New syntheses of cobalt ferrite particles in the range 2-5 nm: Comparison of the magnetic properties of the nanosized particles in dispersed fluid or in powder form

The size of cobalt ferrite particles from 2 to 5 nm is controlled by the reactant concentration in oil-in-water micelles. It is possible to obtain the particles either suspended in the solvent to form ferrofluid or in dry powder. When particles are isolated in a fluid, the interactions between particles are small. The magnetic size is similar to that determined by TEM. The reduced susceptibility increases progressively with the size of the particles and does not reach a plateau. With dry powder made of nanosized particles, the magnetic size is higher than that observed by TEM. Likewise, the reduced susceptibility strongly increases with increasing the particle size to reach a plateau for an average size equal to 3 nm. These are explained by the increase in the attractive interactions between particles. By heating the powder, the reduced remanence, M(r)/M(s), and the coercivity, H-c, increase with the increase in annealing temperature. This is attributed to the increase in the particle size and to the release of the adsorbed surfactant on the particles. The largest size of particles can be obtained either by annealing of the smallest one or from direct synthesis at room temperature. For a given size, the improvement in coercivity with annealing temperature is attributed to a better-defined nanocrystalline structure and to the release of surfactant to the particle interface.