Equilibrium magnetization of a quasispherical cluster of single-domain particles

TL;DR

This study investigates the equilibrium magnetization of finite spherical clusters of single-domain particles, accounting for dipole interactions and anisotropy, revealing deviations from classical models and proposing a modified mean-field theory.

Contribution

It introduces a modified mean-field theory for cluster magnetization that accounts for dipolar interactions and anisotropy effects, improving predictions over classical models.

Findings

Magnetization is lower than classical Langevin predictions.

Modified mean-field theory accurately describes weak field magnetization.

Anisotropy further reduces cluster magnetization, especially at high fields.

Abstract

Equilibrium magnetization curve of a rigid finite-size spherical cluster of single-domain particles is investigated both numerically and analytically. The spatial distribution of particles within the cluster is random. Dipole-dipole interactions between particles are taken into account. The particles are monodisperse. It is shown, using the stochastic Landau-Lifshitz-Gilbert equation that the magnetization of such clusters is generally lower than predicted by the classical Langevin model. In a broad range of dipolar coupling parameters and particle volume fractions, the cluster magnetization in the weak field limit can be successfully described by the modified mean-field theory, which was originally proposed for the description of concentrated ferrofluids. In moderate and strong fields, the theory overestimates the cluster magnetization. However, predictions of the theory can be improved by adjusting the corresponding mean-field parameter. If magnetic anisotropy of particles is additionally taken into account and if the distribution of the particles' easy axes is random and uniform, then the cluster equilibrium response is even weaker. The decrease of the magnetization with increasing anisotropy constant is more pronounced at large applied fields. The phenomenological generalization of the modified mean-field theory, that correctly describes this effect for small coupling parameters, is proposed.