Crossover to the negative dielectric constant in La3+ and Dy3+ doped Co-Zn spinel nanoferrites

The paper is focused on a comprehensive study of cation distribution and impedance spectroscopy analysis of pristine and rare earth ions (La3+ and Dy3+) doped Co0.7Zn0.3Fe2O4 ferrite nanocrystals. The samples have a single phase spinel cubic structure of space group Fd3̅m with a in the range of 0.8400−0.8408 nm as confirmed by Rietveld refinement analysis of the X-ray diffraction (XRD) data. The cation distribution property of these compositions has been thoroughly studied using the data of XRD as well as Raman spectra. It is predicted that Co2+ and Zn2+ ions prefer to occupy both the sites, tetrahedral A and octahedral B, but rare earth ions have a strong preference for B-site. The non-uniform, agglomerated grains of these samples have been observed using the field emission scanning electron microscope. Up to a particular temperature, the samples have shown a positive dielectric constant, but above that, an unusual result of negative dielectric constant is observed; the Nyquist plots and other related parameters change their features in this crossover region. The doped samples have shown higher dielectric constant and lower loss as compared to the parent sample in the frequency range of 104–106 Hz and temperature interval of 25−300 °C. The variation of dielectric modulus confirms the non-Debye type relaxor behavior of the samples. Our reported results would facilitate applications of these materials for a wide range of microwave devices. •Studies on La3+ and Dy3+ ions doped Co0.7Zn0.3Fe2O4 spinel nanoferrites have been made.•The doped samples shown higher ε′and lower tanδ for the temperature interval of 25−300℃ and frequency range of 104−106 Hz.•An unusual negative dielectric constant and semiconducting to metallic transition occurs above 300 °C.•Nyquist plots change from RC to RL type in the crossover region.•The obtained results can have noble potential applications in the field of microwave devices.