Phase transition in magnetite
Since antiquity, magnetite (Fe3O4) has been exploited for its magnetic properties. Nowadays it is under active consideration as the active material in spintronic devices. Surprisingly though, fundamental questions about the charge and spin distribution in this material remain a puzzle.
One such puzzle surrounds the Verwey transition, a phenomenon where the conductivity drops by two orders of magnitude when magnetite is cooled below 121 K. A small band gap is opened in the low-temperature phase and the crystal symmetry is lowered, which has been assumed to be due to some pattern of charge and perhaps spin ordering on the Fe B-sites. We have carried out high-precision first principles calculations of the atomic and electronic structure and have found that magnetite undergoes partial charge and orbital ordering at low temperature, since the associated electron-phonon interaction (a Jahn-Teller distortion) can lower the energy and open a band gap. By using various structural models, we show that lattice distortion is a consequence of frustration in the charge ordering pattern.