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Magnetic excitation spectrum of LuFe2O4 measured with inelastic neutron scattering

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.