Scientific Highlight November 2007
Zener Double Exchange and Charge Ordering in Magnetite
Rob McQueeney (Ames), Mohana Yethiraj (ANSTO), Wouter Montfrooij (U. of Missouri), Toby Perring (ISIS), George Honig and Patty Metcalf (Purdue U.).
Magnetite (Fe3O4) is a mixed-valent system in which 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, on the MAPS time-of-flight spectrometer at ISIS in the UK, show that optical spin waves propagating on the B-site sublattice (~80 meV) 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 D5 symmetry, not all modes, indicating that charge fluctuations are slow and the double exchange is constrained by electron correlations above TV .
Intensity in the [H K0] plane at (a) 130 K and (b) below TV at 110 K. The optical spin waves at 80 meV (in the purple circled region) indicates extremely broad and weak D5 symmetry above Tv; the Δ-5 mode is more well defined at 110K. (c) Calculation of the MAPS spin-wave spectra using a Heisenberg model.
Published in PHYSICAL REVIEW LETTERS, Vol. 99, 246401, 2007