Rotation of Magnetization Derived from Brownian Relaxation in Magnetic Fluids of Different Viscosity Evaluated by Dynamic Hysteresis Measurements over a Wide Frequency Range.

Satoshi Ota,Ryoji Takeda,Tsutomu Yamada,Ryoichi Kitaguchi,Yasushi Takemura

doi:10.3390/nano6090170

Satoshi Ota, Ryoji Takeda + Show 3 more

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https://doi.org/10.3390/nano6090170

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Abstract

The dependence of magnetic relaxation on particle parameters, such as the size and anisotropy, has been conventionally discussed. In addition, the influences of external conditions, such as the intensity and frequency of the applied field, the surrounding viscosity, and the temperature on the magnetic relaxation have been researched. According to one of the basic theories regarding magnetic relaxation, the faster type of relaxation dominates the process. However, in this study, we reveal that Brownian and Néel relaxations coexist and that Brownian relaxation can occur after Néel relaxation despite having a longer relaxation time. To understand the mechanisms of Brownian rotation, alternating current (AC) hysteresis loops were measured in magnetic fluids of different viscosities. These loops conveyed the amplitude and phase delay of the magnetization. In addition, the intrinsic loss power (ILP) was calculated using the area of the AC hysteresis loops. The ILP also showed the magnetization response regarding the magnetic relaxation over a wide frequency range. To develop biomedical applications of magnetic nanoparticles, such as hyperthermia and magnetic particle imaging, it is necessary to understand the mechanisms of magnetic relaxation.

Highlights

Magnetic nanoparticles (MNPs) attract attention for biomedical applications such as hyperthermia treatment and magnetic particle imaging (MPI), exhibiting potential for therapeutic and diagnostic applications [1,2]
ΤN = τ0exp respectively, where η is the viscosity, VH is the hydrodynamic volume of MNPs, τ0 is the attempt time of ~10−9 s associated with gyromagnetic procession, K is the magnetic anisotropic constant, VM is the volume of the core particle, kB is the Boltzmann constant 1.38 × 10−23 J/K, and T is the temperature in Kelvin [3,4]
alternating current (AC) hysteresis loops were measured to reveal the mechanisms of the particle and magnetization rotations associated with magnetic relaxations in magnetic fluids having different viscosities

Summary

Introduction

Magnetic nanoparticles (MNPs) attract attention for biomedical applications such as hyperthermia treatment and magnetic particle imaging (MPI), exhibiting potential for therapeutic and diagnostic applications [1,2]. A reduction of the Brownian and Néel relaxation times with an increasing applied-field intensity was measured according to the alternating current (AC) susceptibility [8]. AC hysteresis loops were measured to reveal the mechanisms of the particle and magnetization rotations associated with magnetic relaxations in magnetic fluids having different viscosities. The AC hysteresis loops indicate that the magnetization-reversal process changes with the field intensity, which is a feature not exhibited by the susceptibility. The relaxation mechanism in a higher field than the field intensity applicable to the linear response theory (LRT) is evaluated by the ILP estimated from AC hysteresis loops. In the measurement of AC hysteresis loops, the waveform of the magnetization signal was not sinusoidal, despite the sinusoidal applied field. The AC hysteresis measurement allows the determination of the magnetic properties of the MNPs suspended in a liquid, fixed to inhibit particle rotation, and added in a cellular environment [20,21,22]

Materials and Methods

Conclusions