Role of La/Ce co-substitution in modulating conductivity and interfacial polarization in hard-soft CoFe2O4/NiFe2O4 spinel ferrite nanocomposites [H/S CoLaxCexFe2-2xO4/NiFe2O4 (x ≤ 0.10) SFNCs]

Abstract Hard/soft spinel ferrite nanocomposites H/S CoLa x Ce x Fe 2−2x O 4 /NiFe 2 O 4 (x ≤ 0.10) SFNCs were successfully prepared using a sol–gel auto-combustion method to systematically investigate the influence of La/Ce co-substitution, frequency, and temperature on their structural, electrical, and dielectric properties. XRD, SEM, TEM, HR-TEM, and EDX analyses confirmed the formation of the spinel phases and nanocomposite morphology, while revealing a systematic reduction in crystallite size from ~ 43.44 nm (x = 0.00) to ~ 26.11 nm (x = 0.10), attributed to rare-earth-ion-induced grain-boundary pinning. Broadband dielectric spectroscopy, complex impedance (Z*), and electric modulus (M*) analyses reveal strong frequency dispersion and thermally activated behavior governed by hopping conduction, space-charge effects, and defect-assisted relaxation. AC and DC conductivity results identify an optimal substitution range (x ≈ 0.06–0.08), where thermally assisted charge transport is maximized and activation energy is minimized. In contrast, excessive substitution (x = 0.10) introduces structural disorder and insulating grain boundaries, leading to reduced mobility and increased resistivity. Dielectric loss and three-dimensional εʺ analyses demonstrate a clear transition from bulk-dominated dissipation at low substitution levels to relaxation-dominated behavior at x ≥ 0.08, associated with enhanced Maxwell–Wagner interfacial polarization. Impedance spectroscopy reveals a crossover from grain-controlled to grain-boundary-dominated transport, confirmed by the emergence of multiple semicircular arcs and equivalent circuit fitting showing a sharp increase in grain-boundary resistance. Complementary modulus analysis highlights strongly non-Debye relaxation with a broad distribution of relaxation times arising from cationic disorder and interfacial heterogeneity. Predominantly, La/Ce co-substitution is shown to be an effective strategy for tuning grain-boundary-controlled dielectric behavior and interfacial polarization in hard/soft ferrite nanocomposites, making these materials promising candidates for EMI shielding and high-frequency electronic applications requiring controlled impedance and reduced energy dissipation.