Excess-iron driven spin glass phase in Fe_1+yTe_1-xSe_x

The iron-chalcogenide superconductor FeTe_1-xSe_x displays a variety of exotic features distinct from iron pnictides. Although much effort has been devoted to understanding the interplay between magnetism and superconductivity near x=0.5, the existence of a spin glass phase with short-range magnetic order in the doping range (x～0.1-0.3) has rarely been studied. Here, we use DC/AC magnetization and (quasi) elastic neutron scattering to confirm the spin-glass nature of the short-range magnetic order in a Fe_1.07Te_0.8Se_0.2 sample. The AC-frequency dependent spin-freezing temperature T_f generates a frequency sensitivity ΔT_f(ω) /T_f(ω) Δlog₁₀ω≈0.028 and the description of the critical slowing down with τ=τ₀(T_f / T_SG)^-zv gives T_SG≈22 K and zv≈10, comparable to that of a classical spin-glass system. We have also extended the frequency-dependent T_f to the smaller time scale using energy-resolution-dependent neutron diffraction measurements, in which the T_N of the short-range magnetic order increases systematically with increasing energy resolution. By removing the excess iron through annealing in oxygen, the spin-freezing behavior disappears, and bulk superconductivity is realized. Thus, the excess Fe is the driving force for the formation of the spin-glass phase detrimental to bulk superconductivity.