Magnetization of nanomagnet assemblies: Effects of anisotropy and dipolar interactions

TL;DR

This paper studies how anisotropy and dipolar interactions influence the magnetization behavior of nanoparticle assemblies, providing analytical and numerical insights into deviations from classical models.

Contribution

It offers new analytical expressions for magnetization considering anisotropy and dipolar interactions, validated by numerical simulations.

Findings

Anisotropy and dipolar interactions cause measurable deviations from Langevin behavior.

Analytical formulas accurately describe magnetization in low and high field regimes.

Numerical Monte Carlo results support the analytical approximations.

Abstract

We investigate the effect of anisotropy and weak dipolar interactions on the magnetization of an assembly of nanoparticles with distributed magnetic moments, i.e., assembly of magnetic nanoparticles in the one-spin approximation, with textured or random anisotropy. The magnetization of a free particle is obtained either by a numerical calculation of the partition function or analytically in the low and high field regimes, using perturbation theory and the steepest-descent approximation, respectively. The magnetization of an interacting assembly is computed analytically in the range of low and high field, and numerically using the Monte Carlo technique. Approximate analytical expressions for the assembly magnetization are provided which take account of the dipolar interactions, temperature, magnetic field, and anisotropy. The effect of anisotropy and dipolar interactions are discussed and the deviations from the Langevin law they entail are investigated, and illustrated for realistic assemblies with the lognormal moment distribution.