Coexistence of incommensurate magnetism and superconductivity inFe1+ySexTe1−x

We have studied the superconducting and magnetic properties of ${\text{Fe}}_{1+y}{\text{Se}}_{x}{\text{Te}}_{1\ensuremath{-}x}$ single crystals $(0\ensuremath{\le}x\ensuremath{\le}0.5)$ by magnetic susceptibility, muon-spin rotation, and neutron diffraction. We find three regimes of behavior: (i) commensurate magnetic order for $x\ensuremath{\lesssim}0.1$, (ii) bulk superconductivity for $x\ensuremath{\sim}0.5$, and (iii) a range $x\ensuremath{\approx}0.25--0.45$ in which superconductivity coexists with incommensurate magnetic order. The results are qualitatively consistent with two-band mean-field models in which itinerant magnetism and extended $s$-wave superconductivity are competing order parameters.