Evidence for local moment magnetism in superconducting FeTe$_{0.35}$Se$_{0.65}$

We investigate the temperature evolution (from 5~K to 300~K) of low energy spin fluctuations in Fe-based superconductor FeTe$_{0.35}$Se$_{0.65}$ (T$_c \sim 14$~K) via inelastic neutron scattering. The magnetic excitation spectrum in the superconducting phase appears qualitatively similar to those observed in other Fe-based superconductors, with a spin gap (at about 5~meV) and a resonance peak at $\hbar\omega\sim6.5$~meV. At higher temperatures, the spectral weight of the low-temperature resonance is found to redistribute to lower energies below the spin gap. A significant moment ($\agt 0.26 \mu_B/$Fe) is found for the integrated spectral weight below merely $\hbar\omega\sim12$~meV, with nearly no temperature dependence up to 300K, indicating existence of strong local moments.