| Summary: | Sol-gel route followed by thermal treatment was used to produce NiFe<sub>2</sub>O<sub>4</sub> doped with transition metal ions (Zn<sup>2+</sup>, Mn<sup>2+</sup>, Co<sup>2+</sup>). The structural, morphological, and magnetic properties of the doped NiFe<sub>2</sub>O<sub>4</sub> were compared with those of virgin NiFe<sub>2</sub>O<sub>4</sub>. The metal-glyoxylates’ formation and decomposition as well as the thermal stability of the doped and virgin ferrites were assessed by thermal analysis. The functional groups identified by Fourier-transform infrared spectroscopy confirmed the decomposition of metal nitrates, the formation and decomposition of precursors, and the formation of the SiO<sub>2</sub> matrix. The X-ray diffraction indicated that the sol-gel synthesis produced single-phase crystalline ferrites in case of virgin, Zn<sup>2+</sup> and Co<sup>2+</sup>-doped Ni-ferrites. By doping with Mn<sup>2+</sup>, several secondary phases derived from the SiO<sub>2</sub> matrix accompanied the crystalline spinel ferrite. The crystallite sizes depended on the annealing temperature and type of doping ion. The gradual increase of lattice parameters suggested the uniform distribution of doping metal ions in the NiFe<sub>2</sub>O<sub>4</sub> lattice. The saturation magnetization, remanent magnetizations, coercivity, and anisotropy were found to depend on the doping ion, annealing temperature, and particle size. The high saturation magnetization values of the obtained nanocomposites make them suitable for a wide range of applications in the field of sensors development and construction.
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