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

TL;DR

This paper models magnetic nanoparticles with Neel surface anisotropy as many-spin systems, revealing how surface effects, internal structure, and elongation influence their effective anisotropy energies and barriers.

Contribution

It demonstrates that the energy of many-spin nanoparticles can be effectively represented by a one-spin potential with uniaxial and cubic anisotropies, dependent on surface and internal properties.

Findings

Surface effects can reverse the sign of uniaxial anisotropy in elongated particles.

Surface effects can change the sign of cubic core anisotropy in symmetric particles.

Competition between core and surface anisotropies introduces a new energy component affecting higher-order anisotropies.

Abstract

Magnetic nanoparticles with Neel 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) In symmetric particles (spherical and truncated octahedral) with cubic core anisotropy surface effects can change its sign. We also show that the competition between the core and surface anisotropies leads to a new energy that contributes to both the 2nd- and 4th-order effective anisotropies.