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

Abstract

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.

Exploring Exchange Bias Coupling in Fe3−δO4@CoO Core–Shell Nanoparticle 2D Assemblies

“…Fe 3− δ O 4 @CoO core–shell nanoparticles have been synthesized by a two‐step seed‐mediated growth method based on the successive thermal decomposition of iron stearate and cobalt stearate as we have recently reported . The functionalization of Fe 3− δ O 4 @CoO core–shell nanoparticles by 12‐azido(dodecyl) phosphonic acid (AP12) was achieved by ligand exchange process from a suspension of nanoparticles coated with oleic acid by adapting successfully the strategy we developed for iron oxide nanoparticles .…”

Drug releasing nanoplatforms activated by alternating magnetic fields

“…[46] However, up to now most works were devoted to pure IONPs [23] because of their proven biocompatibility and ease of synthesis and of tuning of their size within a narrow size distribution. Despite their high potential, the development of doped ferrites was thus limited, their synthesis being generally more complex (chemical heterogeneities) [59][60][61] and their biocompatibility being discussed.…”

Tunnel Magnetoresistance in Self-Assemblies of Exchange-Coupled Core/Shell Nanoparticles

“…[27] In this regard, an exciting possibility is the fabrication of devices based on self-assemblies of exchange coupled core/shell MNPs with tailored magnetic properties. [28] The coercive field in these systems can be finely modified through the interface magnetic coupling [29][30][31][32][33][34], the core size and shell thickness, [35][36][37] or the magnetic anisotropy of the components. [23,[38][39][40] Devices of this type should provide a way to manipulate at will the characteristic switching field of TMR by controlling the magnetic coupling across the core/shell interface.…”