High Frequency AC Magnetic Field Research Articles

Optical Microscopy Using the Faraday Effect Reveals in Situ Magnetization Dynamics of Magnetic Nanoparticles in Biological Samples.

The study of exogenous and endogenous nanoscale magnetic material in biology is important for developing biomedical nanotechnology as well as for understanding fundamental biological processes such as iron metabolism and biomineralization. Here, we exploit the magneto-optical Faraday effect to probe intracellular magnetic properties and perform magnetic imaging, revealing the location-specific magnetization dynamics of exogenous magnetic nanoparticles within cells. The opportunities enabled by this method are shown in the context of magnetic hyperthermia; an effect where local heating is generated in magnetic nanoparticles exposed to high-frequency AC magnetic fields. Magnetic hyperthermia has the potential to be used as a cellular-level thermotherapy for cancer, as well as for other biomedical applications that target heat-sensitive cellular function. However, previous experiments have suggested that the cellular environment modifies the magnetization dynamics of nanoparticles, thus dramatically altering their heating efficiency. By combining magneto-optical and fluorescence measurements, we demonstrate a form of biological microscopy that we used here to study the magnetization dynamics of nanoparticles in situ, in both histological samples and living cancer cells. Correlative magnetic and fluorescence imaging identified aggregated magnetic nanoparticles colocalized with cellular lysosomes. Nanoparticles aggregated within these lysosomes displayed reduced AC magnetic coercivity compared to the same particles measured in an aqueous suspension or aggregated in other areas of the cells. Such measurements reveal the power of this approach, enabling investigations of how cellular location, nanoparticle aggregation, and interparticle magnetic interactions affect the magnetization dynamics and consequently the heating response of nanoparticles in the biological milieu.

Magnetostrictive-piezocatalytic CoFe2O4@UiO-66 nanohybrid and its potential for deep-seated tumor treatment.

Magnetostrictive CoFe2O4 (CFO) nanoparticles were encapsulated within a UiO-66 metal-organic-framework layer to form a CFO@UiO-66 nanohybrid. The deforming of CFO, in response to a high-frequency AC magnetic field, initiates the piezocatalytic property of UiO-66 to generate ˙OH radicals, which can kill cancer cells buried in thick tissues, showcasing bright potential for deep-seated tumor treatment.

High-resistivity anisotropic hot-deformed Nd-Fe-B magnets prepared from DyF3 electrophoretic deposited powders

The electrical resistivities of anisotropic Nd-Fe-B-based magnets were successfully increased in both parallel and perpendicular directions to the c-axis by hot-deforming Nd-Fe-B melt-spun flakes that were coated with DyF3 using electrophoretic deposition. Consequently, the operating temperature under high-frequency AC magnetic field was reduced by 20 °C because of the reduction of eddy current loss. The resistive layer formed at the interface of original ribbons in the hot-deformed magnet was found to be NdF3 rather than DyF3. Transmission electron microscopy and atom probe tomography revealed that liquid Nd-rich intergranular phase reduces DyF3 to form NdF3 at the ribbon interfaces during hot-deformation at elevated temperature. Excess Dy diffuses into the original ribbon flakes through grain boundaries to form a Dy-rich shell in Nd2Fe14B grains, giving rise to a high coercivity of 1.8 T. This study demonstrates an alternative way to solve the thermal demagnetization problem of Nd-Fe-B-based permanent magnets during operation in electric motors via the resistivity increase of the magnet.

Promotion of ferrofluid microchannel flows by gradient magnetic fields

• Ferrofluid Couette-Poiseuille flows subject to gradient magnetic field are analyzed. • Steady gradient magnetic fields promote the ferrofluid Couette-Poiseuille flows. • The backflow is completely suppressed and the flow separation disappears. • The gradient magnetic field enhances the effect of magnetization relaxation. • The gradient magnetic field increases magnetic stress and reduces shear stress. Inspired from the wide engineering applications of ferrofluids, we investigated the ferrofluid microchannel flows subject to a gradient magnetic field applied perpendicular to the channel walls. The ferrohydrodynamic problem encompassing the Fokker-Planck magnetization equation in a Couette-Poiseuille configuration is solved numerically. The velocity and spin profiles, vorticity, magnetization, flow rate, and stresses acting on the moving plate are examined for favorable and unfavorable field gradients, as well as the adding of different pressure gradients. We show that the steady gradient magnetic fields promote the flow of both pure Couette and Couette-Poiseuille flows in the microchannel geometry. The increment of the velocity attains 35% and the resulting volumetric flow rate increases by about 24% at the most. This phenomenon was only observed in ferrofluid flows subject to high frequency AC magnetic fields or temperature gradient field in the past. The backflow occurs in normal Couette-Poiseuille flows can be completely suppressed by the gradient fields and the flow separation disappears correspondingly. The gradient magnetic fields are able to enhance the effect of magnetization relaxation and dramatically increase the magnetic stress and reduce the shear stress by comparison with the cases subject to uniform magnetic fields, yet the tradeoff between them leads to a rise in the total tangential stress acting on the moving plate.

Design and Simulation of Solar-based Induction Heating System

Solar energy is abundantly available energy for the generation of heat, and one the best appliances used for this purpose is Induction heaters. For the heat generation application, the combination of solar energy with the induction heat generation technique is the productive solution. Due to high efficiency and high-power density, resonant converters with soft switching are commonly utilized in domestic induction heating applications. For induction heating applications, a low profile and an improved efficiency resonant inverter design and implementation procedure are presented. Reducing conduction losses by using automotive-grade MOSFET devices against the classical IGBT-based converter. Conversion efficiency can be increased by decreasing switching losses which can be done by reducing switching times of MOSFET devices. Heating the metallic pan with circulating eddy currents induced by a high-frequency AC magnetic field is the principle of the induction cooker. If the load has an electrically conducting base, then the coil within the induction cooker and the load (pan/pot) acts as a transformer. The power will be transferred with high efficiency if the conducting base is ferromagnetic. An efficient control scheme incorporated in class E resonance heating by using solar power is presented. The simulation analysis is done in MATLAB-SIMULINK. Index Terms: induction heating, solar energy, class E resonant converter, MOSFET, MATLAB-SIMULINK

Modulation of the longitudinal pumping in quantum spin systems

The effect of the modulation of a high-frequency ac magnetic field parallel to the dc one (longitudinal pumping) by the lower-frequency ac field acting on quantum spin systems is considered for closed and open systems. For most magnetically ordered spin systems, the modulation causes the oscillation of the threshold of the parametric instability of spin waves as a function of the amplitude and the frequency of the modulation. On the other hand, our results for quantum spin systems with fermionic eigenstates imply that the modulating field causes the combined resonance and the modulation of the magnitude of time-dependent oscillations of the magnetization and the absorbed by the system power of the ac field for the closed system and for the open system in the dynamical and the steady-state regimes.

Magnetic nanoparticle thermometry independent of Brownian relaxation

An improved method of magnetic nanoparticle (MNP) thermometry is proposed. The phase lag ϕ of the fundamental f0 harmonic is measured to eliminate the influence of Brownian relaxation on the ratio of 3f0 to f0 harmonic amplitudes applying a phenomenological model, thus allowing measurements in high-frequency ac magnetic fields. The model is verified by simulations of the Fokker–Planck equation. An MNP spectrometer is calibrated for the measurements of the phase lag ϕ and the amplitudes of 3f0 and f0 harmonics. Calibration curves of the harmonic ratio and tanϕ are measured by varying the frequency (from 10 Hz to 1840 Hz) of ac magnetic fields with different amplitudes (from 3.60 mT to 4.00 mT) at a known temperature. A phenomenological model is employed to fit the calibration curves. Afterwards, the improved method is proposed to iteratively compensate the measured harmonic ratio with tanϕ, and consequently calculate temperature applying the static Langevin function. Experimental results on SHP-25 MNPs show that the proposed method significantly improves the systematic error to 2 K at maximum with a relative accuracy of about 0.63%. This demonstrates the feasibility of the proposed method for MNP thermometry with SHP-25 MNPs even if the MNP signal is affected by Brownian relaxation.

Dynamical quantum phase transitions in presence of a spin bath

We derive an effective time independent Hamiltonian for the transverse Ising model coupled to a spin bath, in the presence of a high frequency AC magnetic field. We show that the spin blocking mechanism that removes the quantum phase transition can be suppressed by the AC field, allowing high tunability of the quantum critical point. Finally, we calculate the phase diagram within the RPA approximation for the case of spin $7/2$ nuclei, appropriate to the $LiHo_{x}Y_{1-x}F_{4}$ system.

Self-Healing of Core-Shell Magnetic Polystyrene Nanocomposites.

High heat generation is reported in core-shell magnetic nanoparticle polystyrene (PS) nanocomposites (3.5, 10 wt %) when they are placed in a high-frequency ac magnetic field. These magnetic nanoparticles with cobalt iron oxide core and manganese iron oxide shell were synthesized and characterized by wide-angle X-ray scattering (WAX), thermal gravimetric analysis (TGA), transmission electron microscopy (TEM), and ac field gradient magnetometery. When placed in a high-frequency ac magnetic field, the thermal energy generated in the magnetic polystyrene nanocomposites resulted in a surface temperature increase. The heat generation is attributed to the contribution of Néel relaxation and hysteresis of the core-shell magnetic nanoparticles in the solid state. The maximum surface temperature increased with increasing nanoparticle content and resulted in melting of the magnetic polystyrene nanocomposite.

Nanomaterial-assisted PCR based on thermal generation from magnetic nanoparticles under high-frequency AC magnetic fields

Here the authors present a nanomaterial-assisted PCR technique based on the use of thermal generation from magnetic nanoparticles (MNPs) under AC magnetic fields. In this approach, MNPs work as internal nano thermal generators to realize PCR thermal cycling. In order to suppress the non-specific absorption of DNA synthetic enzymes, MNPs are decorated with bovine serum albumin (BSA), forming BSA/MNP complexes. Under high-frequency AC magnetic fields, these complexes work as internal nano thermal generators, thereby producing the typical temperature required for PCR thermal cycling, and perform all the reaction processes of PCR amplification in the place of conventional PCR thermal cyclers.

Nonequilibrium magnetic response of anisotropic superparamagnetic nanoparticles and possible artifacts in magnetic particle imaging.

Magnetic responses of superparamagnetic nanoparticles to high-frequency AC magnetic fields with sufficiently large amplitudes are numerically simulated to exactly clarify the phenomena occurring in magnetic particle imaging. When the magnetic anisotropy energy inevitable in actual nanoparticles is taken into account in considering the magnetic potential, larger nanoparticles exhibit a delayed response to alternations of the magnetic fields. This kind of delay is rather remarkable in the lower-amplitude range of the field, where the assistance by the Zeeman energy to thermally activated magnetization reversal is insufficient. In some cases, a sign inversion of the third-order harmonic response was found to occur at some specific amplitude, despite the lack in DC bias magnetic field strength. Considering the attenuation of the AC magnetic field generated in the human body, it is possible that the phases of the signals from nanoparticles deep inside the body and those near the body surface are completely different. This may lead to artifacts in the reconstructed image. Furthermore, when the magnetic/thermal torque-driven rotation of the anisotropic nanoparticles as well as the magnetic anisotropy energy are taken into account, the simulated results show that, once the easy axes are aligned toward the direction of the DC bias magnetic field, it takes time to randomize them at the field-free point. During this relaxation, the third-order harmonic response depends highly upon the history of the magnetic field. This is because non-linearity of the anhysteretic magnetization curve for the superparamagnetic nanoparticles varies with the orientations of the easy axes. This history dependence may also lead to another artifact in magnetic particle imaging, when the scanning of the field-free point is faster than the Brownian relaxations.

Comparative Study of Magnetic Properties of Nanoparticles by High-Frequency Heat Dissipation and Conventional Magnetometry

The rate of heating of 15 nm uniformly-sized magnetic aqueous nanoparticles suspension by high-amplitude and high-frequency ac magnetic field induced by the resonating LC circuit was measured. The results are analyzed in terms of specific energy absorption rate (SAR). Fitting field amplitude and frequency dependences of SAR to the linear response theory, magnetic moment per particles was extracted. The value of magnetic moment was independently evaluated from dc magnetization measurements of a frozen colloid by fitting field-dependent magnetization to a Langevin function. The two methods produced similar results, which are compared to the theoretical expectation for this particle size. Additionally, analysis of SAR curves yielded effective relaxation time.

Magnetic field triggered drug release from polymersomes for cancer therapeutics

Local and temporal control of drug release has for long been a main focus in the development of novel drug carriers. Polymersomes, which can load both hydrophilic and hydrophobic species and, at the same time, be tailored to respond to a desired stimulus, have drawn much attention over the last decade. Here we describe polymersomes able to encapsulate up to 6% (w/w) of doxorubicin (DOX) together with 30% (w/w) of superparamagnetic iron oxide nanoparticles (USPIO; γ-Fe2O3). Upon internalization in HeLa cells and when a high frequency AC magnetic field (14mT at 750kHz) was applied, the developed delivery system elicited an 18% increase in cell toxicity, associated with augmented DOX release kinetics. In order to ensure that the observed cytotoxicity arose from the increased doxorubicin release and not from a pure magnetic hyperthermia effect, polymersomes loaded with magnetic nanoparticles alone were also tested. In this case, no increased toxicity was observed. We hypothesize that the magnetic field is inducing a very local hyperthermia effect at the level of the polymersome membrane, increasing drug release. This approach opens new perspectives in the development of smart delivery systems able to release drug upon demand and therefore, improving treatment control.

Size-Dependent Mechanisms in AC Magnetic Hyperthermia Response of Iron-Oxide Nanoparticles

This paper correlates the magnetic properties of iron-oxide nanoparticles in the size range 5-18 nm with the occurring heating loss mechanisms when magnetic nanoparticle colloidal suspensions are subjected to high-frequency ac magnetic fields. The narrow size distribution of the nanoparticles enabled their clear classification into: 1) the superparamagnetic region (as large as 10 nm) where heating is mainly attributed to Neel relaxation; 2) the intermediate superparamagnetic-ferromagnetic transition region (10-13 nm); and 3) the ferromagnetic region (above 13 nm) where hysteresis losses dominate. The results from specific loss power measurements suggest that for size and concentration optimization, superparamagnetic nanoparticles may release significant amounts of heat to the surroundings, while the hysteresis losses mechanism appears to be much more efficient and the heat transfer provided through may be easier tuned for magnetically driven hyperthermia applications.

Edge pinch instability of oblate liquid metal drops in a transverse AC magnetic field

This paper considers the stability of liquid metal drops subject to a high-frequency AC magnetic field. An energy variation principle is derived in terms of the surface integral of the scalar magnetic potential. This principle is applied to a thin perfectly conducting liquid disk, which is used to model the drops constrained in a horizontal gap between two parallel insulating plates. Firstly, the stability of a circular disk is analysed with respect to small-amplitude harmonic edge perturbations. Analytical solution shows that the edge deformations with the azimuthal wavenumbers m = 2, 3, 4, . . . start to develop as the magnetic Bond number exceeds the critical threshold Bmc = 3π(m + 1)/2. The most unstable is m = 2 mode, which corresponds to an elliptical deformation. Secondly, strongly deformed equilibrium shapes are modelled numerically by minimising the associated energy in combination with the solution of a surface integral equation for the scalar magnetic potential on an unstructured triangular mesh. The edge instability is found to result in the equilibrium shapes of either two- or threefold rotational symmetry depending on the magnetic field strength and the initial perturbation. The shapes of higher rotational symmetries are unstable and fall back to one of these two basic states. The developed method is both efficient and accurate enough for modelling of strongly deformed drop shapes.

The effect of annealing on the structure, magnetic properties and AC heating of CoFe 2O 4 for biomedical applications

Single and nanosized spinel CoFe 2O 4 phase has been prepared successfully by a simple combination of mechanical milling from a mixture of Fe 2O 3 and Co 3O 4 powder precursors followed by a subsequent annealing. X-ray diffraction analysis reveals that the estimated crystallite size of CoFe 2O 4 increases with increasing temperature but remains at the nanoscale, i.e. 85 nm at 900 °C. Moreover, magnetic measurements show that a great enhancement in the saturation magnetization was achieved whereas a large hysteresis loop was observed (i.e.72 emu/g at 900 °C). Evaluation and applicability of CoFe 2O 4 nanoparticles under high frequency AC magnetic field for heating in biomedical applications were examined. It was found that under fixed amplitude (516 Oe) and frequency (229 kHz), the prepared nanoparticles generate significant heat: after 5 s the temperature was around 97 °C for the as-milled powder and reached almost 178 °C for the powder annealed at 900 °C.

Ratchet Effect of Domain Wall Motion by GHz AC Magnetic Field in Asymmetric Sawtooth-Shaped Ferromagnetic Nanowires

We have investigated a ratchet effect of magnetic domain wall in asymmetric sawtooth-shaped ferromagnetic nanowires under high frequency AC magnetic fields by means of micromagnetic simulation. The ratchet phenomenon has been intensively examined during the domain wall propagation with variation of the asymmetric sawtooth geometry of nanowires as well as variation of the AC external field frequency/strength. Very interestingly, it has been found that there is a strong correlation between the spatial frequency of asymmetrical notches and the effective driving frequency/strength of AC external fields. It is demonstrated that number of notches swept by domain wall ratchet motion per each half-cycle of the driving frequency is controlled under a GHz AC driving field.

High frequency magnetic field imaging by frequency modulated magnetic force microscopy

High frequency ac magnetic field from magnetic recording head was successfully imaged by using our newly developed frequency-modulated magnetic force microscopy (FM-MFM), which uses the frequency modulation of cantilever oscillation caused by applying ac magnetic field to a mechanically oscillated cantilever. The FM-MFM has been demonstrated and the experimental results show good agreement with our proposed models for FM-MFM. The amplitude and phase images of an ac magnetic field can be obtained separately by using the FM-MFM technique with a lock-in amplifier. By taking advantage of this technique, the present FM-MFM method opens a possibility to evaluate the magnetic field characteristics of magnetic recording head with high spatial resolution.

Influence of High Frequency AC Magnetic Field on Trapped Magnetic Field

The trapped magnetic field in an HTS bulk is decayed and even erased by the application of an AC magnetic field whose amplitude is much smaller than the maximum trapped magnetic field. In previous experiments, the decay of the trapped magnetic field was shown to be due to the temperature rise of the bulk caused by the AC losses. Especially, in a high frequency magnetic field, AC losses are larger and the decay of the trapped magnetic field is accelerated by this AC loss increase. We investigated the time relationships of the AC loss of the HTS bulk in a high frequency magnetic field using the thermal equilibrium equation.

Edge pinch instability of liquid metal sheet in a transverse high-frequency ac magnetic field

We analyze the linear stability of the edge of a thin liquid metal layer subject to a transverse high-frequency ac magnetic field. The layer is treated as a perfectly conducting liquid sheet that allows us to solve the problem analytically for both a semi-infinite geometry with a straight edge and a thin disk of finite radius. It is shown that the long-wave perturbations of a straight edge are monotonically unstable when the wave number exceeds the critical value k(c) = F0/(gamma l0), which is determined by the linear density of the electromagnetic force F0 acting on the edge, the surface tension gamma, and the effective arclength of edge thickness l0. Perturbations with wavelength shorter than critical are stabilized by the surface tension, whereas the growth rate of long-wave perturbations reduces as similar to k for k --> 0. Thus, there is the fastest growing perturbation with the wave number k max = 2/3 k(c). When the layer is arranged vertically, long-wave perturbations are stabilized by the gravity, and the critical perturbation is characterized by the capillary wave number k(c) = square root of (g rho/gamma), where g is the acceleration due to gravity and rho is the density of metal. In this case, the critical linear density of electromagnetic force is F(0,c) = 2k(c)l0 gamma, which corresponds to the critical current amplitude I(0,c) = 4 square root of (pi k(c) l0L gamma/mu 0) when the magnetic field is generated by a straight wire at the distance L directly above the edge. By applying the general approach developed for the semi-infinite sheet, we find that a circular disk of radius R0 placed in a transverse uniform high-frequency ac magnetic field with the induction amplitude B0 becomes linearly unstable with respect to exponentially growing perturbation with the azimuthal wave number m = 2 when the magnetic Bond number exceeds Bm(c) = B(0)2 R(0)2 / (2 mu 0 l0 gamma) = 3 pi. For Bm > Bm(c), the wave number of the fastest growing perturbation is m(max) = [2Bm/(3 pi)]. These theoretical results agree well with the experimental observations.