Impact of solubility limits and phase segregation on the structural, morphological, and magnetic properties of silver-substituted cobalt ferrite

Silver-substituted cobalt ferrite nanoparticles (Co 1−x Ag x Fe 2 O 4 ; x = 0.0, 0.2, 0.4, and 0.6) were synthesized by the sol–gel auto-combustion method to investigate how Ag solubility limits and phase segregation govern the structural, morphological, and magnetic evolution of cobalt ferrite. While prior literature has extensively focused on narrow, low-doping concentrations where single-phase structures are preserved, the behavior of this system at higher substitution levels and the influence of phase segregation on the coupled structural and magnetic evolution remain insufficiently understood. This work addresses this knowledge gap by systematically investigating the Ag solubility limit and its influence on phase segregation and magnetic evolution. X-ray diffraction revealed the appearance of metallic Ag secondary phases from x = 0.2, indicating that the Ag solubility limit is reached at or below this composition. Consequently, compositions with x > 0.2 behave as Ag/CoFe₂O₄ nanocomposites rather than true substitutional ferrites, resulting in a non-linear variation of the lattice parameter (8.37–8.43 Å). The crystallite size decreased to 19.60 ± 1.56 nm at x = 0.4 because of Zener pinning, accompanied by the maximum microstrain (18.20 × 10⁻ 3 ) and dislocation density (26.03 × 10 14 m⁻ 2 ). The evolution of lattice strain and phase segregation fundamentally altered the magnetic behavior, suppressing the saturation magnetization from 19.54 emu/g for pristine CoFe 2 O 4 to 0.13 emu/g at x = 0.2, followed by 0.35 emu/g at x = 0.4 and a partial recovery to 11.60 emu/g at x = 0.6 as the ferrite phase became stabilized. This study demonstrates that the magnetic response is governed by the interplay between Ag solubility, phase segregation, lattice strain, and ferrite phase stabilization rather than by Ag concentration alone. These findings establish the critical role of Ag solubility limits and phase segregation in governing the structural and magnetic evolution of cobalt ferrite nanoparticles, providing fundamental design principles for ferrite-based magnetic materials and magnetically recoverable functional systems.