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 emerg ing field of magnetic multilayer devices. In many crystalline materials, el ectrons are more or less localized on specific lattice sites. In some such cases, longer range fluctuations in electron density known as charge-densit y waves or charge ordering also occur. Charge ordering is usually associate d 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 temperatu re and above of a new form of charge ordering in thin films of magnetite, w hich has a cubic, rather than a layered, structure. We show that this pheno menon of charge ordering on tetrahedral sites is unrelated to the well-know n low-temperature ordering on octahedral sites that occurs below 120 K. Fro m electron-diffraction patterns we deduce the magnitude and crystallographi c orientation of the fluctuations, and find they are a regular multiple of the underlying structure. In addition, we present electron micrographs in w hich the charge ordering is visible, and which clearly show disorganized bo undaries between regions of regularly arranged charge ordering. We believe that ordering of this type in magnetite may not have been observed previous ly because of insufficient purity and perfection of the materials studied. However, we suspect that similar charge ordering is Likely in other metal o xides of high crystal quality where metal ions have mixed-valence states. [ S0163-1829(99)50318-4].