  We have used inelastic neutron scattering and muon-spin rotation to compare
the low energy magnetic excitations in single crystals of superconducting
Fe1.01Se0.50Te0.50 and non-superconducting Fe1.10Se0.25Te0.75. We confirm the
existence of a spin resonance in the superconducting phase of
Fe1.01Se0.50Te0.50, at an energy of 7 meV and a wavevector of (1/2,1/2,0). The
non-superconducting sample exhibits two incommensurate magnetic excitations at
(1/2,1/2,0)\pm(0.18,-0.18,0) which rise steeply in energy, but no resonance is
observed at low energies. A strongly dispersive low-energy magnetic excitation
is also observed in Fe1.10Se0.25Te0.75 close to the commensurate
antiferromagnetic ordering wavevector (1/2-\delta,0,1/2) where \delta \approx
0.03. The magnetic correlations in both samples are found to be quasi-two
dimensional in character and persist well above the magnetic
(Fe1.10Se0.25Te0.75) and superconducting (Fe1.01Se0.50Te0.50) transition
temperatures.
