  Spin excitations are one of the top candidates for mediating electron pairing
in unconventional superconductors. Their coupling to superconductivity is
evident in a large number of systems, by the observation of an abrupt
redistribution of magnetic spectral weight at the superconducting transition
temperature, Tc, for energies comparable to the superconducting gap. Here we
report inelastic neutron scattering measurements on Fe-based superconductors,
Fe1-x (Ni/Cu)x Te0.5 Se0.5, that emphasize an additional signature. The overall
shape of the low energy magnetic dispersion changes from two incommensurate
vertical columns at T >> Tc to a distinctly different U-shaped dispersion at
low temperature. Importantly, this spectral reconstruction is apparent for
temperature up to ~3Tc. If the magnetic excitations are involved in the pairing
mechanism, their surprising modification on the approach to Tc demonstrates
that strong interactions are involved.
