Superconductivity, weak ferromagnetism and structural instability in (Eu_1−yGd_y)_2−xCe_xCuO₄

We present a study of the transition from superconductivity to weak ferromagnetism as a function of composition in the systems (Eu1-yRy)2-xCexCuO4 with R = Sm and Gd. T(c) decreases linearly from the Sm- to the Eu-compound und further in the Gd-doped samples up to y = 0.1. At this composition we observe in the same sample both superconductivity and weak ferromagnetism. At higher Gd-contents superconductivity is suppressed and the onset temperature for weak ferromagnetism increases slightly with y. Our results confirm that this transition is related to an instability of the T' structure