Systematic Study of Exchange Coupling in Core–Shell Fe_3−δO₄@CoO Nanoparticles

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 Fe_3−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 <i>M</i>_R/<i>M</i>_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

Mastering the Shape and Composition of Dendronized Iron Oxide Nanoparticles To Tailor Magnetic Resonance Imaging and Hyperthermia

The current challenge in the field of nanomedicine is the design of multifunctional nano-objects effective both for the diagnosis and treatment of diseases. Here, dendronized FeO_1–<i>x</i>@Fe_3–<i>x</i>O₄ nanoparticles with spherical, cubic, and octopode shapes and oxidized Fe_3–<i>x</i>O₄ nanocubes have been synthesized and structurally and magnetically characterized. Strong exchange bias properties are highlighted in core–shell nanoparticles (NPs) due to magnetic interactions between their antiferromagnetic core and ferrimagnetic shell. Both <i>in vitro</i> relaxivity measurements and nuclear magnetic resonance (NMR) distribution profiles have confirmed the very good <i>in vitro</i> magnetic resonance imaging (MRI) properties of core–shell and cubic shape NPs, especially at low concentration. This might be related to the supplementary anisotropy introduced by the exchange bias properties and the cubic shape. The high heating values of core–shell NPs and oxidized nanocubes at low concentration are attributed to dipolar interactions inducing different clustering states, as a function of concentration. <i>In vivo</i> MRI studies have also evidenced a clustering effect at the injection point, depending on the concentration, and confirmed the very good <i>in vivo</i> MRI properties of core–shell NPs and oxidized nanocubes in particular at low concentration. These results show that these core–shell and cubic shape dendronized nano-objects are very suitable to combine MRI and hyperthermia properties at low injected doses