Optimization of cobalt concentration for improved magnetic characteristics and stability of Co_xFe_3-xO₄ mixed ferrite nanomagnetic fluids

The present work reports the preparation of stable kerosene-based nanomagnetic fluids (NMFs) with Co_xFe_3-xO₄magnetic nanoparticle dispersion. The Co_xFe_3-xO₄ (0.8≤x ≤ 0) magnetic nanoparticles dispersed NMFs have been synthesized by a wet chemical method. The effect of cobalt concentration variation on magnetic properties has been probed in detail. The anisotropy and magnetic properties are significantly dependent on the concentration of the cobalt which has been investigated by room temperature dc magnetic measurement (M-H). The M-H plot indicates that the optimized concentration of the cobalt in the system leads to an increase in the saturation magnetization of the NMFs from 43.87 emu/g to 61.78 emu/g at an optimized concentration (x = 0.2). This can be correlated to the distribution of cations on tetrahedral and octahedral sites. A 24% Cobalt (Co²⁺) substitution from tetrahedral sites in place of Fe³⁺ ions increases the exchange interaction among tetrahedral and octahedral sites resulting in improvement in the magnetic performance of the NMFs. Although, after the critical concentration (>0.2) of Cobalt doping, the net exchange interaction decreases which result in a lowering of magnetic saturation for the higher concentration of Cobalt doping. Further, the structural properties of these fluids have been investigated by the X-ray diffraction (XRD) technique. The cation distribution of XRD patterns has been identified by the Le Bail Rietveld refinement technique that confirms the single cubic spinel phase belonging to the Fd-3m space group. The structural distribution correlated with the magnetic properties indicates that the variation in the composition of the mixed ferrites systems significantly affects the magnetic behavior. The optimized concentration of doping helps in improving the performance of NMFs which is a key bottleneck in the development of NMF applications such as biomedical, thermoelectric, EMI shielding, and spintronics.