Friedel-like oscillations from interstitial Iron in superconducting Fe _1+yTe _0.62Se _0.38

Using polarized and unpolarized neutron scattering, we show that interstitial Fe in superconducting Fe _1+yTe _1-xSe _x induces a magnetic Friedel-like oscillation that diffracts at Q=(120) and involves >50 neighboring Fe sites. The interstitial >2μ _B moment is surrounded by compensating ferromagnetic four-spin clusters that may seed double stripe ordering in Fe _1+yTe. A semimetallic five-band model with (1212) Fermi surface nesting and fourfold symmetric superexchange between interstitial Fe and two in-plane nearest neighbors largely accounts for the observed diffraction.