The Verwey phase of magnetite — a long-running mystery in magnetism

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 superstructure was recently determined by high energy microcrystal x-ray diffraction. An analysis of 168 frozen phonon modes in the acentric (and hence multiferroic) low temperature magnetite structure is presented here. Differences between the amplitudes of centric and acentric branches of, X and W modes all contribute to the significant off-center atomic distortions in the low temperature structure.