Chemical phase separation of superconductive and ferromagnetic domains in ZnNNi_{3 - X}Co_x

Various ZnN_yNi_3-xCo_x compounds with differing Co content, x, were synthesized, and their magnetic properties were investigated. Uniform solid solutions could not be obtained at low Co content (x < 0.75); instead micrometer-scaled ferromagnetic ZnN_yNi _0.6Co_2.4 domains formed embedded within a superconductive ZnNNi₃ bulk, showing chemical phase separation of superconductive ZnNNi₃ and ferromagnetic ZnN_yNi_0.6Co _2.4. At intermediate levels of Co concentration (0.75 < x < 2), this two-phase separation might persist, and the superconductive behavior was strongly suppressed in this composition region. Only at high Co concentration (x > 2) the uniform ferromagnetic solid solution ZnN_yNi _3-xCo_x (with most likely y = 0.5) formed. The phase separation behavior is intrinsic to the system, reflecting the existence of a miscibility gap in ZnN_yNi_3-xCo_x for the samples with x < 2, and was shown not to be attributable to incomplete synthesis. In the two-phased samples, high-quality granular contact between the superconductor and ferromagnet has been realized, suggesting that the production of useful devices requiring high-quality contacts between superconductors and ferromagnets may be possible by making use of this two-phase situation.