Reaction Time Effect on Magnetic Properties of CoFe ₂ O ₄ Nanoparticles Through Surfactant‐Assisted Hydrothermal Synthesis

CoFe 2 O 4 nanoparticles were synthesized by surfactant‐assisted hydrothermal treatment for 8–12 h to study the role of reaction time on their structural and magnetic properties. XRD revealed increased from ∼14 to 20 nm coherent domains with increasing synthesis time, while the particle sizes determined with TEM were essentially the same (∼20 nm). Mössbauer spectroscopy showed a decrease in Fe 3+ ions at incomplete fivefold octahedral sites and a corresponding increase at fully coordinated sixfold sites, indicating recrystallization of structurally disordered shells and amorphous‐like ultrasmall particles forming near‐surface region. FORC analysis confirmed that the magnetically soft fraction associated with these disordered regions becomes progressively harder and couples to the magnetically hard phase via exchange interactions.