  We report neutron scattering measurements of cooperative spin excitations in
antiferromagnetically ordered BaFe2As2, the parent phase of an iron pnictide
superconductor. The data extend up to ~100meV and show that the spin excitation
spectrum is sharp and highly dispersive. By fitting the spectrum to a linear
spin-wave model we estimate the magnon bandwidth to be in the region of 0.17eV.
The large characteristic spin fluctuation energy suggests that magnetism could
play a role in the formation of the superconducting state.
