Cation Engineering in Spinel M₀.₄Fe₂.₆O₄ (M = Co, Mn, Zn) Ferrofluids: Correlating Crystal Structure, Magnetic properties, and Rheological Response

Abstract

Cation substitution provides an effective route for tailoring the magnetic and rheological properties of spinel ferrite ferrofluids. In this work, Zn₀.₄Fe₂.₆O₄ (ZFO), Mn₀.₄Fe₂.₆O₄ (MFO), and Co₀.₄Fe₂.₆O₄ (CFO) nanoparticles were synthesized under identical conditions and dispersed in kerosene to investigate the influence of divalent-cation chemistry on their structural, magnetic, and flow behaviour. X-ray diffraction confirmed the formation of the spinel ferrite structure, while FTIR and EDX analyses supported the formation of the respective metal–oxygen framework and elemental composition. Magnetic measurements revealed distinct saturation magnetization, coercivity, and effective anisotropy among the ferrofluids, demonstrating the influence of cation substitution on their magnetic characteristics. Rheological measurements under an applied magnetic field of 0.18 T over a shear-rate range of 1–1000 s⁻¹ revealed pronounced shear-thinning behaviour for all three ferrofluids. The experimental viscosity data were satisfactorily described by the Power-law, Cross, and Carreau models. CFO exhibited the highest zero-shear viscosity and longest characteristic relaxation time, indicating comparatively stronger structural stability under low-shear conditions, whereas MFO showed the weakest rheological response. The results establish a clear relationship between cation chemistry, magnetic properties, field-induced nanoparticle organization, and shear-dependent flow behaviour, providing a pathway for tailoring spinel-ferrite ferrofluids for magnetically controlled applications.