  We report neutron inelastic scattering measurements and analysis of the
spectrum of magnons propagating within the Fe2O4 bilayers of LuFe2O4. The
observed spectrum is consistent with six magnetic modes and a single prominent
gap, which is compatible with a single bilayer magnetic unit cell containing
six spins. We model the magnon dispersion by linear spin-wave theory and find
very good agreement with the domain-averaged spectrum of a spin-charge bilayer
superstructure comprising one Fe3+ -rich monolayer and one Fe2+ -rich
monolayer. These findings indicate the existence of polar bilayers in LuFe2O4,
contrary to recent studies that advocate a charge-segregated non-polar bilayer
model. Weak scattering observed below the magnon gap suggests that a fraction
of the bilayers contain other combinations of charged monolayers not included
in the model. Refined values for the dominant exchange interactions are
reported.
