AC Magnetic Field Research Articles

Reversible Alignment of Nanoparticles and Intracellular Vesicles During Magnetic Hyperthermia Experiments

AbstractHeating magnetic nanoparticles (MNPs) with AC (Alternating Current) magnetic fields has received significant attention in recent years, particularly for biomedical uses. However, most studies focus on characterizing the heat release, overlooking the fact that the MNPs in the viscous cell environment constitute a dynamic magnetic colloid whose configuration may evolve over time, particularly if a driving force as the AC field is applied. Aiming to shed light on this matter, in this workthe dynamics of the colloid structure during hyperthermia experiments are studied. By combining various experimental and theoretical tools, it is concluded that the AC field may drive the formation of aligned structures, and the impact that such structures may have on the associated heating is assessed. Remarkably, the results show that those field‐driven structures are highly unstable for small particle sizes, rapidly disassembling upon field removal. Moreover, an analogous behavior in vitro is found, with the AC magnetic field also promoting a reversible alignment of vesicles containing the MNPs within the cells. The results suggest that the observed alignment, both of MNPs and intracellular vesicles, may be a common phenomenon in usual hyperthermia experiments, but unnoticed because of the intrinsic unstable nature of the aligned structures.

AC losses in macroscopic thin-walled superconducting cylinders

Measurements of the ac response represent a valuable method for probing the properties of superconductors. In the surface superconducting state (SSS), a current exceeding the surface critical current I c leads to breakdown of SSS and penetration of external magnetic field into the sample bulk. An interesting free-of-bulk system in SSS is offered by thin-walled cylinders. According to the full penetration of magnetic flux (FPMF) model, each time the instant value of an ac field is equal to a certain critical value, the ac susceptibility χ will exhibit jumps as a function of the ac field amplitude H ac because of the periodic destruction and restoration of SSS in the cylinder wall. Here we study the low-frequency (128–8192 Hz) ac response of thin-walled niobium cylinders under superimposed dc and ac magnetic fields applied parallel to the cylinder axis. In contrast to the FPMF model predictions, experiments reveal a smooth χ(H ac) dependence. To explain the experimental observations, we propose a phenomenological partial penetration magnetic flux (PPMF) model, which assumes that after restoration of the superconducting state, the magnetic fields inside and outside the cylinder are unequal and the magnitude of the penetrating flux is random for every penetration. This model fits very well the experimental data on the temperature dependence of the first harmonic χ 1 for any dc field and ac amplitude.

Theory and mitigation of motional eddy current in high-field eddy current shielding.

Eddy current shielding by a Faraday cage is an effective way to shield alternating-current magnetic fields in scientific instrumentation. In a strong static magnetic field, however, the eddy current in the conductive shield is subject to the Lorentz force, which causes the shield to vibrate. In addition to mechanical issues (e.g., acoustic noise), such vibration induces motional eddy current in the shield that can dominate the original, electromagnetic eddy current to undermine the conductor's shielding capability. In this work, we investigate a method to control motional eddy current by making cut-out patterns in the conductor that follow the electromagnetic eddy current image. This effectively limits the surface current of the plate to a single mode and prevents the proliferation of uncontrolled motion-induced surface currents that disrupts eddy current shielding. After developing a comprehensive theory of magneto-mechanical interaction in a conductive plate, the proposed method was tested on a flat-geometry testbed experiment inside a 3 T magnetic resonance imaging (MRI) magnet. It was found that the magnetic field generated by the motional eddy current was much more localized in space and frequency for a patterned-copper shield compared to a solid copper. The magnetic field of the patterned shield could be accurately predicted from the impedance measurement in the magnet. Implications of our results for improved shielding of gradient fields in high-field MRI are discussed.

Development of flexible, lead-free electrospun fibers comprising PVDF and NiFe2O4 for harvesting mechanical and stray magnetic fields

This research focuses on crafting highly flexible and efficient mechanical and magneto-mechano-electrical (MME) nanogenerators using electrospun poly (vinylidene fluoride)-nickel ferrite (PVDF-NiFe2O4) composite fibers, tailored for self-powered and wireless sensor networks. Nickel ferrite nanoparticles, averaging 40 nm in crystallite size, were synthesized through the autocombustion method and integrated as fillers within the PVDF matrix. Analysis via XRD and FTIR confirmed a greater presence of the ferroelectric β-phase in the PVDF electrospun fibers, accentuated by the incorporation of NiFe2O4. Notably, the mat containing 5 wt% of NiFe2O4 boasted an estimated 86 % β-phase. SEM micrographs depicted the formation of uniform bead-free fiber with a diameter of 900 nm. VSM analysis verified the ferrimagnetic properties of the composite fiber mats, registering a maximum saturation magnetization of 4.6 emu/cm3. Under mechanical tapping, the nanogenerator yielded an output voltage of 16.5 V and a power of 0.8 μW. The study also compared the performance of two MME nanogenerator geometries (rectangular and truncated) under an AC magnetic field of 10 Oe. Remarkably, the MME nanogenerator with a truncated geometry exhibited superior performance, generating 6.2 V, a 50 % increase over the rectangular counterpart.

Heat Generation and Diffusion in an Assembly of Magnetic Nanoparticles: Application to Magnetic Hyperthermia

We investigate the thermal generation and transport properties of an assembly of magnetic nanoparticles embedded in a solid or fluid matrix, subjected to an AC magnetic field. For this purpose, we first build the heat equation for the assembly using the effective thermal transport coefficients obtained within the effective medium approach. In the present calculation, the SAR is obtained from the (linear) dynamic response of the assembly to the AC magnetic field. We numerically solve the extended heat equation and, as a preliminary study, we obtain the space-time profile of the temperature and total power absorbed by the system.

Assessing mechanical and stray magnetic field energy harvesting capabilities in lead-free PVDF/BCT-BZT composites integrated with metglas

The exploration of energy harvesting encompasses a wide array of sources, with a specific focus on recovering mechanical and magnetic energy from overlooked outlets, driving current research endeavours. In this study, we developed highly flexible Magneto-Mechano-Electric (MME) harvesters by combining poly(vinylidene fluoride) (PVDF)/barium calcium zirconium titanate (BCT-BZT) composite films with metglas. The flexible piezoelectric polymer-ceramic composite films were fabricated using a solvent casting technique. The high piezoelectric BCT-BZT fillers were synthesized through a sol-gel reaction route. A thorough analysis was carried out on these composites to evaluate their structural, functional, microstructural, and dielectric properties. The composite film loaded with 20 wt% BCT-BZT filler achieved a 78 % β-phase with a significant enhancement in the dielectric properties, resulting in a high permittivity∼40 and low dielectric loss. Employing these films, we engineered nanogenerators capable of converting mechanical energy into electrical energy, yielding an output voltage of 11 V. Thereafter, to harvest mechanical and stray magnetic fields, we developed a MME generator comprising a magnetic Metglas layer and PVDF-BCT-BZT composite and achieved an output voltage of 3.3 V with a power density of 85 μW/m2 when subjected to an AC magnetic field of 5 Oe. Furthermore, the MME generator has demonstrated the ability to charge various capacitors showcasing its potential for usage in self-powered, non-contact, and implantable devices.

Magnetic nanoparticle‐induced sorbent regeneration for direct air capture

AbstractDirect air capture (DAC) is a promising technology for decarbonization through the removal of CO2 from the atmosphere. In many DAC processes, the regeneration energy used to restore the capture capacity of sorbents accounts for a significant fraction of the energy required by the whole process. Here we report an effective and scalable sorbent regeneration method for liquid DAC solvents based on magnetic nanoparticles (MNPs) heating with AC magnetic fields. MNPs can be directly heated to provide uniform and rapid volumetric heating, as we demonstrate by promoting the release of captured CO2 from an aqueous solution of potassium sarcosinate. Our results showed that 90% of the solvent can be regenerated within 7.5 min of heating through proposed technique. The MNPs and solvent are found to be stable during the regeneration process and the MNPs showed long‐term stability in the CO2‐saturated solvent. Cyclic experiments showed that the nanoparticles can be reused for multiple cycles without performance deterioration. The process is operated in a noncontact mode through electromagnetic waves, making it an adoptable approach for existing carbon capture systems. The MNPs heating provides an effective regeneration strategy for liquid solvents used in carbon capture processes, in particular for DAC.

Facilitated cell probing and hyperthermia treatment by fluorescent DNA engineered cobalt ferrite nanoparticles

BackgroundProbing a cell by fluorescence imaging is very convenient. We have observed that tagging of DNA with some organic dye molecules enhances fluorescence properties. This property of DNA can be utilised to enhance cell imaging quality to track the cell in better way for diagnosis as well as therapy. ResultsWe have shown internalisation of DNA functionalized cobalt-ferrite nanoparticles (CF-DNA NPs) with cells is better compared to free cobalt-ferrite nanoparticles (CF NPs). Therefore, CF NPs were functionalized by DNA (CF-DNA) and tagged them with a fluorophore like RITC for improvement of cell tracking by fluorescence imaging. Cell imaging and cytotoxicity effects were compared by treating the human mammary carcinoma cells (MDA MB-231) with CF NPs and CF-DNA NPs. An investigation was made on applicability of hyperthermia therapy for these particles under AC magnetic field. SignificancesA significant enhancement in fluorescence contrast of cancer cells was observed in case of CF-DNA NPs compared to CF NPs and both the particles show less cytotoxic effect for normal cells. A significant cell death and change in cell morphology after hyperthermia treatment was observed by phase contrast microscope and scanning electron microscope (SEM). Monitoring the treatment in a selective region is helped by this technique with the use of a very small quantity of theranostic material. This new treatment protocol may open a new avenue in cancer therapy.

Relatively Low‐Frequency Magnetic Field Detection System Using Metglas/PZT‐5B Sensor

The magnetoelectric (ME) sensor is a new and promising type of magnetic field sensor with ultrahigh sensitivity. However, there are few reports on the research of real‐time measurement system which can promote its practical application. Herein, a novel real‐time measuring approach for weak AC magnetic fields at relatively low frequency is proposed using Metglas/PZT‐5B ME sensors. The system mainly consists of an oscilloscope, a signal generator, and a program developed with LabVIEW programming. Real‐time measurement of relatively low‐frequency magnetic fields has been achieved by using frequency upconversion methods, simultaneously displaying the frequency and magnitude of the magnetic field. As a result, the real‐time measurement system is able to detect a weak AC magnetic field as low as 0.1 nT@1 Hz, which is promising to push the ME sensor to practical application.

Magnetic Iron Oxide Nanoparticles Coated by Coumarin-Bound Copolymer for Enhanced Magneto- and Photothermal Heating and Luminescent Thermometry.

In this work, we report on the synthesis and investigation of new hybrid multifunctional iron oxide nanoparticles (IONPs) coated by coumarin-bound copolymer, which combine magneto- or photothermal heating with luminescent thermometry. A series of amphiphilic block copolymers, including Coum-C11-PPhOx27-PMOx59 and Coum-C11-PButOx8-PMOx42 bearing luminescent and photodimerizable coumarin moiety, as well as coumarin-free PPhOx27-PMOx57, were evaluated for their utility as luminescent thermometers and for encapsulating spherical 26 nm IONPs. The obtained IONP@Coum-C11-PPhOx27-PMOx59 nano-objects are perfectly dispersible in water and able to provide macroscopic heating remotely triggered by an alternating current magnetic field (AMF) with a specific absorption rate (SAR) value of 240 W.g-1 or laser irradiation with a photothermal conversion efficiency of η = 68%. On the other hand, they exhibit temperature-dependent emission of coumarin offering the function of luminescent thermometer, which operates in the visible region between 20 °C and 60 °C in water displaying a maximal relative thermal sensitivity (Sr) of 1.53%·°C-1 at 60 °C.

Magnetoelectric effect in a Metglas–langatate ring heterostructure for current sensing

The magnetoelectric (ME) effect was observed and investigated in a planar ring-type heterostructure containing mechanically coupled rings of magnetostrictive amorphous alloy Metglas and a piezoelectric single crystal langatate. The structure was excited by a circumferential ac magnetic field in the presence of a circumferential dc bias magnetic field. Due to the absence of demagnetization and the high acoustic quality factor of the langatate, a high ME field conversion coefficient of α E ≈ 53 V (Oe cm)−1 was obtained at the frequency 221 kHz of the fundamental radial acoustic resonance mode. The structure can be used as a non-contact ac or dc current sensor. The sensitivity of the structure to the ac current was 1.6 V A−1, whereas for the dc current it reached 4.2 V A−1.

Theory of electron spin resonance in scanning tunneling microscopy

Electron spin resonance (ESR) spectroscopy in scanning tunneling microscopy (STM) has enabled probing the electronic structure of single magnetic atoms and molecules on surfaces with unprecedented energy resolution, as well as demonstrating coherent manipulation of single spins. Despite this remarkable success, the field could still be greatly advanced by a more quantitative understanding of the ESR-STM physical mechanisms. Here, we present a theory of ESR-STM that quantitatively models not only the ESR signal itself, but also the full background tunneling current, from which the ESR signal is derived. Our theory is based on a combination of Green's function techniques to describe the electron tunneling and a quantum master equation for the dynamics of the spin system along with microwave radiation interacting with both the tunneling current and the magnetic system. We show that this theory is able to quantitatively reproduce the experimental results for a spin-1/2 system (TiH molecules on MgO) across many orders of magnitude in tunneling current, providing access to the relaxation and decoherence rates that govern the spin dynamics due to intrinsic mechanisms and to the applied bias voltage. More importantly, our work establishes that (i) sizable ESR signals, which are a measure of microwave-induced changes in the junction magnetoresistance, require surprisingly high tip spin polarizations, and (ii) the coupling of the magnetization dynamics to the microwave field gives rise to the asymmetric ESR spectra often observed in this spectroscopy. Additionally, our theory provides very specific predictions for the dependence of the relaxation and decoherence times on the bias voltage and the tip-sample distance. Finally, with the help of electromagnetic simulations, we find that the transitions in our ESR-STM experiments, in which the tunnel junction is irradiated by a nearby microwave antenna, can be driven by the ac magnetic field at the junction. Published by the American Physical Society 2024

High-Frequency Hall Effect and Transverse Electric Galvanomagnetic Waves in Current-Biased 2D Electron Systems

We derive the electrodynamic conductivity tensor for 2DESs with dc drift with account for the high-frequency Hall effect (interaction of dc current with ac magnetic field). We demonstrate the limitations of the quasistatic approach which neglects this effect. With the help of electrodynamic conductivity we find a novel two-dimensional transverse electric (TE) electromagnetic mode. This mode is non-reciprocal with dispersion \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\omega = {\\mathbf{k}}{{{\\mathbf{u}}}_{0}}$$\\end{document} and manifests itself in lowering the reflection coefficient of 2DES at the resonance frequency. In addition, we predict birefringence of an incident evanescent TE wave on a 2DES system with drift and find hints of Cerenkov amplification in the low frequency limit. We discuss the limiting cases when the quasistatic approach is suitable.

Synthesis, Surface Modification and Magnetic Properties Analysis of Heat-Generating Cobalt-Substituted Magnetite Nanoparticles.

Here, we present the results of the synthesis, surface modification, and properties analysis of magnetite-based nanoparticles, specifically Co0.047Fe2.953O4 (S1) and Co0.086Fe2.914O4 (S2). These nanoparticles were synthesized using the co-precipitation method at 80 °C for 2 h. They exhibit a single-phase nature and crystallize in a spinel-type structure (space group Fd3¯m). Transmission electron microscopy analysis reveals that the particles are quasi-spherical in shape and approximately 11 nm in size. An observed increase in saturation magnetization, coercivity, remanence, and blocking temperature in S2 compared to S1 can be attributed to an increase in magnetocrystalline anisotropy due to the incorporation of Co ions in the crystal lattice of the parent compound (Fe3O4). The heating efficiency of the samples was determined by fitting the Box-Lucas equation to the acquired temperature curves. The calculated Specific Loss Power (SLP) values were 46 W/g and 23 W/g (under HAC = 200 Oe and f = 252 kHz) for S1 and S2, respectively. Additionally, sample S1 was coated with citric acid (Co0.047Fe2.953O4@CA) and poly(acrylic acid) (Co0.047Fe2.953O4@PAA) to obtain stable colloids for further tests for magnetic hyperthermia applications in cancer therapy. Fits of the Box-Lucas equation provided SLP values of 21 W/g and 34 W/g for CA- and PAA-coated samples, respectively. On the other hand, X-ray photoelectron spectroscopy analysis points to the catalytically active centers Fe2+/Fe3+ and Co2+/Co3+ on the particle surface, suggesting possible applications of the samples as heterogeneous self-heating catalysts in advanced oxidation processes under an AC magnetic field.

Quantum sensing of magnetic fields with molecular spins

Spins are prototypical systems with the potential to probe magnetic fields down to the atomic scale limit. Exploiting their quantum nature through appropriate sensing protocols allows to enlarge their applicability to fields not always accessible by classical sensors. Here we first show that quantum sensing protocols for AC magnetic fields can be implemented with molecular spin ensembles embedded into hybrid quantum circuits. We then show that, using only echo detection at microwave frequency and no optical readout, Dynamical Decoupling protocols synchronized with the AC magnetic fields can enhance sensitivity up to S ≈ 10−10 − 10−9 T Hz−1/2 with a low (4-5) number of applied pulses. These results paves the way for the development of strategies to exploit molecular spins as quantum sensors.

A multi-frequency magnetic particle spectroscopy system for systematic characterization and highly sensitive detection of magnetic nanoparticles

Systematic characterization and highly sensitive detection of magnetic nanoparticles (MNPs) is of great importance to investigate the magnetic properties of the MNPs for biomedical applications. In this study, a multi-frequency magnetic particle spectroscopy (MPS) system is developed to measure the magnetic response of the MNPs in different ac magnetic fields. A model for the description of the output signal of the MPS system is deduced to analyze the signal-to-noise ratio of the MNP harmonics. The noise source, including the Gaussian noise and harmonic distortion from the excitation magnetic field, is quantitatively measured and comprehensively analyzed to investigate their influences on the limit-of-detection (LOD) of the MPS system. The MPS signal and dynamic magnetization curve of the MNPs are measured in different ac magnetic fields with the developed MPS system. In addition, a series of MNP samples with different iron concentrations are prepared for experiments to determine the LOD, indicating an LOD of 2.3 ng in terms of iron. The dependence of the LOD on the Gaussian noise and the harmonic distortion is discussed. The present work indicates that one needs to comprehensively consider the influence factors of the Gaussian noise and harmonic distortion to develop a sensitive MPS system with multi-frequency excitation.

Magnetic Field Sensing via Acoustic Sensing Fiber with Metglas® 2605SC Cladding Wires

Magnetic field sensing has the potential to become necessary as a critical tool for long-term subsurface geophysical monitoring. The success of distributed fiber optic sensing for geophysical characterization provides a template for the development of next generation downhole magnetic sensors. In this study, Sentek Instrument’s picoDAS is coupled with a multi-material single mode optical fiber with Metglas® 2605SC cladding wire inclusions for magnetic field detection. The response of acoustic sensing fibers with one and two Metglas® 2605SC cladding wires was evaluated upon exposure to lateral AC magnetic fields. An improved response was demonstrated for a sensing fiber with in-cladding wire following thermal magnetic annealing (~400 °C) under a constant static transverse magnetic field (~200 μT). A minimal detectable magnetic field of ~500 nT was confirmed for a sensing fiber with two 10 μm cladding wires. The successful demonstration of a magnetic field sensing fiber with Metglas® cladding wires fabricated via traditional draw processes sets the stage for distributed measurements and joint inversion as a compliment to distributed fiber optic acoustic sensors.

Optical fiber weak magnetic sensing system based on strip-ring sensing structure and PGC-Improved algorithm

The detection of weak magnetic fields is integral to applications in geophysical monitoring, biomagnetism, and remote sensing. This paper presents a new sensing unit structure and signal demodulation algorithm for optical fiber weak magnetic sensing system. The system uses a strip-ring sensing unit structure that combines magnetostrictive effects with optical fibers through 3D printing technology. This integration facilitates the conversion of magnetostrictive displacement into optical phase. An improved Phase Generated Carrier (PGC) algorithm is employed to demodulate the phase information, enhancing the signal to noise ratio (SNR) by 4.33 dB compared to the traditional PGC algorithm, and achieving a phase resolution of 29.51 µrad/Hz1/2 at 500 Hz. Experimental results demonstrate that the suggested sensing system can detect a minimum magnetic field of 140 nT, with an AC magnetic field phase resolution of 15.78 nT/Hz1/2 at 500 Hz. The demodulation sensitivity reaches 1.87 × 10−6 rad/nT under a magnetic field of 140–1400 nT. The capability of the proposed system demonstrated great potential as an efficient approach for detecting weak magnetic signals in many practical applications.

Analysis of comprehensive magnetic shielding and optimization design of high-conductive layers in magnetic shielding devices for atomic magnetometer

Since the frequency of the biological magnetic field is DC to 500 Hz, it is important to improve the ability of the magnetic shielding device within this range for the measurement of the biological magnetic field and the reduction of the magnetic noise of the SERF magnetometer. At present, the optimization of AC magnetic field shielding in the frequency range of 0.01 to 500 Hz is rarely considered. To optimize the AC shielding factor, this paper analyzed the influence of the position and structure of the high-conductive layer in the magnetic shielding device and proposed an optimized structure of the high conductive layer. What’s more, the theoretical formula was improved by introducing frequency coefficients, solving the problem of significant differences between theoretical calculation results, FEM results and measurement results. Finally, it was verified by experiments that the optimized high-conductivity layer structure can improve the AC shielding factor of magnetic shielding device.

Magnetic field-assisted water splitting at ternary NiCoFe magnetic Nanocatalysts: Optimization study

This study addresses the enhanced splitting of water into H2 and O2 assisted with applied magnetic field at ternary NiCoFe catalyst. The catalyst's activity towards water electrolysis is enhanced in the presence of AC-induced magnetic field (B = 720 μT) and approaches the performance of Pt/C∥IrO2. That is the overpotential at a current density of 10 mA cm−2 (η10) is reduced by 90 mV and 210 mV for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), respectively. Besides, a significant reduction in the cell voltage of 0.3 V and a power saving of 215.6 kWh during the production of one kg of H2 are obtained when AC magnetic field of 720 μT is applied. A markedly high stability of the catalyst is obtained under magnetic field during a prolonged electrolysis. The magnetohydrodynamic effect is believed to drive water electrolysis at NiCoFe film by reducing the ohmic potential drop across the electrode/solution interface, and increases the rate of charge transfer at the catalyst surface. The results are explained by capacitance measurements revealing maximum capacitance of NiFeCo catalyst in the presence of AC magnetic flux of 720 μT. Thus, favourable adsorption conditions lead to accelerated kinetics of OER and HER.