Investigation of the presence of charge order in magnetite by measurement of the spin wave spectrum

Inelastic neutron scattering results on magnetite (Fe₃O₄) show a large splitting in the acoustic spin wave branch, producing a 7 meV gap midway to the Brillouin zone boundary at q=(0,0,1∕2) and ℏω=43 meV. The splitting occurs below the Verwey transition temperature, where a metal-insulator transition occurs simultaneously with a structural transformation, supposedly caused by the charge ordering on the iron sublattice. The wavevector (0,0,1∕2) corresponds to the superlattice peak in the low symmetry structure. The dependence of the magnetic superexchange on changes in the crystal structure and ionic configurations that occur below the Verwey transition affect the spin wave dispersion. To better understand the origin of the observed splitting, several Heisenberg models intended to reproduce the pair-wise variation of the magnetic superexchange arising from both small crystalline distortions and charge ordering were studied. None of the models studied predicts the observed splitting, whose origin may arise from charge-density wave formation or magnetoelastic coupling.