Characterization of Néel and Brownian Relaxations Isolated from Complex Dynamics Influenced by Dipole Interactions in Magnetic Nanoparticles

Abstract

The magnetization dynamics involved in applying an alternating field are composed of a superposition of Néel and Brownian relaxations. To evaluate the mechanisms of magnetic relaxations, it is necessary to individually evaluate the Néel and Brownian regimes. In this study, by applying a fast responding pulse field, the two-step magnetization response of magnetic nanoparticles dispersed in a fluid in the Brownian regime occurred after the Néel regime. We isolated Néel and Brownian relaxations from an experimentally observed superposition relaxation system by fitting the theoretical calculation to the measured time evolution response of the magnetization, which was in agreement with the susceptibility that was measured through applying an alternating magnetic field. The dependence of Néel and Brownian relaxation's dominance in the superposition system on relaxation times was clearly observed. In particular, the effect of dipole interactions on Néel and Brownian relaxation times were confirmed by changing the particle concentration in a magnetic fluid. Elucidation of the transition process from Néel to Brownian relaxation, which is influenced by the proportion between Néel and Brownian relaxation times and the effect of dipole interactions on the individual relaxation processes, allows us to develop optimal magnetic nanoparticles for a variety of biomedical applications.