Intraplanar Magnetic Excitations in Na0.5CoO2: An Inelastic Neutron   Study
  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. 
J. Wooldridge, G. Balakrishnan, D. McK. Paul, C. Frost, P. Bourges and   M. R. Lees