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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Baykal A., ÜNAL B., Almessiere M., Korkmaz A. D., Slimani Y.

Applied Physics A: Materials Science and Processing, vol.132, no.3, 2026 (SCI-Expanded, Scopus)

  • Publication Type: Article / Article
  • Volume: 132 Issue: 3
  • Publication Date: 2026
  • Doi Number: 10.1007/s00339-026-09421-5
  • Journal Name: Applied Physics A: Materials Science and Processing
  • Journal Indexes: Science Citation Index Expanded (SCI-EXPANDED), Scopus, Chemical Abstracts Core, Chimica, Compendex, INSPEC
  • Keywords: Cole-Cole analysis, Conductivity, Dielectric properties, Hard-soft ferrites, La/Ce Co-Substitution
  • Istanbul Gelisim University Affiliated: Yes

Abstract

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.