Static and Dynamic Magnetism in Underdoped Superconductor BaFe1.92Co0.08As2

We report neutron scattering measurements on single crystals of BaFe1.92Co0.08As2. The magnetic Bragg peak intensity is reduced by 6% upon cooling through T-C. The spin dynamics exhibit a gap of 8 meV with anisotropic three-dimensional interactions. Below T-C additional intensity appears at an energy of similar to 4.5(0.5) meV, similar to previous observations of a spin resonance in other Fe-based superconductors. No further gapping of the spin excitations is observed below T-C for energies down to 2 meV. These observations suggest the redistribution of spectral weight from the magnetic Bragg position to a spin resonance, demonstrating the direct competition between static magnetic order and superconductivity.