AC Magnetization of Immobilized Magnetic Nanoparticles With Different Degrees of Parallel Alignment of Easy Axes
01 Feb 2021-IEEE Transactions on Magnetics (Institute of Electrical and Electronics Engineers Inc.)-Vol. 57, Iss: 2, pp 1-5
TL;DR: In this paper, the authors showed that the magnetic properties of magnetic easy axes are significantly affected by the degree of easy axes' alignment when the strength of the ac excitation field, $H 0, is relatively small.
Abstract: Magnetic nanoparticles (MNPs) are physically immobilized in some biomedical applications. In this case, dynamic ac magnetization that determines MNPs’ performance has strong dependence on the relative direction between the magnetic easy axes of MNPs and the applied excitation field. To study this dependence, we prepared immobilized MNP samples with different degrees of easy axes’ alignment. The degree of alignment was changed by setting the strength of a dc field that is applied during the immobilization process. The magnetic properties of these samples, such as ac magnetization, coercive field, amplitude of third-harmonic signal, and hysteresis loss, are compared with those of a randomly oriented easy axes’ sample. It was shown that these values are significantly affected by the degree of easy axes’ alignment when the strength of the ac excitation field, $H_{0}$ , is relatively small. The alignment effect gradually becomes smaller when $H_{0}$ is increased. These findings are important for immobilized MNP applications in hyperthermia and magnetic particle imaging.
Citations
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TL;DR: It is found that the easy axis alignment of MNPs considerably affects their M–H curve and harmonic spectra, which indicates the importance of this alignment for their biomedical applications.
Abstract: Magnetic nanoparticles (MNPs) have been widely studied for use in biomedical applications such as magnetic particle imaging and magnetic hyperthermia. We studied the effect of easy axis alignment on dynamic magnetization of both immobilized and suspended MNPs. For immobilized samples, easy axes of magnetization were aligned by applying a dc field during immobilization. For suspended samples, the applied ac field was set so that the alignment of the easy axes was caused by the ac field. Magnetization (M–H) curves of MNPs with different degrees of easy axis alignment were measured with an applied ac field. We found that the easy axis alignment of MNPs considerably affects their M–H curve and harmonic spectra, which indicates the importance of this alignment for their biomedical applications. The experimental results were quantitatively explained with numerical simulation that took account of easy axis alignment and core size distribution in the sample. We can express the degree of easy axis alignment using a distribution function of the easy axis angle for both immobilized and suspended cases. The distribution function will be useful for quantitatively evaluating MNP performance in biomedical applications.
17 citations
TL;DR: In this article, the authors studied magnetic properties of suspended and immobilized MNPs when the Neel relaxation time is much shorter than the Brownian and showed that they have different magnetic properties such as AC magnetization curves and harmonic spectra.
Abstract: Magnetic nanoparticles (MNPs) have been widely studied for bio-sensing applications, where suspended and immobilized MNPs can be magnetically distinguished using their different magnetic properties. We study magnetic properties of suspended and immobilized MNPs when the Neel relaxation time is much shorter than the Brownian. We show in both numerical simulation and experiment that they have different magnetic properties such as AC magnetization curves and harmonic spectra even though the dynamic behavior of both MNP types is primarily dominated by Neel relaxation. This difference is caused by the partial alignment of the easy axes in suspended MNPs when an AC magnetic field is applied. We introduce a distribution function for the angle of easy axis alignment. We also show a method to evaluate the distribution function from the measured AC magnetization curve and clarify the relationship between easy axis alignment and the AC field strength. Using the distribution function, we can quantitatively discuss the effect of easy axis alignment on the magnetic properties of suspended MNPs. The obtained results provide a basis for using MNPs in bio-sensing.
8 citations
TL;DR: In this paper , the authors investigated how the AC magnetization curve of MNP depended on the easy-axis angle of magnetization via numerical simulations of the Fokker-Planck equation.
Abstract: Magnetic nanoparticles (MNPs) have been widely studied for hyperthermia applications. We studied the hysteresis loss of immobilized MNPs with partially aligned easy axes. For this purpose, we investigated how the AC magnetization ( M– H) curve of MNP depended on the easy-axis angle of magnetization, β, via numerical simulations of the Fokker–Planck equation. We clarified the dependences of the coercive field, Hc, and the hysteresis loss, A, on β. We, thus, obtained analytical expressions for Hc( β) and A( β) that explain simulation results for a wide range of MNP parameters and excitation conditions. The angle dependences were also discussed on the basis of magnetic moment reversal over the anisotropy energy barrier. We then examined the hysteresis loss of an MNP sample with partial alignment of the easy axes and obtained an expression for the loss by combining A( β) and a distribution function for β. We quantitatively clarified the relationship between the loss and the degree of easy-axis alignment. The loss of immobilized MNPs can be increased by a factor of 2.2 by using easy-axis alignment relative to the case of randomly oriented easy axes. Finally, we examined the alignment of easy axes induced by an AC field in suspended MNPs and showed that the loss of immobilized MNPs with partially aligned easy axes can be used to estimate the loss for suspended MNPs.
3 citations
TL;DR: In this paper , the structural and magnetic properties of an ensemble of single-domain interacting magnetic nanoparticles immobilized in a non-magnetic medium are studied, and features of the orientational texture formed in the sample are analyzed depending on the intensity of the magnetic field αp and interparticle dipole-dipole interactions.
Abstract: This work is devoted to the theoretical study of the structural and magnetic properties of an ensemble of single-domain interacting magnetic nanoparticles immobilized in a non-magnetic medium. This model is typical for describing magnetically active soft materials, "smart" polymer ferrocomposites, which have been applied in science-intensive industrial and biomedical technologies. It is assumed that the ferrocomposite is obtained by solidification of the carrier medium in a ferrofluid under an external magnetic field, the intensity of which is determined by the Langevin parameter αp; after the solidification of the carrier liquid, the nanoparticles retain the spatial distribution and orientation of their easy magnetization axes. The features of the orientational texture formed in the sample are analyzed depending on the intensity of the magnetic field αp and interparticle dipole-dipole interactions. The magnetization of a textured ferrocomposite in the magnetic field α is also investigated. Our results show that in the case of a co-directional arrangement of the considered fields and if α < αp, the ferrocomposites are magnetized much more efficiently than ferrofluids due to their texture. In the fields α > αp, the ferrocomposite is magnetized less efficiently than the ferrofluid due to the internal magnetic anisotropy of the nanoparticles. The analytical expressions presented here make it possible to predict the magnetization of a ferrocomposite depending on its internal structure and synthesis conditions, which is the theoretical basis for the synthesis of ferrocomposites with a predetermined magnetic response in a given magnetic field.
2 citations
TL;DR: In this paper , a model-based approach was proposed to model the magnetization response of a large ensembles of magnetic nanoparticles to magnetic fields which allows determining the spatial distribution of the MNP concentration from measured voltage signals.
1 citations
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397 citations
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175 citations
"AC Magnetization of Immobilized Mag..." refers background in this paper
...Therefore, it is important to quantitatively characterize the dynamic ac magnetization of MNPs to improve the performance in these applications [5]–[14]....
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TL;DR: A review of the development of magnetic particle imaging (MPI) since its introduction in 2005 is presented in this article, where detailed discussions on imaging sequences, reconstruction algorithms, scanner instrumentation and potential medical applications are provided.
Abstract: Tomographic imaging has become a mandatory tool for the diagnosis of a majority of diseases in clinical routine. Since each method has its pros and cons, a variety of them is regularly used in clinics to satisfy all application needs. Magnetic particle imaging (MPI) is a relatively new tomographic imaging technique that images magnetic nanoparticles with a high spatiotemporal resolution in a quantitative way, and in turn is highly suited for vascular and targeted imaging. MPI was introduced in 2005 and now enters the preclinical research phase, where medical researchers get access to this new technology and exploit its potential under physiological conditions. Within this paper, we review the development of MPI since its introduction in 2005. Besides an in-depth description of the basic principles, we provide detailed discussions on imaging sequences, reconstruction algorithms, scanner instrumentation and potential medical applications.
167 citations
TL;DR: In this paper, it was shown that the magnetic dynamics of an assembly of nanoparticles dispersed in a viscous liquid differs significantly from the behavior of the same assembly of particles immobilized in a solid matrix.
Abstract: It is shown that the magnetic dynamics of an assembly of nanoparticles dispersed in a viscous liquid differs significantly from the behavior of the same assembly of nanoparticles immobilized in a solid matrix. For an assembly of magnetic nanoparticles in a liquid two characteristic mode for stationary magnetization oscillations are found that can be called the viscous and magnetic modes, respectively. In the viscous mode, which occurs for small amplitude of the alternating magnetic field H0 as compared to the particle anisotropy field Hk, the particle rotates in the liquid as a whole. In a stationary motion the unit magnetization vector and the director, describing the spatial orientation of the particle, move in unison, but the phase of oscillations of these vectors is shifted relative to that of the alternating magnetic field. Therefore, for the viscous mode the energy absorption is mainly due to viscous losses associated with the particle rotation in the liquid. In the opposite regime, H0 ≥ Hk, the dir...
155 citations
TL;DR: The results presented demonstrate that cellular internalization can disable Brownian relaxation, which has significant implications for designing suitable nanoparticles for intracellular hyperthermia applications and highlights the possibility that particles could be released in reusable form from degrading cells followinghyperthermia treatment, and subsequently reabsorbed by viable cells.
Abstract: Magnetization relaxation mechanisms strongly influence how magnetic nanoparticles respond to high-frequency fields in applications such as magnetic hyperthermia. The dominant mechanism depends on the mobility of the particles, which will be affected in turn by their microenvironment. In this study AC susceptometry was used to follow the in situ magnetic response of model systems of blocked and superparamagnetic nanoparticles, following their cellular internalization and subsequent release by freeze-thaw lysis. The AC susceptibility signal from internalized particles in live cells showed only Neel relaxation, consistent with measurements of immobilized nanoparticle suspensions. However, Brownian relaxation was restored after cell lysis, indicating that the immobilization effect was reversible and that nanoparticle integrity was maintained in the cells. The results presented demonstrate that cellular internalization can disable Brownian relaxation, which has significant implications for designing suitable nanoparticles for intracellular hyperthermia applications. Further to this, the results highlight the possibility that particles could be released in reusable form from degrading cells following hyperthermia treatment, and subsequently reabsorbed by viable cells.
112 citations
"AC Magnetization of Immobilized Mag..." refers background in this paper
...There are some situations in which MNPs become physically immobilized when used in the aforementioned applications [15], [16]....
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