  Inelastic neutron scattering measurements on single crystals of
superconducting BaFe1.84Co0.16As2 reveal a magnetic excitation located at
wavevectors (1/2 1/2 L) in tetragonal notation. On cooling below TC, a clear
resonance peak is observed at this wavevector with an energy of 8.6(0.5) meV,
corresponding to 4.5(0.3) kBTC. This is in good agreement with the canonical
value of 5 kBTC observed in the cuprates. The spectrum shows strong dispersion
in the tetragonal plane but very weak dispersion along the c-axis, indicating
that the magnetic fluctuations are two-dimensional in nature. This is in sharp
contrast to the anisotropic three dimensional spin excitations seen in the
undoped parent compounds.
