Crystal structure and the effect of oxygen on the superconductivity and weak magnetism in RE_1.5Ce_0.5RuSr₂Cu₂ O₁₀ (RE = Eu, Gd)

The superconducting tetragonal RE_1.5Ce_0.5RuSr₂Cu₂ O₁₀ (RE = Eu, Gd) system is the first known Cu-O based material in which superconductivity (SC) and weak-ferromagnetism (W-FM) coexists at T_M > T_c (T_c = 32, 42 K, and T_M = 122, 180 K for RE = Eu and Gd respectively). Superconductivity is confined to the CuO₂ planes, and the magnetic ordering is due to the Ru sublattice. The as-prepared sample (ASP), is an under-doped SC compound. The hole doping in the CuO₂ planes can be controlled with appropriate variation of oxygen concentrations, obtained by annealing the ASP sample under various oxygen pressures. Oxygen treatment (for RE = Gd) shifts both T_c and T_M (T_c = 52 K for RE = Gd at 600 atm). On the other hand, quenching the ASP sample to ambient temperature, suppresses SC and enhances the W-FM properties of the Ru sublattice. X-ray diffraction studies performed at various low temperatures, indicate that the tetragonal structure is preserved (down to 10 K), and no anomalies in the a and c lattice parameters are observed at T_c and T_M.