Cobalt Concentration-Dependent Structural and Magnetic Transitions in Nanocrystalline Ni0.9−xZn0.1CoxFe2O4 Ferrites

Nanocrystalline Ni_0.9−xZn_0.1Co_xFe₂O₄ (x = 0.2, 0.4, 0.6, 0.8) ferrites were synthesized via the sol–gel method, yielding a cubic single-phase spinel structure (Fd−3m), as confirmed through Rietveld refinement. The crystallite size was observed to increase from 7 to 10 nm with higher Co substitution. Magnetic characterization revealed a pronounced dependence on Co concentration, with zero-field-cooled and field-cooled magnetization curves exhibiting bifurcation indicative of magnetic relaxation phenomena. The Ni_0.5Zn_0.1Co_0.4Fe₂O₄ composition exhibited a blocking temperature of 191 K, a Curie temperature of 207 K, and a transition from ferromagnetic ordering at 5 K to superparamagnetic behavior at 300 K. Similarly, Ni_0.3Zn_0.1Co_0.6Fe₂O₄ demonstrated a transition at 251 K with retained ferromagnetic ordering at 5 K. In contrast, Ni_0.1Zn_0.1Co_0.8Fe₂O₄ displayed magnetic irreversibility and a paramagnetic state at 300 K. Raman spectroscopy further corroborated the inverse spinel structure, revealing characteristic vibrational modes at ~ 460 cm⁻¹ and 680 cm⁻¹. These findings underscore the pivotal role of Co substitution in modulating the structural and magnetic properties of Ni_0.9−xZn_0.1Co_xFe₂O₄ nanocrystals, particularly their temperature-dependent magnetic phase transitions. Collectively, the results highlight the influence of Co concentration on the structural and magnetic properties of Ni_0.9−xZn_0.1Co_xFe₂O₄ nanocrystals, particularly their temperature-dependent magnetic transitions.