The effect of samarium substitution on the structure, conduction mechanism, and dielectric relaxation of MnNi-nanospinel ferrites

The incorporation of Sm into Mn–Ni spinel ferrites to form Mn0.5Ni0.5SmxFe2−xO4 (x ≤ 0.10) nanospinel ferrites (Sm → MnNi (x ≤ 0.10) NSFs) was achieved via the sol–gel method. The product was subjected to different characterization techniques to investigate its structure and morphology. A cubic crystal structure and a sphere morphology were confirmed by X-ray Powder Diffractometry (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM), respectively. The electrical and dielectric spectra of the Sm → MnNi (x ≤ 0.10) NSFs were systematically examined using dielectric spectroscopy, electrical conductivity, and complex impedance analyses. The AC conductivity spectra reveal both frequency- and temperature-dependent dispersions, which are consistent with Jonscher's power law (JPL), whereas the Arrhenius analysis of the DC conductivity discloses two thermally activated regimes dominated by localized and long-range small polaron-hopping processes. The calculated activation energies ranged from 10 to 190 meV in the LT region and 420 to 530 meV in the HT region, confirming the dual conduction mechanism. Furthermore, dielectric constant and loss spectra demonstrated Maxwell–Wagner interfacial polarization and Koop's phenomenological behavior, with Sm3+ ion-substitution ratios most strongly influencing the grain boundary resistance and polarization dynamics. In addition, the complex electrical modulus and Nyquist plots reveal single semicircular arcs, indicating grain-boundary-dominant conduction and non-Debye relaxation. Interestingly, moderate Sm3+ ion-substitution ratios, ranging from 0.050 to 0.075, improve lattice distortion, lower the activation energy (EA), and enhance hopping conduction; conversely, excessive substitution ratios, such as 0.10, lead to defect clustering, significantly reducing electrical performance. All these findings strongly highlight the critical role of rare earth (RE) element substitution in shaping conduction and dielectric relaxation in Mn–Ni NSFs.