A\nDetailed Investigation of the Onion Structure of\nExchanged Coupled Magnetic Fe3−δO4@CoFe2O4@Fe3−δO4 Nanoparticles

Nanoparticles that combine several\nmagnetic phases offer wide perspectives\nfor cutting edge applications because of the high modularity of their\nmagnetic properties. Besides the addition of the magnetic characteristics\nintrinsic to each phase, the interface that results from core–shell\nand, further, from onion structures leads to synergistic properties\nsuch as magnetic exchange coupling. Such a phenomenon is of high interest\nto overcome the superparamagnetic limit of iron oxide nanoparticles\nwhich hampers potential applications such as data storage or sensors.\nIn this manuscript, we report on the design of nanoparticles with\nan onion-like structure which has been scarcely reported yet. These\nnanoparticles consist of a Fe3−δO4 core covered by a first shell of CoFe2O4 and\na second shell of Fe3−δO4, e.g.,\na Fe3−δO4@CoFe2O4@Fe3−δO4 onion-like structure.\nThey were synthesized through a multistep seed-mediated growth approach\nwhich consists consists in performing three successive thermal decomposition\nof metal complexes in a high-boiling-point solvent (about 300 °C).\nAlthough TEM micrographs clearly show the growth of each shell from\nthe iron oxide core, core sizes and shell thicknesses markedly differ\nfrom what is suggested by the size increasing. We investigated very\nprecisely the structure of nanoparticles in performing high resolution\n(scanning) TEM imaging and geometrical phase analysis (GPA). The chemical\ncomposition and spatial distribution of atoms were studied by electron\nenergy loss spectroscopy (EELS) mapping and spectroscopy. The chemical\nenvironment and oxidation state of cations were investigated by 57Fe Mössbauer spectrometry, soft X-ray absorption spectroscopy\n(XAS) and X-ray magnetic circular dichroism (XMCD). The combination\nof these techniques allowed us to estimate the increase of Fe2+ content in the iron oxide core of the core@shell structure\nand the increase of the cobalt ferrite shell thickness in the core@shell@shell\none, whereas the iron oxide shell appears to be much thinner than\nexpected. Thus, the modification of the chemical composition as well\nas the size of the Fe3−δO4 core\nand the thickness of the cobalt ferrite shell have a high impact on\nthe magnetic properties. Furthermore, the growth of the iron oxide\nshell also markedly modifies the magnetic properties of the core–shell\nnanoparticles, thus demonstrating the high potential of onion-like\nnanoparticles to accurately tune the magnetic properties of nanoparticles\naccording to the desired applications.