Iron oxide nanoparticles (NPs) with average sizes in the range 4−28 nm have been obtained by varying different synthesis parameters of the thermal decomposition of an iron precursor (iron stearate) in the presence of surfactants in high boiling solvents. The synthesis parameters affect the NPs nucleation and growth steps, by modifying the stability of iron stearate on which depend the monomer formation and concentration, in agreement with the LaMer model. The monomer formation, which is reaction time and/or temperature dependent, is thus found to vary mainly as a function of the nature of solvents and ligands. The structural and magnetic characterizations of NPs with sizes in the range 5−20 nm confirm that the composition of NPs evolves from the maghemite for small sizes (typically <8 nm) up to a core of rather stoichiometric magnetite surrounded by an oxidized shell for large sizes (>12 nm) via a perturbed oxidized state for intermediate sizes. The values of saturation magnetization lower than those of bulk magnetite and maghemite were found to be related to this composition evolution and to the presence of oxidation defects, surface spin canting and volume spin canting as a function of NPs diameter. Small NPs presented mainly a surface spin canting. NPs with large sizes display M s which depends on their oxidized shell thickness, defects and surface spin canting. NPs with intermediate sizes display a surface and in particular a volume spin canting due to a disordered structure induced by a perturbed oxidation state in these NPs.

Most studies on the synthesis of nanoparticles are currently focused on the controlled synthesis of new morphologies, including core-shell structures, which are expected to exhibit new magnetic properties for uses in spintronics and recording media applications. In this study, the structure, morphology, and composition of cubic-shaped nanoparticles are carefully investigated and compared to those of spherically shaped nanoparticles through the use of a combination of techniques: X-ray diffraction (XRD) and transmission electronic microscopy (TEM) combined with more sensitive techniques such as scanning transmission electron microscopy-high-angle annular dark field (STEM-HAADF) imaging, electron tomography, and holography. While spherically shaped nanoparticles (NPs) crystallize with the spinel structure, cubic-shaped NPs can be described as a cubic core of w€ ustite surrounded by a spinel shell. Stresses are observed at the core-shell interface and within the spinel shell due to the epitaxial growth and oxidation mechanisms of the w€ ustite phase. Furthermore, magnetic measurements displayed an exchange bias coupling between the antiferromagnetic (AFM) core and the ferrimagnetic (FIM) shell structure of cubic-shaped nanoparticles. It is shown that the magnetic properties are influenced by stresses generated by the oxidation of w€ ustite and, also exhibit variations depending upon the evolution of this core-shell structure as a function of the oxidation time.

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