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The gadolinium-doped nickel ferrites with the chemical composition of NiFe2−xGdxO4 (0.00 £ x £ 0.25) ferrites were synthesized by sol–gel auto-combustion. Spinel ferrite phase formation was analyzed using XRD analysis. The dielectric constant, loss tangent, and the AC conductivity were investigated. The dielectric properties with Gd doping content were studied from 100 Hz to 1 MHz frequency range from room temperature to 400 °C. These results were very well in agreement with Maxwell–Wagner interfacial polarization. The dielectric properties were observed to depend on the frequency and the Gd doping concentration. According to Koop theory, spinel ferrites exhibit a beneficial trend whereby their dielectric constant drops as frequency increases. This behavior is supported by the various cationic distributions within the spinel structure. The use of these materials in medium-frequency devices is reflected in the largest magnitude of the tangent loss at low frequencies. Thus, projected spinel nanoferrites could be advantageous for microwave and sophisticated electronics devices. [ABSTRACT FROM AUTHOR]