Highly crystalline CoFe(2)O(4) nanoparticles with different diameters ranging from 2.4 to 6.1 nm have been synthesized by forced hydrolysis in polyol. The size can be controlled through adjusting the nominal water/metal molar ratio. X-ray diffraction, transmission electron microscopy, x-ray absorption spectroscopy and (57)Fe Mössbauer spectrometry were employed to investigate the structure and the microstructure of the particles produced. Magnetic measurements performed on these particles show that they are superparamagnetic with a size-dependent blocking temperature. At 5 K, high saturation magnetization (~85 emu g(-1)) approaching that of the bulk was found for the larger particles, whereas a very large coercivity (14.5 kOe) is observed for the 3.5 nm sized particles.

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Ultrafine cobalt ferrite nanoparticles were prepared by forced hydrolysis in diethyleneglycol and, after centrifugation, dispersed in the same solvent to form a sol in which a layered hydroxyacetate nickel salt was precipitated. The as-obtained composite was moderately calcined in air to form nanoparticles constituted by a ferrimagnetic CoFe2O4 core embedded in well-crystallized antiferromagnetic NiO. Magnetic properties such as coercivity, hysteresis loop shift, and blocking temperature were analyzed regarding exchange bias features by comparing the magnetic data measured on these particles to those obtained on fresh CoFe2O4 particles mechanically dispersed in a diamagnetic Al2O3 matrix.

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