Post-synthesis annealing of coprecipitated CoFe2O4 nanoparticles in silica matrix

Cobalt ferrite nanoparticles (d similar to 11 nm, sigma(d) = 0.5) produced by coprecipitation at room temperature are heat treated up to 1000 degrees C after a preliminary dispersion in a sol-gel silica matrix to avoid aggregation and coarsening. This protected annealing allows for a significant increase of the crystallinity of the particles, as demonstrated by combined x-ray diffraction and transmission electron microscopy experiments. A large increase in the coercive field value is reported, from 1.0 to 1.6 Tesla after annealing at 600 degrees C. This enhanced coercivity can be explained by the cumulative effect of an increased magnetic anisotropy and of a decreased saturation magnetization (M-s). Mossbauer spectroscopy experiments show that these evolutions of the M-s and anisotropy constant (K) values originate from a small increase in canting angles and in inversion degree, i.e. a displacement of Co2+ ions from tetrahedral to octahedral sites of the spinel lattice. This study emphasizes the little impact of an improved crystallinity on saturation magnetization and canting angle values in coprecipitated CoFe2O4 nanoparticles and highlights that the main source of magnetic disorder is associated with the distribution of Co and Fe within the cationic sites.