Effect of annealing temperature on structure and magnetic properties of sol–gel synthesized Co_0.8Fe_2.2O₄/SiO₂ nanocomposites

Using SiO₂ as amorphous matrix, Co_0.8Fe_2.2O₄/SiO₂ nanocomposites were synthesized by sol–gel method. The effect of annealing temperature on structure, cation distribution, and magnetic properties was investigated by X-ray diffraction (XRD), Mössbauer spectroscopy, and vibrating sample magnetometer (VSM). All Co_0.8Fe_2.2O₄ in obtained composites exhibit cubic spinel structure after annealed at 500 °C and above. The particle size of Co_0.8Fe_2.2O₄ increases from 5 to 25 nm as the annealing temperature increases from 500 to 1100 °C. Analysis of Mössbauer and VSM reveal that higher annealing temperature induces the migration of Fe³⁺ from octahedral B to tetrahedral A sites, which results in the enhancement in magnetic hyperfine field H_in. While the increasing content of Co²⁺ at B sites and the weakened ‘pinning’ effect of domain wall result in the increase of coercivity H_c. In addition, larger Co_0.8Fe_2.2O₄/SiO₂ particles exhibit higher saturation magnetization M_s due to the relative decrease in magnetically disordered content on the Co_0.8Fe_2.2O₄ surface.