  Inelastic neutron scattering measurements mapping the in-plane magnetic
interactions of Na0.5CoO2 reveal dispersive excitations at points above an
energy gap Eg = 11.5(5) meV at the superstructural Bragg reflections. The
excitations are highly damped, broadening with increasing energy, and disappear
at hw ~ 35 meV, a strong indication that the magnetism is itinerant. Tilting
into the ac plane reduces the value of Eg by 25%, suggesting that the
dispersion along c is significant and the magnetic correlations are
three-dimensional, as seen at the higher doping levels.
