Exchange‐Biased Fe_3−xO₄‐CoO Granular Composites of Different Morphologies Prepared by Seed‐Mediated Growth in Polyol: From Core–Shell to Multicore Embedded Structures

Abstract

Magnetically contrasted granular hetero‐nanostructures are prepared by seed‐mediated growth in polyol, properly combining two oxide phases with different magnetic order, ferrimagnetic (F) partially oxidized magnetite Fe3−xO4 and antiferromagnetic (AF) cobalt oxide. Spinel Fe3−xO4 nanoparticles are first synthesized and then used as seeds for rock salt CoO nanocrystals growth. Three different hetero‐nanostructure designs are realized, acting on the content ratio between the seeds and the deposit's precursors during the synthesis. For all of them, the spinel and the rock salt phases are confirmed by X‐ray diffraction and high‐resolution transmission electron microscopy. Both phases are obtained in high‐crystalline quality with a net epitaxial relationship between the two crystallographic lattices. Mössbauer spectrometry confirms the cobalt cation diffusion into the spinel seeds, giving favorable chemical interfacing with the rock salt deposit, thus prevailing its heterogeneous nucleation and consequently offering the best condition for exchange‐bias (EB) onset. Magnetic measurements confirm EB features. The overall magnetic properties are found to be a complex interplay between dipolar interactions, exchange anisotropy at the F/AF interface, and magnetocrystalline anisotropy enhancement in the F phase, due to Co2+ diffusion into iron oxide's crystalline lattice. These results underline the powerfulness of colloidal chemistry for functional granular hetero‐nanostructured material processing.

Giant Exchange‐Bias in Polyol‐Made CoFe2O4‐CoO Core–Shell Like Nanoparticles

“…Note that measured value of µ 0 H E = 365 mT on the composite particles is significantly higher than the best values recently reported on oxide‐based granular composites called “giant exchange‐bias” systems, like NiFe 2 O 4 ‐CoO, SrFe 12 O 19 @CoO, CoFe 2 O 4 @Co 3 O 4, and others summarized in Table SI‐1 in the Supporting Information. These results are very promising and make our engineered CFO‐CO composites particularly valuable as starting powder for the production of exchange‐biased oxide‐based consolidates.…”

Magnetic Properties of Fe3O4/CoFe2O4 Composite Nanoparticles with Core/Shell Architecture

“…The synthesis of nanostructures with the core/shell architecture is a rather complicated procedure, because it must provide a high crystallinity degree in both the core and the shell, guarantee a highquality epitaxy, and minimize the mixing processes at the core/shell interface. That is why when producing Fe 3 O 4 /CoFe 2 O 4 composite nanoparticles, we account for the results of previous works performed by our and other research groups [9][10][11][12][13]. In our previous works, the combined application of X-ray diffraction and Mössbauer spectroscopy to 57 Fe nuclei allowed us to draw conclusion that, among the methods used to synthesize magnetic nanoparticles (MNPs), the method of co-precipitation from a diethylene glycol (DEG) solution makes it possible to fabricate magnetite nanoparticles with the least amount of the maghemite and goethite phases [10,12].…”

Magnetic-Assisted Treatment of Liver Fibrosis

“…Advanced co-precipitation methods are performed at high temperature and pressure, by hydrothermal surface treatment, or hydrothermal routes [93,94,95], as well as in non-aqueous medium by solvothermal methods [96,97,98]. The polyol process is another interesting method which is a cost-effective and easily scalable method to produce MNPs of high quality and variety morphology, from simple pseudo-spherical to multi-core nanoflowers of core-shell MNPs [99,100,101]. In the polyol process, solvents also play the role of a reducing agent and a surfactant.…”