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<p>Hard/soft spinel ferrite nanocomposites H/S CoLa<sub>x</sub>Ce<sub>x</sub>Fe<sub>2−2x</sub>O<sub>4</sub>/NiFe<sub>2</sub>O<sub>4</sub> (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&#xa0;nm (x = 0.00) to ~ 26.11&#xa0;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.</p>

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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]

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  • 发表日期：

    2026年2月27日

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  • A. Baykal,
  • B. Ünal,
  • M. A. Almessiere,
  • A. Demir Korkmaz,
  • Y. Slimani

摘要

Hard/soft spinel ferrite nanocomposites H/S CoLaxCexFe2−2xO4/NiFe2O4 (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.