Preparation and characterization of ultra-small, monodisperse Co Fe3-O4 nanoparticles

The application of magnetite nanoparticles to functional magnetic materials requires the ability to tune their magnetic properties. Doping of transition metals offers the ability to control the magnetic and electron transport properties of magnetite nanoparticles. Herein, we describe a synthetic approach to producing ultra-small (∼6 nm), monodisperse Co-doped magnetite (CoxFe3-xO4) nanoparticles, capped with oleic acid, using a modified co-precipitation method under mild conditions. For stoichiometric amounts of Co corresponding to x = 0.1, 0.3, and 0.5, we measured compositions of Co0.12Fe2.88O4, Co0.33Fe2.67O4, and Co0.58Fe2.42O4, respectively, using XPS analysis. Although a minor impurity of α-FeOOH was detected, this impurity was found to reduce significantly with increasing Co content. The resulting CoxFe3-xO4 nanoparticles showed significant increase in both the coercive field, and the blocking temperatures with increasing Co content, due to increased magnetic anisotropy. Magnetic characterization further revealed saturation magnetization values higher than those reported for CoxFe3-xO4 nanoparticles of similar size, monodispersity and Co content, prepared by other methods.