Non-stoichiometric Ni0.5Zn0.5Fe2−xO4−3/2x ferrites over a wide range of x = 0∼0.8 are synthesized by a sol-gel processing. Phase evolution, crystal structure and crystallite size of spinel ferrites are dependent on annealing temperature and the amount of Fe deficiency. The crystallite size of spinel increases with annealing temperature and grows faster in stoichiometric ferrites than that of non-stoichiometric. Fe deficiency results in the partial reduction of spinel ferrite to zincite ZnO. XRD indicates that the crystallization temperature of ZnO is increased to about 700 °C. Zincite reduces the number of ferrite crystallites and disfavors the growth of spinel ferrites. The lattice parameters decrease with Fe deficiency and are insensitive to the variation in composition in the samples annealed at lower temperature due to the segregation of ZnO and lattice expansion in the ultrafine crystallites.

NiZn ferrite–SiO2 composites are synthesized by a sol–gel method. The chemical and structural changes of gels during heat treatment are analyzed using DTA, infrared spectra (IR) spectra and XRD. Several kinetic models in predicting the formation of ferrite are assessed. The result indicates that the rate of ferrite formation in sol–gel derived composites is best represented by Blum and Li's equation, dx/dt=(k−x)/t. Variation of initial permeability with composition is discussed and possible magnetic mechanisms are investigated. Grain size and the continuity of magnetic circuit are the main factors to determine magnetic properties of ferrite composite materials.