Stepwise seed-mediated growth of multiphase spinel@Wüstite Core–Shell nanoparticles with exchange-coupled magnetic properties

A series of exchange-coupled magnetic nanoparticles featuring multiple magnetic phases arranged in a core-shell architecture was synthesized through a three-step seed-mediated growth process. Iron, cobalt, and nickel precursors were sequentially thermally decomposed in high-boiling-point solvents (approximately 300 °C), enabling the successive growth of CoO and NiO shells on Fe₃O₄ nanoparticle's cores. The structural and chemical characteristics of these nanoparticles were thoroughly investigated using a combination of advanced analytical techniques, including scanning transmission electron microscopy (STEM) in high-angle annular dark-field imaging mode (HAADF), electron energy-loss spectroscopy STEM (EELS-STEM), and X-ray magnetic circular dichroism (XMCD). After each thermal decomposition step, the nanoparticle size progressively increased, accompanied by noticeable changes in morphology, indicating significant surface reconstruction. Furthermore, the chemical structure proved to be more complex than initially anticipated. The high synthesis temperature promoted cation interdiffusion at the interfaces as well as partial dissolution–recrystallization processes at the nanoparticle surface. An intermixed Co/Ni shell, composed of a combination of wüstite and spinel phases, was found to influence the magnetic properties of the nanoparticles through exchange bias coupling. This effect is directly correlated with the relative proportions of Co and Ni cations incorporated within the shell.