Synthesis and Exchange Bias in γ-Fe₂O₃/CoO and Reverse CoO/γ-Fe₂O₃ Binary Nanoparticles

Abstract

γ-Fe2O3/CoO and reverse CoO/γ-Fe2O3 binary nanoparticles have been prepared by a facile, two-step chemical route through the thermal decomposition of Co(acac)2 and Fe(acac)3 in hot oleyl amine. The synthesis is based on heterogeneous nucleation and growth of the second component on seed nanoparticles of the first one. The binary particles have been characterized by X-ray diffraction, transmission electron microscopy, and cognate techniques. The heteroparticles are very stable in nonpolar organic solvents due the organic coating that is formed in situ. The presence of the antiferromagnetic CoO phase leads to unidirectional anisotropy, which is shown to originate from both magnetically compensated and uncompensated interfaces.

Exchange Bias Effects in Iron Oxide-Based Nanoparticle Systems

“…Exchange bias has been observed in different types of iron oxide-based core/shell nanoparticles, such as Fe/γ-Fe 2 O 3 [27,28,29], Fe/Fe 3 O 4 [30,31], FeO/Fe 3 O 4 [32,33,34], Fe 3 O 4 /γ-Fe 2 O 3 [35], γ-Fe 2 O 3 /CoO [36], MnFe 2 O 4 /γ-Fe 2 O 3 , and CoFe 2 O 4 /γ-Fe 2 O 3 [73]. The EB effect has been mainly associated with exchange anisotropy originating from exchange coupling between the shell and the core.…”

“…The magnetic properties of the core/shell MNPs are dependent both on the core and the shell size, shape, composition, interface roughness, etc. [27,28,29,30,31,32,33,34,35,36,73,74,75,76]. These parameters can be easily tuned during the synthesis process and a rich variety of magnetic core/shell systems can be produced by a proper choice of the materials and dimensions of both the core and the shell.…”

“…Therefore, the surface spins of the γ-Fe 2 O 3 shell of 2–4 nm thickness are expected to be largely disordered, contributing significantly to the EB (more precisely, the M-H loop shift) in the case of Fe/γ-Fe 2 O 3 MNPs with diameters less than 10 nm [28,29]. These observations shed light on the different origins of EB in between the magnetite- and maghemite-based core/shell MNP systems [27,28,29,30,31,32,33,34,35,36,73]. …”

“…Recently, iron/iron oxide core/shell nanoparticle systems have been exploited for such applications because the combination of the high magnetization of the core (Fe) and the chemical stability and biocompatibility of the shell (Fe 3 O 4 or γ-Fe 2 O 3 ) lead to more suitable overall properties than either material alone [24,25,26]. These systems also provide excellent models for probing the roles played by interface and surface spins and their impacts on EB in exchange-coupled nanostructures, inspiring a large body of work on core/shell Fe/γ-Fe 2 O 3 [27,28,29], Fe/Fe 3 O 4 [30,31], FeO/Fe 3 O 4 [32,33,34], Fe 3 O 4 /γ-Fe 2 O 3 [35], γ-Fe 2 O 3 /CoO [36], and Au/Fe 3 O 4 particles [37,38,39]. Oxidization-driven migration of metal atoms from the core to the shell of iron/iron oxide nanoparticle systems has been shown to occur via the Kirkendall effect, leading to a morphological transformation into hollow nanoparticles, where the additional (inner) surface area strongly affects magnetic properties including EB [40,41,42,43,44].…”

The influence of oxidation process on exchange bias in egg-shaped FeO/Fe3O4 core/shell nanoparticles

“…Since 1956 EB was observed in many core-shell nanoparticles with T C >T N (Cure Temperature of FM is higher than Néel's Temperature of AF [6]. In recent studies it was found that exchange coupling in AF/FiM nanoparticles strongly depends on the core size, the shell thickness and the shape or crystal quality [7,8].…”