Synthesis and Characterization of CoFe₂O₄ Nanoparticles Dispersed in a Silica Matrix by a Sol−Gel Autocombustion Method

A very fast self-combustion reaction was applied for the first time to the synthesis of a series of magnetic CoFe 2 O 4 −SiO 2 nanocomposites in a wide range of compositions (from 5 to 50 wt % CoFe 2 O 4 ). Combining a gelation method that adopts metal nitrates, citric acid and tetraethoxysilane as precursors with controlled thermal treatments, a wide variety of samples with properties finely modulated were obtained. Particle formation and evolution of the structural and magnetic properties with the temperature were investigated by thermal analysis, X-ray diffraction, transmission electron microscopy, high-resolution transmission electron microscopy, nitrogen physisorption, IR, and magnetic susceptibility measurements. It was shown that the formation of metal ammonium carboxylate complexes takes place in all the gel precursors, contributing to the control of nucleation and growth of the nanoparticles. Spherical particles with narrow particle size distribution, uniformly dispersed in the matrix, are obtained in the diluted samples (N5, N10, N15), while nanoparticles with more irregular shapes form in samples with high CoFe 2 O 4 content (N30, N50). Magnetic properties evolve with CoFe 2 O 4 amounts and thermal treatments. The samples treated at 900 °C exhibit superparamagnetic behavior, with the expected dependence of the magnetic relaxation on particle size.