Nickel ferrite nanoparticles of very small size were prepared by sol-gel combustion and co-precipitation techniques. At the same annealing temperature sol-gel derived particles had bigger crystallite size. In both methods, crystallite size of the particles increased with annealing temperature. Sol-gel derived nickel ferrite particles were found to be of almost spherical shape and moderate particle size with a narrow size distribution; while co-precipitation derived particles had irregular shape and very small particle size with a wide size distribution. Nickel ferrite particles produced by sol-gel method exhibited more purity. Sol-gel synthesized nanoparticles were found to be of high saturation magnetization and hysteresis. Co-precipitation derived nickel ferrite particles, annealed at 400°C exhibited superparamagnetic nature with small saturation magnetization. Saturation magnetization increased with annealing temperature in both the methods. At the annealing temperature of 600°C, co-precipitation derived particles also became ferrimagnetic.

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A series of nickel-cadmium ferrite, Ni 0.9 Cd 0.1 Gd x Fe 2−x O 4 (x = 0, 0.1, 0.2 and 0.3), was prepared through the sol-gel technique. Samples were characterized using x-ray diffraction (XRD), Fourier transform infrared spectroscopy, a wavelength dispersive x-ray fluorescence spectrometer, a transmission electron microscope and a vibrating sample magnetometer. XRD analysis confirmed the formation of single-phase spinel structure in all the samples. The effect of Gd 3+ doping on the crystalline phase, crystallite size and magnetic properties was investigated in detail. Gadolinium doping improved the crystallinity and induced compressive strain in Ni-Cd mixed ferrite. The lattice parameter was observed to increase for minor Gd 3+ doping (up to x = 0.1); however, further doping resulted in a reduction of the lattice constant. The saturation magnetization decreased and coercivity increased with an increase in gadolinium content.

A series of samarium-substituted cobalt ferrites (CoFe2−xSmxO4 with x=0.00, 0.05, 0.10, 0.15, 0.20, 0.25) was synthesized by the sol-gel method. The structural characterizations of all the prepared samples were done using XRD and FTIR. These studies confirmed the formation of single-phase spinel structure in all the compositions. The increase in the value of lattice parameter with increase in samarium concentration suggests the expansion of unit cell. The Hall-Williamson analysis is used for estimating the average crystallite size and lattice strain induced due to the substitution of samarium in the prepared samples. Crystallinity and the crystallite size are observed to increase with the concentration of samarium. The surface morphology and particle size of a typical sample were determined using SEM and TEM respectively. The substitution of samarium strongly influences the magnetic characteristics, and this is confirmed from the magnetization measurements at room temperature.

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