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We investigate the spatial distribution of spin orientation in magnetic nanoparticles consisting of hard and soft magnetic layers. The nanoparticles are synthesized in a core–shell spherical morphology where the target stoichiometry of the magnetically hard, high anisotropy layer is CoFe2O4 (CFO), while the synthesis protocol of the lower anisotropy material is known to produce Fe3O4. The nanoparticles have a mean diameter of ∼9.2–9.6 nm and are synthesized as two variants: a conventional hard/soft core–shell structure with a CFO core/FO shell (CFO@FO) and the inverted structure FO core/CFO shell (FO@CFO). High-resolution electron microscopy confirms the coherent spinel structure across the core–shell boundary in both variants, while magnetometry indicates the nanoparticles are superparamagnetic at 300 K and develop a considerable anisotropy at reduced temperatures. Low-temperature M vs H loops suggest a multistep reversal process. Small angle neutron scattering (SANS) with full polarization analysis reveals a considerable alignment of the spins perpendicular to the field even at fields approaching saturation. The perpendicular magnetization is surprisingly correlated from one nanoparticle to the next, though the interaction is of limited range. More significantly, the SANS data reveal a pronounced difference in the reversal process of the magnetization parallel to the field for the two nanoparticle variants. For the CFO@FO nanoparticles, the core and shell magnetizations appear to track each other through the coercive region, while in the FO@CFO variant, the softer Fe3O4 core reverses before the higher anisotropy CoFe2O4 shell, consistent with expectations from mesoscale magnetic modeling. These results highlight the interplay between interfacial exchange coupling and anisotropy as a means to tune the composite properties of the nanoparticles for tailored applications including biomedical/theranostic uses.
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Effect of Oleylamine on the Surface Chemistry, Morphology, Electronic Structure, and Magnetic Properties of Cobalt Ferrite Nanoparticles
The influence of oleylamine (OLA) concentration on the crystallography, morphology, surface chemistry, chemical bonding, and magnetic properties of solvothermal synthesized CoFe2O4 (CFO) nanoparticles (NPs) has been thoroughly investigated. Varying OLA concentration (0.01–0.1 M) resulted in the formation of cubic spinel-structured CoFe2O4 NPs in the size-range of 20–14 (±1) nm. The Fourier transform spectroscopic analyses performed confirmed the OLA binding to the CFO NPs. The thermogravimetric measurements revealed monolayer and multilayer coating of OLA on CFO NPs, which were further supported by the small-angle X-ray scattering measurements. The magnetic measurements indicated that the maximum saturation (MS) and remanent (Mr) magnetization decreased with increasing OLA concentration. The ratio of maximum dipolar field (Hdip), coercivity (HC), and exchanged bias field (Hex) (at 10 K) to the average crystallite size (Dxrd), i.e., (Hdip/Dxrd), (HC/Dxrd), and (Hex/Dxrd), increased linearly with OLA concentration, indicating that OLA concurrently controls the particle size and interparticle interaction among the CFO NPs. The results and analyses demonstrate that the OLA-mediated synthesis allowed for modification of the structural and magnetic properties of CFO NPs, which could readily find potential application in electronics and biomedicine.
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- Award ID(s):
- 1827745
- PAR ID:
- 10420035
- Date Published:
- Journal Name:
- Nanomaterials
- Volume:
- 12
- Issue:
- 17
- ISSN:
- 2079-4991
- Page Range / eLocation ID:
- 3015
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.3390/nano12173015
