Influence of Dy³⁺ and Cu substitution on the structural, electrical and dielectric properties of CoFe₂O₄ nanoferrites

In this work, Dy³⁺ and Cu doped cobalt ferrites with general chemical formula Co_0.8−xDy_xCu_0.2Fe₂O₄ (where x = 0.0, 0.1, 0.3 and 0.5) have been synthesized via sol–gel route. The cubic phase confirmation and chemical bonding were revealed using X-ray diffraction and Fourier transform infrared spectroscopy respectively. Thermal stability of as-prepared samples was checked by thermo-gravimetric and differential thermal analysis. The surface morphology was studied by scanning electron microscopy. Raman spectroscopy was used for further confirmation of the single-phase cubic spinel structure of the samples. The average crystallite size was found to decrease from 17.5 to 12.4 nm and the lattice constant was increased from 8.3564 to 8.3811 Å on incorporation of Dy³⁺ ions. The dc electrical resistivity in the temperature range of 303–393 K shows the semiconducting nature of the as-prepared samples. The activation energies for different samples were estimated from the Arrhenius plot and found to be in range of 0.25–0.30 eV. The dielectric constant (ε′), ac conductivity (σ_ac) and dielectric loss (tanδ) have been analyzed in the frequency range of 42 Hz - 5 MHz at room temperature. All the dielectric parameters were found to decrease on adding Dy³⁺ ions. The variation of dielectric properties ε′, tanδ, and σ_ac with frequency indicates the typical Maxwell–Wagner type dielectric behavior due to interfacial (space charge) polarization and the exchange of electrons among Fe²⁺ and Fe³⁺ ions. Electron paramagnetic resonance measurements of as-prepared ferrite nanoparticles show the weak super-exchange interactions which cause the large g-value and broadening of the resonance line as compared to the free electron g-value. The prepared ferrites have high dielectric permittivity and low loss making them promising materials for the applications in high frequency memory storage devices.