On the exact crystal structure of exchange-biased Fe₃O₄–CoO nanoaggregates produced by seed-mediated growth in polyol

Abstract

The nucleation and growth mechanism of exchange-biased magnetically contrasted highly crystallized and perfectly epitaxied Fe 3 O 4 -CoO nanoaggregates was investigated using local probe analysis, namely 57 Fe Mössbauer and L 2,3 Co XMCD spectroscopies, and advanced magnetometry. The granular nanocomposites were produced by a two-step chemical route. Spinel iron oxide nanoparticles were first synthesised by forced hydrolysis of iron acetate in a polyol and then used as seeds for rock-salt cobalt monoxide nanocrystalline growth. In one case, this crystallochemical step is carried out by separating the seeds, washing and drying them before mixing them in a fresh polyol solution of cobalt precursors. In another case, it is conducted by introducing, directly in the seed reaction solution, the cobalt precursors in the same Fe/Co nominal atomic ratio. It appears that the composite formation proceeds differently in the two cases. It is driven, in the ex situ route, by Co 2+ cation diffusion into the spinel lattice leading to the formation of Co x Fe 3−x O 4 core single crystals around which CoO nanocrystals start growing. Conversely, in the in situ route, it is driven by Co 2+ cation reactivity with unreacted dissolved Fe 3+ , conducted to the coating of the preformed Fe 3 O 4 cores by a CoFe 2 O 4 -like thin interlayer, itself decorated by outer CoO nanocrystals. These reaction schemes are well confirmed by magnetometry experiments. The low-temperature magnetic properties of the produced composites are consistent with the establishment of an exchange-bias feature into hard-

Exchange‐Biased Fe3−xO4‐CoO Granular Composites of Different Morphologies Prepared by Seed‐Mediated Growth in Polyol: From Core–Shell to Multicore Embedded Structures

“…The value associated with Fe 3+ ions in the A and B sites, in contrast, was found larger than that for pure magnetite, and smaller than that for CoFe 2 O 4 . Moreover, the observed ferrous ions' atomic ratio in composites is about 8%, lower than the expected 33% for pure magnetite, suggesting the diffusion of Co 2+ ions in IO lattice during CO growth, replacing progressively in the spinel B sites, Fe 3+ vacancies and leading a decreasing Co 2+ gradient concentration, from their surface to the inner core of IO seeds. This concentration gradient is consistent with a final chemical composition of Co x Fe 3− x O 4 ‐CoO instead of Fe 3 O 4 ‐CoO.…”

“…This epitaxy is a key issue for CO heterogeneous nucleation and a promoting factor for seed‐mediated growth in the polyol reaction solution. Experimentally, it can be correlated to the large average crystal size inferred from XRD analysis for each phases, the so‐called XRD crystal size being in fact the average coherent crystallographic length . Fourth, for all the explored samples, there are no evidences of any structural defects, like dislocations, stacking faults, or surface amorphization.…”

“…In contrast pristine particles, used as seeds, exhibit a lattice parameter slightly smaller than that of Fe 3 O 4 . This is attributed to their partial oxidation in air, making their chemical composition closer to that of maghemite γ‐Fe 2 O 3 than that of Fe 3 O 4 . The chemical composition evolution of IO in the final composite powders may derive from two different paths.…”

Magnetic Traits in CoO‐Core@CoFe2O4‐shell Like Nanoparticles

“…Since polyol process is reputed for the production of MCM particles of epitaxial quality,, and for maintaining grain size uniformity, we used this technique to prepare core‐shell NPs based on submicrometer pseudo‐single CoO crystals coated by a poly‐ and nanocrystalline CoFe 2 O 4 shell. We complemented these investigations by studying the magnetic properties of single‐crystal CoFe 2 O 4 particles ( aCFO ) of the same size as the ferrite spinel crystals which make up the shell of the CoO‐core@CoFe 2 O 4 ‐shell particles ( CO‐CFO ).…”

Combining X‐Ray Whole Powder Pattern Modeling, Rietveld and Pair Distribution Function Analyses as a Novel Bulk Approach to Study Interfaces in Heteronanostructures: Oxidation Front in FeO/Fe3O4 Core/Shell Nanoparticles as a Case Study

“…Although transmission electron microscopy (TEM) and scanning TEM (STEM) imaging, particularly when combined with electron energy loss spectroscopy (EELS) analysis, can give an accurate account of the morphology and composition of the particles, they have the drawback that typically only a small number of particles can be analyzed. Thus, novel approaches to investigate the structure of core/shell nanoparticles using “bulk” methods are being proposed, such as anomalous small‐angle X‐ray scattering (ASAXS), X‐ray magnetic circular dichroism (XMCD), or resonant inelastic X‐ray spectroscopy (RIXS) with MCD analysis . However, when only one type of ion is involved in the structure (e.g., different oxidation states of the same ion, e.g., FeO/Fe 3 O 4 core/shell nanoparticles or different polymorphs of the same compound, e.g., γ‐Fe 2 O 3 vs ε‐Fe 2 O 3 ) the characterization by STEM‐EELS, RIXS‐MCD, or ASAXS remains rather complex.…”