Effect of Cobalt Doping Concentration on the Crystalline Structure and Magnetic Properties of Monodisperse Co_<i>x</i>Fe_3–<i>x</i>O₄ Nanoparticles within Nonpolar and Aqueous Solvents

In this work, we investigate the effect of cobalt substitution on the size evolution, crystal structure, and magnetic properties of Fe₃O₄ nanoparticles. Monodisperse Co_<i>x</i>Fe_3–<i>x</i>O₄ nanoparticles were prepared, using a one-step method, by direct heating process of iron­(III) and cobalt­(II) acetylacetonates in high-boiling-point inert organic solvent. The quantities of precursors added were based on stoichiometric Fe/Co ratio of desired ferrite. Elemental analyses ICP-AES evidenced successful cobalt doping. The doped particles showed a cobalt-deficient composition. Transmission electron microscopy demonstrated the large changes of particle size as a function of cobalt doping. The magnetization measurements showed an unchanged saturation magnetization only up to <i>x</i> = 0.24, beyond which it significantly decreased. To make the as-synthesized nanoparticles suitable for biomedical applications, oleic acid ligands are exchanged with caffeic acid molecules leading to stable nanoparticles in physiological conditions