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In this research a sol-gel auto-combustion route has been proposed to synthesize nickel-zinc ferrite nanocrystalline powder, using metal nitrates, citric acid as fuel and ammonia as pH adjusting agent. The influence of pH value of the solution on phase evolution, crystallite size and morphology of as-burnt powders were investigated by XRD, SEM and TEM techniques. The results revealed that with pH=7 the single phase nickel-zinc ferrite nanocrystalline powders with crystallite size of about 27nm were formed directly after auto combustion process.

Nanocrystalline Ni–Zn–Al spinel ferrite was synthesized via citrate-gel auto combustion method. The as-prepared powders have been separated into two batches in which one batch of powders were sintered at 1000${}^{\circ }$C for 4 h and the other batch were pressed into pellets and were sintered at the same temperature. Sintering of the samples was done in air atmosphere followed by natural cooling to room temperature. The heat treated powders have then been characterized using TG–DTA, XRD, SEM and TEM for thermal, structural and microstructural aspects while the DC electrical resistivity measurements were carried out on the sintered pellets. The X-ray diffraction patterns displayed the formation of the spinel phase for all powders and the lattice parameter was obtained using Bragg’s law. The crystallite size for all compositions were found to be in nano dimensions and obtained from the Williamson–Hall method. TG–DTA analysis of the undoped ${\mathrm{\text{Ni}}}_{0.5}{\mathrm{\text{Zn}}}_{0.5}{\mathrm{\text{Fe}}}_2{\mathrm{\text{O}}}_4$ indicated the formation of the spinel phase is around 400${}^{\circ }$C while almost uniform microstructure with a more or less spherical grains has been noticed in the SEM micrograph. An enhancement in the DC electrical resistivity ($\ge 10^8\mathrm{\Omega }$-cm) has been observed in ${\mathrm{\text{Ni}}}_{0.5}{\mathrm{\text{Zn}}}_{0.5}{\mathrm{\text{Fe}}}_2{\mathrm{\text{O}}}_4$ synthesized using this technique in comparison with that processed through conventional ceramic technique and a modification in the resistivity has been observed on substituting ${\mathrm{\text{Al}}}^{3+}$ in place of ${\mathrm{\text{Fe}}}^{3+}$. High electrical resistivity makes these ferrites suitable for high-frequency applications due to possible reduction of the eddy current losses. The observed variation in resistivity has been discussed on amendments in structure, microstructure and unavailability of ${\mathrm{\text{Fe}}}^{3+}$ ions with increasing ${\mathrm{\text{Al}}}^{3+}$ ions in the light of existing understanding. The decrease in resistivity with increasing temperature confirms the semiconducting behavior of all samples. Activation energies for conduction were obtained from the slope of the log $\rho$ versus $1/T$ plots and observed to be in the range of 0.6–0.45 eV. The variation in the activation energy for conduction followed a similar trend as the DC resistivity. The drift mobility decreases with increasing ${\mathrm{\text{Al}}}^{3+}$ ions concentration and increases with increasing temperature.