superconductivity has been claimed below T_c ∼46 K.At room pressure, Sm, Eu and Gd seem to be the only RE elements that accept to enter into the structure of RuSr₂GdCu₂O₈. However high pressures and temperatures allows one to replace gadolinium by other lanthanide cations, although it is difficult to get pure samples.In order to better understand this, rather unusual behaviour, we have performed some substitutions in the title compound.The synthesis at high pressures (∼60 kbar) and high temperatures (∼1100 °C) of the title new ruthenates has been achieved and the materials compared to the gadolinium analogue. The structure remains tetragonal (Space group: P 4/ m m m) and the cell parameters linearly decrease following the lanthanide contraction.We have succeeded in replacing Gd by Tb, Dy, Y, Ho and Er, magnetic susceptibility measurements reveal the existence of interesting phenomena. The microstructure of these compounds has been studied by high-resolution electron microscopy.Also, at room pressure, we have replaced up to 10% calcium for strontium and observed that, in consonance with the difference in ionic radii, the unit cell contracts. On the other hand, the calcium-doped sample shows a positive magnetic susceptibility at 5 K.