Cobalt ferrite (CoFe2O4) possesses excellent chemical stability, good mechanical hardness and a large positive first order crystalline anisotropy constant, making it a promising candidate for magneto-optical recording media. In addition to the precise control on the composition and structure of the ferrite, the success of its practical application relies on the capability of controlling crystal size within the superparamagnetic and single domain limits. Accordingly, cobalt ferrite nanocrystals were prepared by a modified aqueous processing route under boiling conditions. The synthesis protocols were modified to achieve a fine control on crystal growth and final size at the nanoscale by tuning of the solution oversaturation. These conditions permitted to achieve room-temperature coercivity as high as 4.6 kOe (from typical 600 Oe) in 12-14 nm CoFe2O4 crystals.
Magnetization-magnetic field profiles for cobalt ferrite synthesized before and after acidic treatment. The coercivity was as high as 7.9 kOe in the acidic treated sample.
Furthermore, our most recent results evidenced that the coercivity of the cobalt ferrite nanocrystals could be enhanced even further. A post-acidic treatment of the ferrite nanocrystals caused their coercivity to rise up to 8 kOe at room temperature. The corresponding average crystallite size was 20 nm. To our knowledge, this is the highest coercivity value reported so far in the scientific literature. This value is even above the theoretically predicted maximum of 5.3 kOe. This type of nanosize material can be considered a very promising material for data storage.