Magnetic nanoparticles (MNPs) have been widely studied for use in biomedical application with the magnetic anisotropy constant K playing an important role in determining the performance. We estimated K near room temperature from the coercive field Hc of an AC magnetization (M–H) curve. First, we performed numerical simulation of the AC M–H curve of immobilized MNPs and clarified the dependencies of Hc on the MNP parameters and excitation conditions. Based on the simulation result, we obtained an analytical expression for Hc that was more general and included the previously obtained expression; and in addition, it could be applied to an MNP sample with a core-size distribution. Next, we measured the AC M–H curves of two commercial MNP samples and determined the dependencies of Hc on the amplitude and frequency of the excitation field. The dependencies agreed reasonably well with the analytical results. The K value was evaluated to obtain the best fit between the measured and analytical Hc, and the obtained K values were consistent with those estimated using other methods. The temperature dependence of K near room temperature was also determined. The present method will provide a useful tool to estimate the K value of MNPs.

Evaluation of effective magnetic anisotropy constant of magnetic nanoparticles from coercive field of AC magnetization curve

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.

https://aip.scitation.org/doi/pdf/10.1063/5.0090915

Dependence of hysteresis loss of immobilized magnetic nanoparticles on the easy-axis angle: Effect of easy-axis alignment

For the success of biomedical applications of magnetic iron oxide nanoparticles (MION), such as magnetic hyperthermia and magnetic particle imaging, it is essential to understand the orientational dynamics of MION in a complex fluid under an alternating field. Here, using the magnetic linear dichroism (MLD) measurement, we directly observed the orientational behavior of MION in a hydrogel under a damped oscillating magnetic field (DOMF) of 33 kHz in frequency. Hydrophobically modified ethoxylated urethane (HEUR) is examined as the network polymer because the mesh size of the network is controllable with its concentration. We used two MIONs: a bare MION (MION1) and a MION coated with an amphiphilic polymer (MION2). Where the mesh size of the gel network is larger than the particle's hydrodynamic diameter, MION1 in the hydrogel rotates in the same manner in a simple solution, although the macroscopic rheological property of the medium is quite different. Meanwhile, the orientational behavior of MION2 is dramatically changed by the addition of HEUR molecules even below the minimum gelation concentration, indicating that MION2 is associated with the flower micelles of HEUR. By analyzing the MLD waveform, the orientational behavior of MION1 in the HEUR gel under a DOMF can be explained with single-mode relaxation, whereas that of MION2 is complicated; a rapid partial rotation near the particle and a whole slow rotation of the particle-flower micelle associate are superimposed. It is hard to distinguish this difference in orientational behaviors from the dynamic magnetization curve because the dominant magnetization reversal process is Néel rotation, the rotation of the magnetic moment in the particle. The MLD measurement is a potential tool for optimizing biomedical techniques utilizing MIONs and for nanorheology or colloid science in a complex matrix such as a hydrogel or cytoplasmic matrix.

Orientational Dynamics of Magnetic Iron Oxide Nanoparticles in a Hydrogel: Observation by Magnetic Linear Dichroism under Oscillating Field.

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.

Structure and magnetization of a magnetoactive ferrocomposite.

Reducing nanoparticle (NP) dosage for hyperthermia therapy has remained a great challenge. In this work, efficiencies of alternating current (AC) magnetic field and near-infrared (NIR) heating are simultaneously enhanced by Zn and Co co-doping of magnetite NPs. The optimum magnetic anisotropy for maximized loss power under each magnetic field is achieved by tuning the doping concentration. The specific loss power of Zn0.3 Co0.08 Fe2.62 O4 @SiO2 NPs reaches 2428 W g-1 under an AC field of 27 kA m-1 at 430 kHz; 12 296 W g-1 under NIR laser irradiation at 808 nm and 2.5 W cm-2 ; and an unprecedented value of 14 724 W g-1 under dual mode. These values far exceed what has been achieved previously in iron oxide NPs. Ex vivo experiments on sacrificial mice show that while the NP dosage is substantially reduced to that used for magnetic resonance imaging, the surface body temperature of the mice reaches 50 °C after exposure to both AC field and laser irradiation under field parameters and laser intensity below safety limits. This nanoplatform is thus promising for multi-modal local hyperthermia therapy.

Simultaneous Enhancement of Magnetothermal and Photothermal Responses by Zn, Co Co-Doped Ferrite Nanoparticles.

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.

Difference in AC magnetization between suspended and immobilized magnetic nanoparticles in Néel-relaxation dominant case: Effect of easy axis alignment in suspended nanoparticles

Jinnya Gotou

Papers

Difference in AC magnetization between suspended and immobilized magnetic nanoparticles in Néel-relaxation dominant case: Effect of easy axis alignment in suspended nanoparticles