Herein, we report the synthesis of nanoparticles and doping of Cu-doped Co–Zn ferrites using the auto-combustion sol–gel synthesis technique. X-ray diffraction studies confirmed the single-phase structure of the samples with space group Fd3m and crystallite size in the range of 20.57–32.69 nm. Transmission electron microscopy micrographs and selected area electron diffraction patterns confirmed the polycrystalline nature of the ferrite nanoparticles. Energy-dispersive X-ray spectroscopy revealed the elemental composition in the absence of any impurity phases. Fourier-transform infrared studies showed the presence of two prominent peaks at approximately 420 cm−1 and 580 cm−1, showing metal–oxygen stretching and the formation of ferrite composite. X-ray photoelectron spectroscopy was employed to determine the oxidation states of Fe, Co, Zn, and Cu and O vacancies based on which cationic distributions at tetrahedral and octahedral sites are proposed. Dielectric spectroscopy showed that the samples exhibit Maxwell–Wagner interfacial polarization, which decreases as the frequency of the applied field increases. The dielectric loss of the samples was less than 1, confirming that the samples can be used for the fabrication of multilayer inductor chips. The ac conductivity of the samples increased with increasing doping and with frequency, and this has been explained by the hopping model. The hysteresis loops revealed that coercivity decreases slightly with doping, while the highest saturation magnetization of 55.61 emu/g was obtained when x = 0.1. The magnetic anisotropic constant was found to be less than 0.5, which suggests that the samples exhibit uniaxial anisotropy rather than cubic anisotropy. The squareness ratio indicates that the samples are useful in high-frequency applications.

M-type barium hexaferrites (BaM) with the substitution of Ce–Dy ions were synthesized using the sol-gel auto-ignition method. The prepared materials were explored for their application as a permanent magnet and microwave absorbing material. The structural properties, phase evaluation, micro-strain, morphological analysis, magnetic behaviour, microwave absorbing properties and optical properties were studied by employing various techniques. The structural parameters and phase identification obtained by Rietveld refinement confirmed the formation of an M-type hexaferrite structure for pure BaM, whereas Ce–Dy substitution induced secondary phases of cubic CeO2 and ortho DyFeO3. Crystallite size obtained from Williamson–Hall plots increased from 27.1 nm to 30.8 nm with the introduction of Ce–Dy ions in BaM. The nanocrystalline nature of the prepared samples was confirmed using scanning and transmission electron microscopy techniques. Fourier transform infrared spectra of all the samples were recorded in the wavenumber range of 400–4000 cm−1 and also supported the x-ray diffraction findings by confirming the formation of samples with hexaferrite structures. Coercivity of the BaM hexaferrites improved from 4430 to 5721 Oe with the Ce–Dy substitution. A Ce–Dy substituted BaM hexaferrite sample of 3 mm thickness showed a maximum reflection loss of −16.3 dB around 16.7 GHz. Permittivity and permeability studies were carried out to understand the microwave absorption behaviour.

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