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Comparisons of characteristic timescales and approximate models for Brownian magnetic nanoparticle rotations.

Authors
  • Reeves, Daniel B1
  • Weaver, John B1
  • 1 Department of Physics and Astronomy, Dartmouth College , Hanover, New Hampshire 03755, USA.
Type
Published Article
Journal
Journal of Applied Physics
Publisher
AIP Publishing
Publication Date
Jun 21, 2015
Volume
117
Issue
23
Pages
233905–233905
Identifiers
PMID: 26130846
Source
Medline
Language
English
License
Unknown

Abstract

Magnetic nanoparticles are promising tools for a host of therapeutic and diagnostic medical applications. The dynamics of rotating magnetic nanoparticles in applied magnetic fields depend strongly on the type and strength of the field applied. There are two possible rotation mechanisms and the decision for the dominant mechanism is often made by comparing the equilibrium relaxation times. This is a problem when particles are driven with high-amplitude fields because they are not necessarily at equilibrium at all. Instead, it is more appropriate to consider the "characteristic timescales" that arise in various applied fields. Approximate forms for the characteristic time of Brownian particle rotations do exist and we show agreement between several analytical and phenomenological-fit models to simulated data from a stochastic Langevin equation approach. We also compare several approximate models with solutions of the Fokker-Planck equation to determine their range of validity for general fields and relaxation times. The effective field model is an excellent approximation, while the linear response solution is only useful for very low fields and frequencies for realistic Brownian particle rotations.

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