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Effective anisotropies and energy barriers of magnetic nanoparticles with Néel surface anisotropy

Yanes, R., Chubykalo-Fesenko, O., Kachkachi, H., Garanin, D. A., Evans, R., Chantrell, R. W. and Evans, Richard Francis Llewelyn (2007) Effective anisotropies and energy barriers of magnetic nanoparticles with Néel surface anisotropy. Physical Review B (PRB). 064416. ISSN 1098-0121

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Abstract

Magnetic nanoparticles with Néel surface anisotropy, different internal structures, surface arrangements, and elongation are modeled as many-spin systems. The results suggest that the energy of many-spin nanoparticles cut from cubic lattices can be represented by an effective one-spin potential containing uniaxial and cubic anisotropies. It is shown that the values and signs of the corresponding constants depend strongly on the particle's surface arrangement, internal structure, and shape. Particles cut from a simple cubic lattice have the opposite sign of the effective cubic term, as compared to particles cut from the face-centered cubic lattice. Furthermore, other remarkable phenomena are observed in nanoparticles with relatively strong surface effects. (i) In elongated particles surface effects can change the sign of the uniaxial anisotropy. (ii) In symmetric particles (spherical and truncated octahedral) with cubic core anisotropy surface effects can change the sing of the latter. We also show that the competition between the core and surface anisotropies leads to a new energy that contributes to both the second- and fourth-order effective anisotropies. We evaluate energy barriers ΔE as functions of the strength of the surface anisotropy and the particle size. The results are analyzed with the help of the effective one-spin potential, which allows us to assess the consistency of the widely used formula ΔE/V= K∞ +6 Ks /D, where K∞ is the core anisotropy constant, Ks is a phenomenological constant related to surface anisotropy, and D is the particle's diameter. We show that the energy barriers are consistent with this formula only for elongated particles for which the surface contribution to the effective uniaxial anisotropy scales with the surface and is linear in the constant of the Néel surface anisotropy. © 2007 The American Physical Society.

Item Type: Article
Copyright, Publisher and Additional Information: © 2007 The American Physical Society. This is an author produced version of a paper published in Physical Review B. Uploaded in accordance with the publisher's self-archiving policy.
Keywords: Core-shell nanoparticles, Cobalt particles, Gamma-FE2O3, Dichroism, Origin, Condensed Matter Physics
Institution: The University of York
Academic Units: The University of York > Physics (York)
The University of York
The University of York > Biology (York)
Depositing User: Ms Diana Hilmer
Date Deposited: 24 Oct 2007 14:22
Last Modified: 28 Jul 2015 18:59
Published Version: http://dx.doi.org/10.1103/PhysRevB.76.064416
Status: Published
Refereed: Yes
Related URLs:
URI: http://eprints.whiterose.ac.uk/id/eprint/3423

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