Ferri-ferromagnetic nanocomposites CoFe 2 O 4 CoFe 2 were prepared via reduction of the cobalt ferrite CoFe 2 O 4 into a hydrogen atmosphere and by heat treatments. This preparation method yielded powders with relative volume fraction of CoFe 2 O 4 and CoFe 2 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/m 3 ) for the maximum energy product ( ) max was obtained for the sample containing 91% in volume of CoFe 2 . This ( ) max is 19% higher than the value measured in pure CoFe 2 O 4 . 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 CoFe 2 O 4 and the soft ferromagnet CoFe 2 are effectively exchange coupled and that their magnetization reverses cooperatively in the range of compositions investigated.

Fe10Ag90 granular alloys have been prepared using a sol-gel process, sintered at 300 °C and annealed at temperatures between 400 °C and 700 °C. The mean size of the iron particles, obtained from X-ray diffraction, is 30.0 ± 0.7 nm. Due to the existence of a distribution of particle sizes in these samples, both blocked (BL) and superparamagnetic (SPM) particles are present simultaneously, as confirmed by magnetization measurements at room temperature. AC susceptibility measurements as a function of temperature reveal a magnetic phase transition at about 770 °C, indicating the presence of particles exhibiting bulk behavior, in the samples annealed above 550 °C. The presence of these particles has been attributed to an atomic diffusion process between the grains, forming bulk-like multiple-domain Fe particles having Curie temperatures near that of bulk alpha-Fe phase (T C = 770 °C)