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

TL;DR

This study investigates charge order in magnetite by analyzing spin wave spectra through inelastic neutron scattering, revealing a gap related to structural changes below the Verwey transition.

Contribution

The paper provides experimental evidence of spin wave splitting in magnetite and models the effects of structural distortions, suggesting the splitting may originate from charge-density waves or magnetoelastic effects.

Findings

Observed a 7 meV gap in spin wave spectrum below Verwey transition

Models based on superexchange variations do not fully explain the splitting

Possible origin of splitting includes charge-density waves or magnetoelastic coupling

Abstract

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 E = 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, we have constructed a series of Heisenberg models intended to reproduce the pairwise variation of the magnetic superexchange arising from both small crystalline distortions and charge ordering. We find that none of the models studied predicts the observed splitting, whose origin may arise from charge-density wave formation or magnetoelastic coupling.