The verwey phase of magnetite - A long-running mystery in ferrites

Abstract

Magnetite (Fe3O4) is the original magnetic material and the parent of ferrite magnets, with modern applications ranging from spintronics to MRI contrast agents. At ambient temperatures magnetite has a cubic spinel-type crystal structure, but it undergoes a complex structural distortion and becomes electrically insulating below the 125 K Verwey transition. The electronic ground state of the Verwey phase has been unclear for over 70 years as the low temperature structure was unknown, but the full low temperature superstructure was recently determined by high energy microcrystal x-ray diffraction. There are 168 frozen phonon modes in the acentric (and hence multiferroic) low temperature magnetite structure. The ground state was found to be Fe2+/Fe3+ charge ordered and Fe2+ orbital ordered to a first approximation, but an unexpected localization of electrons in three-Fe 'trimeron' units was discovered. This description is supported by band structure calculations. This brief review will summarise recent progress on understanding the ground state structure of the Verwey phase of magnetite.