High‐resolution transmission electron microscopy shows a perfect epitaxy between the face‐centered cubic lattices of the spinel core and the rock‐salt shell. 57Fe Mössbauer spectrometry evidences an evolution of the cation distribution among the spinel lattice in the cobalt ferrite core during the core–shell NPs processing. The major purpose is to study systematically the characteristics of the as‐produced powders, making emphasis on their internal crystallographic arrangement and their magnetic properties. These evidences are further corroborated by the materials magnetic properties.Ĭobalt ferrite ferrimagnetic nanoparticles (NPs) are prepared and used in this work as seeds to grow a thin antiferromagnetic poly‐ and nanocrystalline CoO shell. Conversely, the cation-exchange is boosted if the iron oxide phase is structurally prone to vacancies, like wüstite, and the shell where the iron cations have been partly substituted becomes quite thicker. Moreover, the starting phase of iron oxide strongly dictates the number of iron cations that could be replaced: if it is structurally free of vacancies, like magnetite, the maximum amount of exchanged cations is low, and only affecting the nanoparticles’ most external, disordered layers. We show here that it unavoidably leads to core/shell structures with only the shell undergoing the cation-exchange. Surprisingly, we found results quite discordant from the few ones so far published in exploiting again this approach. Such a synthetic strategy was however seldom applied to iron oxide magnetic nanoparticles, where the substitution of iron with diverse transition element cations was described as occurring in their whole volume. In the last years a novel synthetic approach based on cation-exchange has been reported, being one of its main advantages that to keep nanoparticles shape and size unaltered. Tuning the nanoparticles magnetic behavior via the control of their features has always been challenging, since these features are mostly intertwined. the formation of a Co-doped interfacial layer in the nanoparticles. Element-specific XMCD measurements showed the fine coupling of Fe and Co cations which agree with Co-Ferrite in each sample, e.g. The coercive field (HC) was increased by core reduction while the blocking temperature (TB) was increased by a larger core. In addition, the magnetic properties were also influenced by the core size. The Co-Ferrite FiM shells resulted in better enhancement of Eeff than a CoO AFM shell. Each core-shell nanoparticle displayed enhanced effective magnetic anisotropy energy (Eeff) in comparison to pristine Fe3-δO4 nanoparticles because of magnetic coupling at the core-shell interface. The structural properties of nanoparticles were correlated to their magnetic properties which were investigated by SQUID magnetometry. ![]() The operating conditions influenced significantly the oxidation rate of Fe²⁺ in the core as well as the occupancy of Oh sites by Fe²⁺ and Co²⁺ cations. The distribution of Fe and Co elements in the crystal structure was described accurately by XAS and XMCD. The structural properties of core-shell nanoparticles were investigated by a wide panel of techniques such as HAADF, STEM and XRD. The formation of the Co-Ferrite shell was also found to happen through two different mechanisms: the diffusion of Co or the growth at the iron oxide surface. ![]() Depending on the thermal stability and the concentration of Co precursor, we were able to control the formation of a hard ferrimagnetic (FiM) Co-Ferrite shell or an antiferromagnetic (AFM) CoO shell at the surface of a soft FiM Fe3-δO4 core. Here, we report on the design of core-shell nanoparticles by performing the successive thermal decomposition of Fe and Co complexes. Exchange coupled core-shell nanoparticles present a high potential to tune adequately the magnetic properties for specific applications such as nanomedicine or spintronics.
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