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Using neutron inelastic scattering, we investigate the role of interstitialiron on the low-energy spin fluctuations in powder samples ofFe_{1+x}Te_{0.7}Se_{0.3}. We demonstrate how combining the principle ofdetailed balance along with measurements at several temperatures allows us tosubtract both temperature-independent and phonon backgrounds from S(Q,\omega)to obtain purely magnetic scattering. For small values of interstitial iron(x=0.009(3)), the sample is superconducting (T_{c}=14 K) and displays a spingap of 7 meV peaked in momentum at wave vector q_{0}=(\pi,\pi) consistent withsingle crystal results. On populating the interstitial iron sites, thesuperconducting volume fraction decreases and we observe a filling in of thelow-energy magnetic fluctuations and a decrease of the characteristic wavevector of the magnetic fluctuations. For large concentrations of interstitialiron (x=0.048(2)) where the superconducting volume fraction is minimal, weobserve the presence of gapless spin fluctuations at a wave vector ofq_{0}=(\pi,0). We estimate the absolute total moment for the various samplesand find that the amount of interstitial iron does not change the totalmagnetic spectral weight significantly, but rather has the effect of shiftingthe spectral weight in Q and energy. These results show that thesuperconducting and magnetic properties can be tuned by doping small amounts ofiron and are suggestive that interstitial iron concentration is also acontrolling dopant in the Fe_{1+x}Te_{1-y}Se_{y} phase diagram in addition tothe Te/Se ratio.

Spin fluctuations and superconductivity in powders of Fe_1+xTe_0.7Se_0.3 as a function of interstitial iron concentration