Post-synthesis annealing of coprecipitated CoFe2O4 nanoparticles in silica matrix

Abstract Cobalt ferrite nanoparticles ( d  ∼ 11 nm, σ d  = 0.5) produced by coprecipitation at room temperature are heat treated up to 1000 °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 °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 Co 2+ 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 CoFe 2 O 4 nanoparticles and highlights that the main source of magnetic disorder is associated with the distribution of Co and Fe within the cationic sites.