Statistical Mechanics of Nonuniform Magnetization Reversal

TL;DR

This paper develops an analytical model for the thermally activated magnetization reversal in quasi-1D ferromagnetic systems, revealing how soliton pair nucleation influences coercivity in magnetic recording media.

Contribution

It provides an analytical expression for the energy barrier of soliton-antisoliton pairs across all external field values, extending the understanding of magnetization reversal beyond existing models.

Findings

Analytical energy barrier expression valid for all external fields.

Reversal rate reduces to soliton nucleation in the overdamped double sine-Gordon model.

Predicted coercivity is lower than traditional Néel and Brown theories.

Abstract

The magnetization reversal rate via thermal creation of soliton pairs in quasi-1D ferromagnetic systems is calculated. Such a model describes e.g. the time dependent coercivity of elongated particles as used in magnetic recording media. The energy barrier that has to be overcome by thermal fluctuations corresponds to a soliton-antisoliton pair whose size depends on the external field. In contrast to other models of first order phase transitions such as the phi^4 model, an analytical expression for this energy barrier is found for all values of the external field. The magnetization reversal rate is calculated using a functional Fokker-Planck description of the stochastic magnetization dynamics. Analytical results are obtained in the limits of small fields and fields close to the anisotropy field. In the former case the hard-axis anisotropy becomes effectively strong and the magnetization reversal rate is shown to reduce to the nucleation rate of soliton-antisoliton pairs in the overdamped double sine-Gordon model. The present theory therefore includes the nucleation rate of soliton-antisoliton pairs in the double sine-Gordon chain as a special case. These results demonstrate that for elongated particles, the experimentally observed coercivity is significantly lower than the value predicted by the standard theories of N\'eel and Brown.