Critical defect threshold governing magnetic anisotropy in Ce-substituted Zn–Mg spinel ferrites​

Rare-earth substitution in spinel ferrites often produces complex structural and magnetic responses that cannot be explained by simple compositional scaling. In this work, Zn0.5Mg0.5CexFe2-xO₄ (x = 0.0–0.10) nanoferrites were investigated to identify the mechanistic origin of their magnetic anisotropy evolution. Structural, spectroscopic, and magnetic measurements reveal the existence of a critical defect threshold near x ≈ 0.075. Up to this composition, Ce incorporation progressively increases lattice distortion, microstrain, and oxygen vacancy concentration, accompanied by a rise in the Fe²⁺/Fe³⁺ ratio and an increase in the Ce³⁺/Ce⁴⁺ ratio derived from XPS analysis. These defect-mediated electronic changes weaken metal–oxygen bond stiffness and modify Fe–O–Fe superexchange geometry, as evidenced by FTIR vibrational softening and Rietveld-refined structural parameters. Electron spin resonance measurements further reveal a pronounced shift in the Landé g-factor, indicating enhanced spin–orbit coupling and internal magnetic fields. Consequently, the effective magnetocrystalline anisotropy (Keff) and coercivity reach a maximum at x ≈ 0.075. Beyond this threshold, further Ce substitution leads to excessive defect accumulation, structural disorder, and partial magnetic dilution, resulting in a reduction of anisotropy despite the continued increase in Fe²⁺ fraction. These results demonstrate that magnetic anisotropy in Ce-modified Zn–Mg ferrites is governed by a defect-controlled structural relaxation mechanism rather than simple ionic substitution. The identification of this critical defect threshold provides a strategy for tuning magnetic anisotropy in spinel ferrite nanostructures for electromagnetic and sensing applications.