Interplay of superconductivity and magnetism in oxy-chalcogen cuprates YBaSrCu$_{3}$O$_{x}$Se$_{y}$

In YBa$_{2}$Cu$_{3}$O$_{7}$ superconductors we substituted one atom of Sr for Ba, and simultaneously doped the composition by Se. The resulting substance demonstrates two superconducting transitions: at 34K and 12K. Moreover, at 18K it reveals the Wohlleben effect, jumping from a diamagnetic to paramagnetic response, while keeping the resistance zero. At about 4K, a ferromagnetic state sets up yielding a re-entrance effect, noticeable as an upturn in magnetic susceptibility and, in some samples, in resistivity. This substance behaves very differently from the reported YBaSrCu$_{3}$O$_{7}$ or YBa$_{2}$Cu$_{3}$O$_{x}$Se$_{y}$: simultaneous presence of Se- and Sr-ions yields magnetic moments in the lattice. We present data on DC and AC magnetization, on heat capacity and resistivity in magnetic fields, on crystalline phases, as well as on composition. We also discuss possible mechanisms responsible for the observed effects.