A Detailed Investigation of the Onion Structure of Exchanged Coupled Magnetic Fe_3−δO₄@CoFe₂O₄@Fe_3−δO₄ Nanoparticles

Abstract

Nanoparticles which combine several magnetic phases offer wide perspectives for cutting edge applications because of the high modularity of their magnetic properties. Besides the addition of the magnetic characteristics intrinsic to each phase, the interface that results from core-shell and, further, from onion structures leads to synergistic properties such as magnetic exchange coupling. Such a phenomenon is of high interest to overcome the superparamagnetic limit of iron oxide nanoparticles which hampers potential applications such as data storage or sensors. In this manuscript, we report on the design of nanoparticles with an onion-like structure which have been scarcely reported yet. These nanoparticles consist in a Fe3- O4 core covered by a first shell of CoFe2O4 and a second shell of Fe3- O4, e.g. a Fe3- O4@CoFe2O4@Fe3- O4 onion-like structure. They were synthesized by a multi-step seed mediated growth approach which consists to perform three successive thermal decomposition of a metal complexes in a high boiling point solvent (about 300 °C). Although TEM micrographs clearly show the growth of each shell from the iron oxide core, core sizes and shell thicknesses markedly differ from what is suggested by the size increase. We investigated very precisely the structure of nanoparticles in performing high resolution (scanning) TEM imaging and geometrical phase analysis (GPA). The chemical composition and spatial distribution of atoms were studied by electron energy loss spectroscopy (EELS) mapping and spectroscopy. The chemical environment and oxidation state of cations were investigated by Mössbauer spectrometry, soft X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD). The combination of these techniques allowed us to estimate the increase of Fe 2+ content in the iron oxide core of the core@shell structure and the increase of the cobalt ferrite shell thickness in the core@shell@shell one, while the iron oxide shell appears to be much thinner than expected. Thus, the modification of the chemical composition as well as the size of the Fe3- O4 core and the thickness of the cobalt ferrite shell have a high impact on the magnetic properties. Furthermore, the growth of the iron oxide shell also markedly modifies the magnetic properties of the core-shell nanoparticles, thus demonstrating the high potential of onion-like nanoparticles for tuning accurately the magnetic properties of nanoparticles according to the desired applications.

A detailed investigation of the core@shell structure of exchanged coupled magnetic nanoparticles after performing solvent annealing

“…Although the lattice mismatch (<1%) between the spinel and the wüstite phases is small, the growth of CoO from different facets of the Fe 3− δ O 4 core seems to result in strains that propagates from the core–shell interface as we observe recently in Fe 3− δ O 4 @CoFe 2 O 4 @Fe 3− δ O 4 nanoparticles. 43…”

“…We performed the solvent thermal annealing of Fe 3- O 4 @CoO nanoparticles in order to simulate the experimental conditions prior to the seed-mediated growth of an additional shell of magnetic material as we reported recently. 32,43 We studied the structure and the magnetic properties of a Fe 3- O 4 @CoO nanoparticles after thermal annealing to 290 °C in a high boiling temperature solvent. The morphology, the size, the chemical composition and the crystal structure of nanoparticles were studied before and after thermal annealing by using advanced analysis techniques such as HR-TEM, XAS, XMCD and DANES.…”

“…Although the lattice mismatch (< 1 %) between the spinel and the wüstite phases is small, the growth of CoO from different facets of the Fe 3- O 4 core seems to result in strains that propagates from the core-shell interface as we observe recently in Fe 3- O 4 @CoFe 2 O 4 @Fe 3- O 4 nanoparticles. 43 Infrared spectroscopy showed typical bands of metal oxide nanoparticles coated with oleic acid (Figure 4a). Bands at 2922 cm -1 ( as C-H) and 2952 cm -1 ( s C-H) correspond to the hydrocarbon chains of oleic acid.…”

Aberration-corrected transmission electron microscopy with Zernike phase plates

“…Figure 3a shows a HRTEM image of Fe 3-δ O 4 NPs on an aC support film close to Gaussian focus. Lattice fringes are visible in most NPs and the power spectrum reveals reflections rings at 0.3 nm (022), 0.25 nm (113), 0.16 nm (115) and 0.15 nm (044) corresponding to different lattice distances of the magnetite spinel crystal structure [16]. Figure 3d shows a HRTEM image of the identical sample area as in (a), which has been obtained directly after insertion of a previously centered Zernike PP, without changing any additional imaging parameter.…”

“…We present experimental results using unheated Zernike PPs with varying cut-on frequencies applied on different nanomaterials, e.g. Fe 3-δ O 4 magnetite nanoparticles (NPs) [16]. The effect of beam parallelity (convergence) is studied.…”