Preparation and magnetic study of the CoFe2O4-CoFe2 nanocomposite powders

Ferri-ferromagnetic nanocomposites CoFe₂O₄-CoFe₂ were prepared via reduction of the cobalt ferrite CoFe₂O₄ into a hydrogen atmosphere and by heat treatments. This preparation method yielded powders with relative volume fraction of CoFe₂O₄ and CoFe₂ in the range 0-0.91. The structure and the room temperature magnetization of the samples were analyzed by X-ray diffraction, scanning electron microscope (SEM) and transmission electron microscope (TEM), and by vibrating sample magnetometry, respectively. It was found that the saturation magnetization of the nanocomposite powders increases with the volume fraction of the ferromagnetic phase while their coercivity decreases. The highest value (5 kJ/m³) for the maximum energy product (BH)_max was obtained for the sample containing 91% in volume of CoFe₂. This (BH)_max is 19% higher than the value measured in pure CoFe₂O₄. The magnetic field dependence of the magnetization did also behave as the nanocomposite powders were single-phase materials. This result indicates that the hard ferrimagnet CoFe₂O₄ and the soft ferromagnet CoFe₂ are effectively exchange coupled and that their magnetization reverses cooperatively in the range of compositions investigated.