Macrospin model of an assembly of magnetically coupled core-shell nanoparticles

Kons, Corisa; Srikanth, H.; Phan, M. H.; Arena, D. A.; Pereiro, Manuel

doi:10.1103/physrevb.106.104402

Cited by 3 publications

(2 citation statements)

References 19 publications

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“…In addition to anisotropy and Zeeman contributions, the Hamiltonian includes intraparticle interactions between the macrospins, angular-dependent dipolar energies between the total magnetization of different NPs, and Heisenberg interactions between NPs. Please refer to the Methods section and ref for a more complete description of the model.…”

Section: Results and Analysismentioning

confidence: 99%

“…A similar slow approach to saturation is recovered for the CFO@ FO variant, but the mesoscale model suggests a weak coercivity (nonzero remanence) which is not observed in the 300 K data. Additional details on calculations of the temperature-dependent, ensemble-averaged magnetization, along with a fully atomistic calculation of the spins from a single NP, are presented in ref 54.…”

Section: ■ Introductionmentioning

confidence: 99%

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Influence of Hard/Soft Layer Ordering on Magnetization Reversal of Bimagnetic Nanoparticles: Implications for Biomedical/Theranostic Applications

Kons

Krycka

Almanza-Robles

et al. 2023

ACS Appl. Nano Mater.

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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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Section: Results and Analysismentioning

confidence: 99%

Section: ■ Introductionmentioning

confidence: 99%

Influence of Hard/Soft Layer Ordering on Magnetization Reversal of Bimagnetic Nanoparticles: Implications for Biomedical/Theranostic Applications

Kons

Krycka

Almanza-Robles

et al. 2023

ACS Appl. Nano Mater.

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Magnetization reversals in core–shell sphere clusters: finite-element micromagnetic simulation and machine learning analysis

Park,

Kim

2023

Sci Rep

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Recently developed permanent magnets, featuring specially engineered microstructures of inhomogeneous magnetic phases, are being considered as cost-effective alternatives to homogeneous single-main-phase hard magnets composed of Nd2Fe14B, without compromising performance. In this study, we conducted a comprehensive examination of a core–shell sphere cluster model of Ce-substituted inhomogeneous Nd2-δCeδFe14B phases versus homogeneous magnetic phases, utilizing finite-element micromagnetic simulation and machine learning methods. This involved a meticulous, sphere-by-sphere analysis of individual demagnetization curves calculated from the cluster model. The grain-by-grain analyses unveiled that these individual demagnetization curves can elucidate the overall magnetization reversal in terms of the nucleation and coercive fields for each sphere. Furthermore, it was observed that Nd-rich spheres exhibited much broader ranges of nucleation and coercive field distributions, while Nd-lean spheres showed relatively narrower ranges. To identify the key parameter responsible for the notable differences in the nucleation fields, we constructed a machine learning regression model. The model utilized numerous hyperparameter sets, optimized through the very fast simulated annealing algorithm, to ensure reliable training. Using the kernel SHapley Additive eXplanation (SHAP) technique, we inferred that stray fields among the 11 parameters were closely related to coercivity. We further substantiated the machine learning models’ inference by establishing an analytical model based on the eigenvalue problem in classical micromagnetic theory. Our grain-by-grain interpretation can guide the optimal design of granular hard magnets from Nd2Fe14B and other abundant rare earth transition elements, focusing on extraordinary performance through the careful adjustment of microstructures and elemental compositions.

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Effect of the Core–Shell Exchange Coupling on the Approach to Magnetic Saturation in a Ferrimagnetic Nanoparticle

Komogortsev,

Stolyar,

Mokhov

et al. 2024

Magnetochemistry

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The generally accepted model of the magnetic structure of an iron oxide core‒shell nanoparticle includes a single-domain magnetically ordered core surrounded by a layer with a frozen spin disorder. Due to the exchange coupling between the shell and core, the spin disorder should lead to nonuniform magnetization in the core. Suppression of this inhomogeneity by an external magnetic field causes the nonlinear behavior of the magnetization as a function of the field in the region of the approach to magnetic saturation. The equation proposed to describe this effect is tested using a micromagnetic simulation. Analysis of the approach to magnetic saturation of iron oxide nanoparticles at different temperatures using this equation can be used to estimate the temperature evolution of the core‒shell coupling energy and the size of the uniformly magnetized nanoparticle core and the temperature behavior of this size.

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Macrospin model of an assembly of magnetically coupled core-shell nanoparticles

Cited by 3 publications

References 19 publications

Influence of Hard/Soft Layer Ordering on Magnetization Reversal of Bimagnetic Nanoparticles: Implications for Biomedical/Theranostic Applications

Influence of Hard/Soft Layer Ordering on Magnetization Reversal of Bimagnetic Nanoparticles: Implications for Biomedical/Theranostic Applications

Magnetization reversals in core–shell sphere clusters: finite-element micromagnetic simulation and machine learning analysis

Effect of the Core–Shell Exchange Coupling on the Approach to Magnetic Saturation in a Ferrimagnetic Nanoparticle

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