Evidence for charge ordering at room temperature in Fe₃O₄

A previously unknown form of charge ordering has been observed in magnetite, Fe₃O₄, a material that is currently studied for possible use in the emerging field of magnetic multilayer devices. In many crystalline materials, electrons are more or less localized on specific lattice sites. In some such cases, longer range fluctuations in electron density known as charge-density waves or charge ordering also occur. Charge ordering is usually associated with structures that have a well-defined two-dimensional layer structure, and the periodicity of the charge ordering is often irregularly related to the basic structural repeat unit. We report observations at room temperature and above of a new form of charge ordering in thin films of magnetite, which has a cubic, rather than a layered, structure. We show that this phenomenon of charge ordering on tetrahedral sites is unrelated to the well-known low-temperature ordering on octahedral sites that occurs below 120 K. From electron-diffraction patterns we deduce the magnitude and crystallographic orientation of the fluctuations, and find they are a regular multiple of the underlying structure. In addition, we present electron micrographs in which the charge ordering is visible, and which clearly show disorganized boundaries between regions of regularly arranged charge ordering. We believe that ordering of this type in magnetite may not have been observed previously because of insufficient purity and perfection of the materials studied. However, we suspect that similar charge ordering is likely in other metal oxides of high crystal quality where metal ions have mixed-valence states.