Cobalt-ferrite nanoparticles: Structure, cation distributions, and magnetic properties

Cobalt-ferrite nanoparticles have been synthesized in water-in-oil microemulsions (reversed micelles) with varying cation composition. The microenvironment provides a template effect that controls the size and particle shape. Transmission electron microscopy reveals that the particles are nanospheres with particle size ranging from 12 to 18 nm. X-ray diffraction results indicate that at low Co2+:Fe2+ ratio (1:10 and 1:5) in the precursor, the particles retain an essentially ferrite structure (gamma-Fe2O3). However, the cobalt-ferrite phase (CoFe2O4) forms upon further increase of the Co2+ content. The materials are found to exhibit superparamagnetism. The blocking temperatures and coercivities are dependent on the Co2+:Fe2+ ratio in the system. (C) 2000 American Institute of Physics. [S0021- 8979(00)77708-5].