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.

Although single magnetic domain nanoparticles are very promising for many applications, size reduction usually results in low magnetic anisotropy and unblocked domain at room temperature, e.g., superparamagnetism. An alternative approach is core–shell nanoparticles featured by exchange bias coupling between ferro­(i)­magnetic [F­(i)­M] and antiferromagnetic (AFM) phases. Although exchange bias coupling has been reported for very diverse core–shell nanoparticles, it is difficult to compare these studies to rationalize the effect of many structural parameters on the magnetic properties. Herein, we report on a systematic study which consists of the modulation of the shell structure and its influence on the exchange bias coupling. A series of Fe3−δO4@CoO core–shell nanoparticles has been synthesized by seed-mediated growth based on the thermal decomposition technique. The variation of Co reactant concentration resulted in the modulation of the shell structure for which thickness, crystallinity, and interface with the iron oxide core strongly affect the magnetic properties. The thickest CoO shell and the largest F­(i)­M/AFM interface led to the largest exchange bias coupling. Very high values of coercive field (19 000 Oe) and M R/M S ratio (0.86) were obtained. The most stricking results consist of the increase of the coercive field while exchange field vanishes when the CoO thickness decreases: it is ascribed to the diffusion of Co species in the surface layer of iron oxide which generates to some extent cobalt ferrite and induces hard/soft exchange coupling between ferrimagnetic phases.

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