Tuning the superconducting and magnetic properties in Fe_ySe_0.25Te_0.75 by varying the Fe-content

The superconducting and magnetic properties of Fe${}_y$Se${}_{0.25}$Te${}_{0.75}$ single crystals ($0.9\le y\le 1.1$) were studied by means of x-ray diffraction, SQUID magnetometry, muon spin rotation, and elastic neutron diffraction. The samples with $y<1$ exhibit coexistence of bulk superconductivity and incommensurate magnetism. The magnetic order remains incommensurate for $y\ge 1$, but with increasing Fe content superconductivity is suppressed and the magnetic correlation length increases. The results show that the superconducting and the magnetic properties of the Fe${}_y$Se${}_{1-x}$Te${}_x$ can be tuned not only by varying the Se/Te ratio but also by changing the Fe content.