Magnetite (Fe3O4) is a mixed valent system where electronic
conductivity occurs on the B-site (octahedral) iron sublattice of the spinel
structure. Below TV=122 K, a metal-insulator transition occurs which is
argued to arise from the charge ordering of 2+ and 3+ iron valences on the
B-sites (Verwey transition). Inelastic neutron scattering measurements show
that optical spin waves propagating on the B-site sublattice (∼80 meV) are
shifted upwards in energy above TV due to the occurrence of B-B
ferromagnetic double exchange in the mixed valent metallic phase. The double
exchange interaction affects only spin waves of Δ5 symmetry, not all
modes, indicating that valence fluctuations are slow and the double exchange is
constrained by electron correlations above TV.Comment: 4 pages, 5 figure