Tuning of the superconducting and ferromagnetic behavior by oxygen and hydrogen in Eu_1.5Ce_0.5RuSr₂Cu₂O_10-δ

Eu_1.5Ce_0.5RuSr₂Cu₂O_10-δ is the first known Cu-O based material in which superconductivity (SC) and ferromagnetism (FM) coexist (T_c = 32 K, T_M = 122 K). SC is confined to the CuO₂ planes, and the magnetic ordering is due to the Ru sublattice. Macroscopic magnetization and resistivity measurements are combined with microscopic scanning tunneling spectroscopy (STS) studies. For the as-prepared (ASP) sample, the normal-state resistivity is characteristic of an under-doped SC compound. The hole doping in the CuO₂ planes can be controlled with appropriate variation of the oxygen concentrations, obtained by annealing the ASP sample under various oxygen pressures. The effect of oxygen treatment is to shift both T_c and T_M up to 49 and 225 K, respectively (when annealed under 150 atm). For a sample annealed under 75 atm, two SC transitions are found at 32 and 46 K. On the other hand, when hydrogen atoms are loaded, they occupy interstitial sites and suppress SC and enhance the FM properties of the Ru sublattice. This effect is reversible: namely, by depletion of hydrogen, SC is restored and T_M drops back to its original value. STS measurements show that the ASP sample exhibits a variety of SC gaps, which tend to grow with oxygen content, while hydrogen charging induces phase separation into SC, normal and insulator regions, whose relative abundance depends on the hydrogen concentration.