Adv. Mater. 21, 1 (2009)

M. Alexe, M. Ziese, D. Hesse, P. Esquinazi, K. Yamauchi, T. Fukushima, S. Picozzi and U. Gösele

Abstract

Recently, multiferroic materials, that is, materials that possesstwo or more ferroic properties, such as spontaneous electricpolarization and magnetization in the same phase, have attracteda renewed interest of the scientific community.Since thediscovery of an unusual two-orders-of-magnitude increase inresistivity at around 120 K by Verwey, magnetite has beenstudied intensively. While the magnetic properties of Fe3O4 arerelatively well known and understood, the electronic properties,mostly at temperatures below the Verwey transition TV, are stillunder debate. Above TV, magnetite has an inverse spinelstructure in which the iron occupies both the octahedrally andtetrahedrally coordinated cation positions. Relevant for theproperties of magnetite seem to be the Fe2þand Fe3þions,which equally occupy the octahedrally coordinated cation sites (inthe oxygen octahedra). At room temperature, Fe3O4 is aferrimagnetic material with a critical temperature as high as860 K and metallic behavior. At the Verwey temperature,magnetite undergoes a first-order phase transition from apseudocubic to a monoclinic structure. The concurrent metal–insulator transition has been related to a charge-ordering patternconsisting of an alternation of the Fe2þand Fe3þions on theoctahedrally coordinated sites. Magnetite was also one of thefirst materials studied regarding the magnetoelectric (ME) effect,that is, inducing an electrical polarization by an externally appliedmagnetic field and vice-versa. Early studies on the ME effecthave suggested the existence of a spontaneous polarization attemperatures below 38 K, but final proof of ferroelectricity inmagnetite has not yet been given.