Enhanced dielectric and soft magnetic properties of rare earth (Sm, Er) co-substituted cobalt ferrites nanocrystals

Abstract

Nanocrystalline rare earth (Sm, Er) co-substituted cobalt ferrites CoFe_2-2xSm_xEr_xO₄ (0 $\le x\le 0.10$ referred to as SECF) was synthesized via the sol-gel method. X-ray diffraction (XRD) patterns confirmed the formation of a spinel structure along with the segregation of orthoferrites (REFeO₃) phases from x ≥ 0.06 samples. The nanocrystallite ranging from 18 to 72 nm was determined using the Williamson-Hall method (WH). Rietveld refinement of the XRD patterns reveals the changes in crystal parameters and the redistributions of cations with co-substitution. The Fourier transform infrared (FTIR) and Raman spectra confirmed the formation of a spinel phase. High-resolution X-ray photoelectron spectroscopy (XPS) was used to investigate the elemental states of Fe, Co, Sm, Er and O defects. The magnetic properties in the SECF samples transition from hard ferrites to soft ferrites with co-substitution, attributed to the strain-induced effect weakening the magnetic interaction and reducing Co²⁺ ions at the octahedral sites. Electron spin resonance (ESR) spectra reveal the interplay of the magnetic interaction and a deviation of the Lande g-factors with co-substitution. The dielectric properties (ε′ and ε′′) were enhanced with co-substitution from x ≥ 0.06 samples. The ac conductivity displayed two distinct regions (I and II) with frequency, consistent with Jonscher's double power law (JDPL), except for the x = 0.10 sample, which follows Jonscher's single power law (JPL). The temperature-dependent frequency exponents (s₁ and s₂) suggest two conduction mechanisms: correlated barrier hopping (CBT) and non-overlapping small polaron tunneling (NSPT). The activation energies (E_a) obtained from multiple impedance representations were consistent with the electron hopping in Fe²⁺/Fe³⁺. The Nyquist plots indicate the dominance of the grain boundary resistance (R_gb) to the electrical response.