Direct interplay between superconductivity and ferromagnetism in Fe_1+y(Te_0.5Se_0.5)

Two ferromagnetic phases have been identi fied in the Fe-excess Fe_1+y(Te_0.5Se_0.5) compounds, where the nonstoichiometric Fe ions occupy the interstitial sites that are slightly below the center positions of the Se square sublattice. The low-temperature magnetic phase, which coexists with superconductivity, consists the ordering of the lattice as well as the excess Fe ions. Thermal loosening of the superconducting pairs allows the electrons to flow from the superconducting layers to the interstitial Fe sites, which alters the numbers of Fe²⁺ ion on the lattice and on the excess sites. It is this valence change of the magnetic sites that drives the system evolves into its magnetic phase in the normal state. Existence of direct interplays between superconductivity and ferromagnetism in Fe_1+y(Te_0.5Se_0.5) is demonstrated.