Surface anisotropy in ferromagnetic nanoparticles

Abstract

The effect of surface anisotropy on the magnetic ground state of a ferromagnetic nanoparticle is investigated using atomic Monte Carlo simulation for spheres of radius R=6a and R=15a, where a is the interatomic spacing. It is found that the competition between surface and bulk magnetocrystalline anisotropy imposes a “throttled” spin structure where the spins of outer shells tend to orient normal to the surface while the core spins remain parallel to each other. For large values of surface anisotropy, the spins in sufficiently small particles become radially oriented either inward or outward in a “hedgehog” configuration with no net magnetization. Implications for FePt nanoparticles are discussed.

Inhomogeneous states in a small magnetic disk with single-ion surface anisotropy

“…Recently, the stable three-dimensional analog of vortices, hedgehog configuration has been discovered for a ball-shaped particle with strong normal border anisotropy by numeric calculations. 38 On the other hand, for the superfluid 3 He-A, which is defined in terms of our model by use of the infinitely strong surface anisotropy and isotropic volume properties, the true minimum constitutes less symmetric state (boojum, or fountain) with one surface singularity and without the symmetry plane. 30,37 In our case the "boojum-like" distribution appears only for non-stable saddle point, which separates the vortex and non-singular states.…”

Surface effects in the magnetic properties of crystalline 3 nm ferrite nanoparticles chemically synthesized

“…Evidences of distinct magnetic configuration of small nanoparticles were found by Monte Carlo simulations on nanoparticles with radius 6 times or 15 times grater than the interatomic distance, showing a "throttled" spin structure, with the surface spins oriented normally to the surface while the core spins are parallel to each other. 19 In Ref. 5, a speromagneticlike structure was observed for ␥-Fe 2 O 3 phosphate-coated nanoparticles with diameter of 3-10 nm as result for the interplay between surface anisotropy, exchange, and interparticle interactions.…”

Influence of surface anisotropy on the hysteresis of magnetic nanoparticles

“…Moreover, current experimental techniques do not allow to probe sur-face microscopic structure in geometries such as those of a nanoparticle, a fact that forces to assume phenomenological models for surface anisotropy. Monte Carlo (MC) simulations constitute a benchmark to probe different models since, by modeling magnetic ions as classical [5,6,7,8] or Ising spins [9,10,11], the interatomic interactions, the local magnetic anisotropy directions and the values of anisotropy constants can be easily varied while taking into account the exact geometry of the underlying lattice of magnetic ions. Here, we will present results of MC simulations aimed to understand the influence of surface anisotropy on the magnetic properties and hysteresis of a single nanoparticle, neglecting the effects of interactions with other particles.…”