Carrier motion origins and dual-permittivity behavior in FeNi2O4: Dielectric resonance, epsilon-near-zero, polarization effects, and possible advanced electromagnetic applications

Negative permittivity in oxide materials can be achieved through mechanisms such as dielectric resonance or plasma oscillations, while positive permittivity typically arises from various polarization effects. In this study, a spinel nickelate with the nominal composition FeNi₂O₄ was synthesized using the auto-combustion method. The material exhibits both positive and negative permittivity behaviors, attributed to dielectric polarization and dielectric resonance mechanisms. Notably, FeNi₂O₄ demonstrates a tunable negative permittivity, with a maximum value of ε′ ≈ −270 observed around 10 MHz, suggesting the potential for epsilon-near-zero (ENZ) behavior. Beyond 10 MHz, FeNi₂O₄ exhibits a Lorentz-like permittivity response, making it a promising candidate for advanced electromagnetic applications such as metamaterials and microwave absorbers. Temperature-dependent studies reveal that increasing temperature leads to changes in resonance frequency, permittivity magnitude, and a transition in negative permittivity behavior due to the combined effects of dipole resonance and free electron dynamics. To understand the electrical transport mechanisms in FeNi₂O₄, various conduction models are employed. DC conductivity measurements confirm semiconducting behavior, primarily governed by electronic and polaronic hopping processes. Specifically, non-adiabatic small polaron hopping is characterized by an activation energy of E_a = 1.01 eV, while Mott's variable range hopping (VRH) corresponds to a disordered energy E_a = 0.377 eV. Additionally, Shklovskii–Efros (SE)-type VRH conduction is evident, associated with the formation of a soft Coulomb gap of approximately Δ = 1.2 eV. The frequency-dependent conductivity spectra reveal that cationic interactions and material response are influenced by temperature. To analyze these behaviors, experimental data are interpreted using Bruce's model, Jonscher's universal power law, and the Summerfield scaling formalism. At high frequencies, the conductivity trends follow superlinear behavior and the nearly constant loss (NCL) regime.