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

Yanes, R., Garanin, D.A., Evans, R., Chantrell, R.W., Chubykalo-Fesenko, O. and Kachkachi, H. (2007) Effective anisotropies and energy barriers of magnetic nanoparticles with Néel's surface anisotropy. Physical Review B (PRB). Art. No. 064416. ISSN 1098-0121

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Abstract

Magnetic nanoparticles with Néel surface anisotropy, different internal structures, surface arrangements and elongation are modelled 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 elongation. 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) The competition between the core and surface anisotropies leads to a new energy that contributes to both the 2nd¡ and 4th¡order effective anisotropies. We also evaluate energy barriers Delta E 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 Delta E/V = K8 +6Ks/D, where K8 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.

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
Academic Units:	The University of York > Physics (York)
Depositing User:	Ms Diana Hilmer
Date Deposited:	24 Oct 2007 14:22
Last Modified:	19 Feb 2013 11:34
Published Version:	http://dx.doi.org/10.1103/PhysRevB.76.064416
Status:	Published
Refereed:	Yes
Related URLs:	http://dx.doi.org/10.1103/PhysRevB.76.06...
URI:	http://eprints.whiterose.ac.uk/id/eprint/3423

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