Possible large spin-phonon coupling in magnetite

Recent inelastic neutron scattering measurements on magnetite (Fe₃O₄) below the metal-insulator (Verwey) transition reveal a large gap (7 meV) forming in the middle of the acoustic spin wave branch at q=(0,0,1/2) and E=43 meV. The wavevector (0,0,1/2) corresponds to the main superlattice reflection of the low symmetry monoclinic structure below T_V and has been described as a charge-ordering wavevector. Detailed studies of Heisenberg models for the spin wave spectrum were performed assuming that the superexchange is modified to reflect crystallographic symmetry lowering due to either atomic distortions or charge ordering. None of the models studied introduced a significant gap in the acoustic spin wave branch. Another possible source is large spin-phonon coupling that results in the mixing of a phonon and spin wave near (0,0,1/2). We have evidence for the existence of such a phonon mode. Our results show a flat optical phonon branch that cuts through the acoustic spin wave precisely at the gap. The presence of strong spin-phonon coupling below the Verwey transition may further complicate our understanding of the Verwey problem.