Sol–gel synthesis and characterization of single-phase Ni ferrite nanoparticles dispersed in SiO2 matrix

Nanoparticles of NiFe2O4 dispersed in SiO2 (25 wt%) matrix were synthesized by sol–gel method using tetraethyl orthosilicate (TEOS), as a precursor for SiO2. The sol–gel method for nanocomposites normally provides multi-phase nanoparticles. We investigated by a synopsis of different analysis methods, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and SQUID-magnetometry, how the various chemical phases are transformed to a single-phase spinel structure during the various stages of annealing from 300 to 900 °C. We have developed a full phase diagram of chemical phases as a function of annealing temperature. The average particle size lies in the range 16–27 nm. The chemical phases formed below 900 °C are NiFe, NiO, γ-Fe2O3, α-Fe2O3, and NiFe2O4, respectively. The role of the TEOS prepared SiO2 matrix is to restrict the particle size in a small range in order to rule out particle size effects. In the mid-infrared, a shift of the vibrational Fe–O bond is observed from 568 to 586 cm−1 for annealing between 500 and 700 °C which indicates an increasing NiFe2O4 phase formation. A systematic study of coercivity field (ranging from 32 to 200 Oe) and saturation magnetic moment (ranging from 12.2 to 32.1 emu/g) for differently annealed samples supports our findings about the evolution of single-phase NiFe2O4 at 900 °C. The opposite trend of saturation magnetic moment and coercivity with respect to annealing temperature clearly separates the different phases of metallic, antiferromagnetic, and finally single-phase spinel NiFe2O4.