Magnetization switching in a Heisenberg model for small ferromagnetic particles

TL;DR

This paper explores how small ferromagnetic particles switch their magnetization direction under thermal activation, analyzing the transition from coherent rotation to nucleation depending on anisotropy, particle size, and magnetic field strength.

Contribution

It provides a detailed study of the crossover from coherent rotation to nucleation in a Heisenberg model, including theoretical analysis and Monte Carlo simulations.

Findings

Identification of energy barriers for magnetization reversal

Theoretical prediction of reversal mechanisms based on anisotropy and size

Monte Carlo simulations confirming the crossover behavior

Abstract

We investigate the thermally activated magnetization switching of small ferromagnetic particles driven by an external magnetic field. For low uniaxial anisotropy the spins can be expected to rotate coherently, while for sufficient large anisotropy they should behave Ising-like, i.e., the switching should then be due to nucleation. We study this crossover from coherent rotation to nucleation for the classical three-dimensional Heisenberg model with a finite anisotropy. The crossover is influenced by the size of the particle, the strength of the driving magnetic field, and the anisotropy. We discuss the relevant energy barriers which have to be overcome during the switching, and find theoretical arguments which yield the energetically favorable reversal mechanisms for given values of the quantities above. The results are confirmed by Monte Carlo simulations of Heisenberg and Ising models.