Magnetocrystalline and Surface Anisotropy in CoFe₂O₄ Nanoparticles

The effect of the annealing temperature T_ann on the magnetic properties of cobalt ferrite nanoparticles embedded in an amorphous silica matrix (CoFe₂O₄/SiO₂), synthesized by a sol-gel auto-combustion method, was investigated by magnetization and AC susceptibility measurements. For samples with 15% w/w nanoparticle concentration, the particle size increases from ~2.5 to ~7 nm, increasing T_ann from 700 to 900 °C. The effective magnetic anisotropy constant (K_eff) increases with decreasing T_ann, due to the increase in the surface contribution. For a 5% w/w sample annealed at 900 °C, K_eff is much larger (1.7 × 10⁶ J/m³) than that of the 15% w/w sample (7.5 ×10⁵ J/m³) annealed at 700 °C and showing comparable particle size. This indicates that the effect of the annealing temperature on the anisotropy is not only the control of the particle size but also on the core structure (i.e., cation distribution between the two spinel sublattices and degree of spin canting), strongly affecting the magnetocrystalline anisotropy. The results provide evidence that the magnetic anisotropy comes from a complex balance between core and surface contributions that can be controlled by thermal treatments.