Superconductivity in Magnetically Ordered Ru_1-xIr _xSr₂GdCu₂O₈ Compounds

We have performed a systematic study of the structural, transport, and magnetic properties of polycrystalline samples of the magnetic superconductors Ru_1-xIr_xSr₂GdCu₂O₈; x = 0, 0.02, 0.05, 0.10, and 0.15. The samples were prepared by solid state reaction and sintered at 1060 °C for 72 h under O₂ flow. X-ray diffraction analysis shows that all samples are nearly single phase and that the lattice parameters are independent of Ir content. Transport properties measurements revealed that the Ru substitution by Ir results in a decrease of T_c,onset from ∼ 50 K (x = 0) to ∼ 30 K (x = 0.10). Further addition of Ir (x > 0.10) causes an evolution from metallic to nonmetallic behavior of p(T). We have also found that magnetic order develops in the undoped Ru-1212 materials near T_M ∼ 130 K. This temperature decreases linearly with increasing Ir content at the rate of ∼ -1.6 K / Ir at.%, suggesting that Ir effectively substitutes Ru in the RuO planes. A subtle drop in ρ(T) is observed close to T_M, probably due to the suppression of the spin-flip scattering. The magnetoresistivity measurements revealed that the temperature T_c,zero, in which ρ(T) ∼ 0, decreases rapidly for low applied magnetic fields (H < 2 T), and that this drop becomes much less pronounced in higher magnetic fields (2 ≤ H ≤ 18 T). The appreciable broadening of p(T) curves at low magnetic fields is reminiscent of the behavior in high-T_c materials showing granular behavior, as for example in Sm_1.85Ce_0.15CuO_4-v.