Cobalt ferrite nanopowders were successfully synthesized by the coprecipitation method and subsequent calcination at 873 and 1073 K. The effects of the thermal treatments on the crystal structure, particle size and magnetic properties of the nano-compounds were investigated. The particle sizes were determined from transmission electron microscopy, and an increase in sizes with the increment in calcination temperature was observed. The mean particle sizes were 29 and 42 nm, for samples calcined at 873, and 1073 K, respectively. By X-ray diffraction, it was determined that the nanoparticles crystallized in the cubic spinel structure. Additionally, Fourier transform infrared spectroscopy studies confirm the presence of spinel metal oxide. The magnetization measurements as a function of the temperature and the applied magnetic field suggested that a large part of the nanoparticles calcined at 873 K present a superparamagnetic behavior at room temperature, while those calcined at 1073 K are mainly in the blocked regime at temperatures below 320 K. Besides remarkably high coercivities of approximately 10.7 and 12.4 kOe were observed at low temperatures, for the nanopowders calcined at 873 and 1073 K, respectively.
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