  Inelastic neutron scattering measurements have been performed on underdoped
Ba(Fe1-xCox)2As2 (x = 4.7%) where superconductivity and long-range
antiferromagnetic (AFM) order coexist. The broad magnetic spectrum found in the
normal state develops into a magnetic resonance feature below TC that has
appreciable dispersion along c-axis with a bandwidth of 3-4 meV. This is in
contrast to the optimally doped x = 8.0% composition, with no long-range AFM
order, where the resonance exhibits a much weaker dispersion [see Lumsden et
al. Phys. Rev. Lett. 102, 107005 (2009)]. The results suggest that the
resonance dispersion arises from interlayer spin correlations present in the
AFM ordered state.
