Ferrite nanoparticles, particularly nickel-zinc ferrite nanoparticles, are novel materials for high-frequency applications. Nanoparticles with a composition of Ni 0.5 Zn 0.5 Fe 2 O 4 were prepared by two different processes, namely the co-precipitation and simplified sol-gel methods. Powder x-ray diffraction (XRD) patterns confirmed the single-phase spinel structure for the as-prepared samples. Samples were sintered at 555 and 755 • C, after which the structural, electrical and magnetic properties were studied. The crystallite sizes, as determined from XRD data, increased with sintering temperature. The dc electrical resistivity measurements were performed as a function of temperature, with the two-probe method in the temperature range from room temperature to 450 • C. The activation energy and drift mobility were calculated from the temperature-dependent dc electrical resistivity measurements. The dielectric constant and dielectric loss tangent for all the samples were determined as a function of frequency, and the frequency range used was from 20 Hz to 3 MHz at room temperature. The samples prepared using the simplified sol-gel method have lower dielectric constant values compared to those of the samples prepared using the co-precipitation method, and those prepared by the former method are more suitable for high-frequency applications. For the magnetic properties, a vibrating sample magnetometer was used. Saturation magnetization and coercivity increased with an increase in sintering temperature.

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Cr-doped Ni-Zn ferrite nanoparticles having the general formula Ni 0.5 Zn 0.5 Cr x Fe 2−x O 4 (x = 0.1, 0.3, 0.5) were prepared by the simplified sol-gel method. The structural and dielectric properties of the samples sintered at 750 ± 5 • C were studied. X-ray diffraction (XRD) patterns confirm the single-phase spinel structure of the prepared samples. The crystallite size calculated from the most intense peak (3 1 1) using the Debye-Scherrer formula was 29-34 nm. Scanning electron microscope images showed that the particle size of the samples lies in the nanometer regime. The dielectric constant (ε r ), dielectric loss tangent (tan δ) and ac electrical conductivity (σ ac ) of nanocrystalline Cr-Ni-Zn ferrites were investigated as a function of frequency and Cr concentration. The dependence of ε r , tan δ and σ ac on the frequency of alternating applied electric field is in accordance with the Maxwell-Wagner model. The effect of Cr doping on the dielectric and electric properties was explained on the basis of cations distribution in the crystal structure.

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