Size-control and size-selection at the nanosize level: Production of CoFe₂O₄ nanoparticles of unusual high coercivity at room temperature

Among the various ferrite materials, cobalt ferrite (CoFe₂O ₄) is well known to possess excellent chemical stability and good mechanical hardness. Furthermore, the large positive first order crystalline anisotropy constant has made this ferrite a promising candidate for magneto-optical recording media. However, applications of CoFe₂O ₄ are often limited due to the lack of a synthesis technique capable to achieve a suitable control over particle size distribution, with particle diameters between the superparamagnetic and single domain sizes range. This control would permit to attain high coercivity and moderate magnetization at room temperature, which is critical for application of this material in actual devices. Here, we introduce a solution chemistry technique that involves a 'seeding step' followed by a size-selective particles separation that permits a fine size classification of single domain CoFe₂O₄ nanoparticles. The size-selected 40-nm particles reported room temperature coercivity as high as 4.6 kOe and a squareness ratio of 0.66.