Larmor precession and Debye relaxation of single-domain magnetic nanoparticles

TL;DR

This paper provides analytical solutions for the behavior of single-domain magnetic nanoparticles under AC magnetic fields, clarifying phenomenological models and explaining experimental observations related to power dissipation and susceptibility.

Contribution

It introduces exact analytical solutions for steady states in magnetic nanoparticles under AC fields, validating and explaining phenomenological models like Rosensweig's chord susceptibility.

Findings

Analytical solutions confirm the use of chord susceptibility for loss calculations.

Power dissipation is identical for circular and linear fields in RMS sense.

Approximation accuracy is limited to certain magnetic field strengths.

Abstract

The numerous phenomenological equations used in the study of the behaviour of single-domain magnetic nanoparticles are described and some issues clarified by means of qualitative comparison. To enable a quantitative \textit{application} of the model based on the Debye (exponential) relaxation and the torque driving the Larmor precession, we present analytical solutions for the steady states in presence of circularly and linearly polarized AC magnetic fields. Using the exact analytical solutions, we can confirm the insight that underlies Rosensweig's introduction of the "chord" susceptibility for an approximate calculation of the losses. As an important consequence, it can also explain experiments, where power dissipation for both fields were found to be identical in "root mean square" sense. We also find that this approximation provides satisfactory numerical accuracy only up to magnetic fields for which the argument of the Langevin function reaches the value 2.8.