Effects of spin non-collinearities in magnetic nanoparticles

Abstract

In a many-spin approach that takes account of the internal structure, microscopic interactions and single-site anisotropies, we investigate the effect of spin non-collinearities induced by the boundary and surface anisotropy on the behaviour of individual magnetic nanoparticles. Through analytical and numerical calculations, we show that there are mainly two regimes separated by some critical value of the surface anisotropy constant $K_s$ which controls the intensity of spin non-collinearities: i) the so called Stoner-Wohlfarth or Néel-Brown regime of a macrospin undergoing a coherent switching, ii) the many-spin regime where the strong spin non-collinearities invalidate the coherent mechanism, and where the particle's magnetic state and switching mechanisms can no longer be modeled by a macrospin. For small-to-intermediate values of $K_s$, and within two models of surface anisotropy (transverse and Néel), the behaviour of the nanoparticle can be modeled by that of a macrospin with an effective potential energy containing a uniaxial and cubic anisotropy terms. This effective spin model provides a crossover between the two regimes.

arXiv comment6 pages, 6 figs, Invited Paper at III Joint European Magnetic Symposia (JEMS), San Sebastian (Spain), 26-30 June 06

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• Magnetic neutron scattering from spherical nanoparticles with Neel surface anisotropy: Analytical treatment
• Spatial magnetization profile in spherical nanomagnets with surface anisotropy: Green's function approach
• Ferromagnetic resonance of a two-dimensional array of nanomagnets: Effects of surface anisotropy and dipolar interactions
• Single-particle versus collective effects in assemblies of nanomagnets: screening
• Surface-induced reduction of the switching field in nanomagnets
• Including nanoparticle shape into macrospin models