Effects of spin non-collinearities in magnetic nanoparticles

TL;DR

This paper investigates how boundary and surface anisotropies induce spin non-collinearities in magnetic nanoparticles, revealing two regimes of behavior and proposing an effective macrospin model for intermediate cases.

Contribution

It introduces a detailed analysis of spin non-collinearities caused by surface anisotropy and develops an effective macrospin model bridging different magnetic regimes.

Findings

Two regimes separated by a critical surface anisotropy constant.

Effective macrospin model with uniaxial and cubic anisotropy terms.

Surface anisotropy influences switching mechanisms and magnetic states.

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 N\'eel-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 N\'eel), 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.