DC bias-dependent dielectric response and superlinear AC conduction in nanocrystalline Mn–Zn spinel ferrites

This study investigates the structural, morphological, and electrical properties of nanocrystalline Mn0.9​Zn0.1​Fe2−x​Lax​O4​ (x=0.00, 0.05, 0.10, and 0.15) ferrites synthesized via the co-precipitation method. X-ray diffraction (XRD) patterns confirm the formation of a single-phase cubic spinel structure for all compositions. Structural parameters, including lattice constant and unit cell volume, initially decrease with La substitution up to x=0.10, followed by a slight expansion at x=0.15. The crystallite size reaches a minimum of 6.9 nm at the x=0.10 threshold, reflecting a state of maximum lattice distortion. Magnetic measurements reveal near-superparamagnetic behavior with dominant thermally assisted moment relaxation, as evidenced by squareness ratios far below the single-domain limit (Mr/Ms ≈ 0.04–0.05), with saturation magnetization showing a sharp minimum at x = 0.10 and partial recovery at x = 0.15. The dc bias–dependent dielectric and electrical properties of the samples were investigated by complex impedance spectroscopy in the 1 kHz–10 MHz frequency range under 0–40 V bias. Nyquist analysis revealed a near-ideal yet non-Debye relaxation characterized by high Cole–Cole α values (0.88–0.95), indicating grain-boundary-dominated charge dynamics. The x = 0.10 composition exhibited reduced grain resistance, suggesting a critical substitution level for enhanced charge transport. AC conductivity followed Jonscher's power law but displayed a systematic superlinear frequency exponent (1.23 ≤ s ≤ 1.36) for all compositions and bias voltages. The persistence of s > 1 confirms the emergence of a Superlinear Power-Law (SLPL) regime, deviating from the conventional sublinear response typically reported for Mn–Zn ferrites. The superlinear behavior, observed even in the undoped sample, is attributed to intrinsic nanostructural disorder and localized carrier dynamics. These results highlight the role of defect structure and microstructural heterogeneity in governing high-frequency charge transport in spinel ferrites. © 2026 Elsevier Ltd and Techna Group S.r.l. All rights are reserved, including those for text and data mining, AI training, and similar technologies.