AC Magnetic Field Research Articles

Novel Biocompatible Magnetoelectric MnFe2 O4 Core@BCZT Shell Nano-Hetero-Structures with Efficient Catalytic Performance.

Magnetoelectric (ME) small-scale robotic devices attract great interest from the scientific community due to their unique properties for biomedical applications. Here, novel ME nano hetero-structures based on the biocompatible magnetostrictive MnFe2 O4 (MFO) and ferroelectric Ba0.85 Ca0.15 Zr0.1 Ti0.9 O3 (BCZT) are developed solely via the hydrothermal method for the first time. An increase in the temperature and duration of the hydrothermal synthesis results in increasing the size, improving the purity, and inducing morphology changes of MFO nanoparticles (NPs). A successful formation of a thin epitaxial BCZT-shell with a 2-5nm thickness is confirmed on the MFO NPs(77 ± 14nm) preliminarily treated with oleic acid (OA) or polyvinylpyrrolidone (PVP), whereas no shell is revealed on the surface of pristine MFO NPs. High magnetization is revealed for the developed ME NPs based on PVP- and OA-functionalized MFO NPs (18.68 ± 0.13 and 20.74 ± 0.22 emug-1 , respectively). Moreover, ME NPs demonstrate 95% degradation of a model pollutant Rhodamine B within 2.5 h under an external AC magnetic field (150 mT, 100Hz). Thus, the developed biocompatible core-shell ME NPs of MFO and BCZT can be considered as a promising tool for non-invasive biomedical applications, environmental remediation, and hydrogen generation for renewable energy sources.

Hybrid NV Center Magnetometer With Directional Vector Resolution Based on Magnetic Flux Concentrators

Since the introduction of Hall magnetometers, different types of magnetometers have been widely used. Among them, magnetometers based on nitrogen-vacancy (NV) centers in diamonds are promising room-temperature solid-state sensors. Their sensitivity to low-frequency (< 10 Hz) magnetic fields has been reported to be currently at the nT level. Still, limitations in the detectable spatial range hinder their potential applications in medical imaging, earth sciences, and navigation. The most recent sensitivity enhancement technique involves using high-permeability flux concentrators with a unique magnetic field amplification structure that can cause the NV magnetometer to lose its vector detection capabilities. To address this limitation, we proposed a hybrid magnetometer based on a diamond-based hetero-NV center, which collects magnetic flux over a larger area and concentrates it in the diamond magnetometer, from which the magnetic field vector can be calculated by measuring the specific magnetic field direction vector. The low-frequency sensitivity of 90 pT∙Hz-1/2 under conventional ambient conditions has been solved by inserting NV-doped diamond blocks between four permalloy concentrators in a triangular structure, which solves the vector deficiency problem of traditional magnetic concentrators. High-sensitivity real-time variation acquisition of the magnetic field signal has been achieved by applying an AC magnetic field in combination with modulation and demodulation methods.

Vortex dynamics induced by scanning SQUID susceptometry

We measured the local magnetic response of a niobium thin film by applying a millitesla-scale AC magnetic field using a micron-scale field coil and detecting the response with a micron-scale pickup loop in a scanning superconducting quantum interference device (SQUID) susceptometry measurement. Near the film's critical temperature, we observed a step-like nonlinear and dissipative magnetic response due to the dynamics of a small number of vortex-antivortex pairs induced in the film by the local applied AC field. We modeled the dynamics of the measurement using a combined two-dimensional London-Maxwell and time-dependent Ginzburg-Landau approach, allowing us to construct a detailed real-space picture of the vortex motion causing the observed dissipative response. This work pushes scanning SQUID susceptometry of two-dimensional superconductors beyond the regime of linear response and lays the foundation for microscopic studies of vortex dynamics and pinning in superconducting devices and more exotic materials systems.

From spectral analysis to hysteresis loops: a breakthrough in the optimization of magnetic nanomaterials for bioapplications

An innovative method is proposed to determine the most important magnetic properties of bioapplication-oriented magnetic nanomaterials exploiting the connection between hysteresis loop and frequency spectrum of magnetization. Owing to conceptual and practical simplicity, the method may result in a substantial advance in the optimization of magnetic nanomaterials for use in precision medicine. The techniques of frequency analysis of the magnetization currently applied to nanomaterials both in vitro and in vivo usually give a limited, qualitative picture of the effects of the active biological environment, and have to be complemented by direct measurement of the hysteresis loop. We show that the very same techniques can be used to convey all the information needed by present-day biomedical applications without the necessity of doing conventional magnetic measurements in the same experimental conditions. The spectral harmonics obtained analysing the response of a magnetic tracer in frequency, as in magnetic particle spectroscopy/imaging, are demonstrated to lead to a precise reconstruction of the hysteresis loop, whose most important parameters (loop’s area, magnetic remanence and coercive field) are directly obtained through transformation formulas based on simple manipulation of the harmonics amplitudes and phases. The validity of the method is experimentally verified on various magnetic nanomaterials for bioapplications submitted to ac magnetic fields of different amplitude, frequency and waveform. In all cases, the experimental data taken in the frequency domain exactly reproduce the magnetic properties obtained from conventional magnetic measurements.

New synthesis route for high quality iron oxide-based nanorings: Structural and magnetothermal evaluations

The present study reports on the successful fabrication of high quality monocrystallite magnetite (Fe3O4) nanorings, based on the transformation of monocrystallite hematite (α-Fe3O4) nanorings, using a wet and mild-temperature-mediated chemical route, as an alternative to the traditional solid–solid annealing methods. Additionally, attempts to fabricate ferrite-based nanorings with different stoichiometry through metal-ion (Mn2+, Zn2+, Co2+, and Fe2+) doping and using solid–solid reaction by annealing the hematite nanorings is herein reported. The nanoring morphology is confirmed by electron microscopy micrographs whereas typical magnetic vortex signature is revealed in magnetic hysteresis cycle measurements recorded at room temperature, via collapsing of open loop at low magnetic fields. Rietveld refinement analysis revealed fabrication of single-phase (except one sample revealing two phases) and high-quality crystalline nanomaterials, identified with standard ICDD cards. The as-fabricated samples were tested in regard to their performance for magnetic hyperthermia application, using AC magnetic field amplitude of 100, 150, 200, and 240 Oe while setting frequency at 330 kHz. The recorded temperature versus time curves were successfully explained by the Box-Lucas method.

Development of capacitive‐coupled hall‐type MHD generator

AbstractWe have demonstrated a capacitively coupled Hall‐type MHD generator using ECR plasma. To clarify the characteristics of the fabricated MHD generator, we measured the power generation characteristics as a function of magnetic field strength using a DC Hall‐type MHD power generation experiment. The results showed that the output power decreased due to magnetic pressure at the higher magnetic field. However, the output power corresponded to the theoretical value at the lower magnetic field. An AC Hall‐type MHD power generation experiment was conducted using an AC magnetic field. As a result, full‐wave rectification voltage was observed as per theory. Finally, capacitively coupled Hall‐type MHD power generation experiments were conducted, and full‐wave rectified waveforms were observed as in AC Hall‐type MHD power generation. These waveforms were similar to the output waveforms predicted from theory. These results show that the capacitively coupled Hall‐type MHD generator is feasible.

Influence of the ac magnetic field on the elastic and electric characteristics of a quantum paramagnet

It is shown that electric, piezoelectric, and elastic characteristics of the orthorhombic quantum paramagnet depend on the applied ac magnetic field, collinear to the dc one. The dependence is due to the renormalization of the quadrupole spin susceptibility. In the dynamical regime, the quadrupole susceptibility oscillates with the frequency of the ac field, and with the Rabi-like frequency. In the steady-state regime, the quadrupole susceptibility oscillates with the frequency of the ac field only about the renormalized with respect to the initial one value. That renormalization can change the values of the electric, piezoelectric, and elastic characteristics of the system.

МОДЕЛЮВАННЯ ПРОЦЕСУ ІНДУКЦІЙНОГО НАГРІВУ ДЛЯ СИСТЕМ МАГНІТНОЇ ГІПЕРТЕРМІЇ

The paper is devoted to the challenges of applying the induction heating (IH) for magnetic hyperthermia. The analysis of the results of previous studies has shown that within the biologically safe range of AC magnetic fields, insufficient induction heating power still appears to be one of the key problems for the successful clinical application of magnetic hyperthermia. In this paper, several possible effective circuit design solutions for the IH system are proposed, and their influence on the parameters of the heating processes of ferrites and ferromagnets is investigated. The model of the induction heater created in COMSOL Multiphysics allowes to simulate the distribution of current density and temperature in the heater. The developed model ensures better assessment of the processes occurring in living tissues and enables to simulate the impact of the magnetic particle material type and size on the temperature of heating and power consumption of the device.

Effect of a DC transport current on the AC loss in no-insulation ReBCO racetrack coils exposed to AC parallel magnetic field at 77 K and 4.2 K

ReBCO coils are developed as DC field coils in linear motor systems to increase the force density, in favor of permanent magnets. Such coils have to sustain a relatively large heat load stemming from the AC magnetic field environment in which they operate. The use of no or partial turn-to-turn insulation can make them more stable against the effects of local heating. Conversely, the radial electrical connections in no-insulation (NI) coils allow for large coupling currents, causing additional AC loss on top of the already significant heat load. Here we report on the AC loss in sub-scale NI, 4 mm wide single-tape, ReBCO racetrack coils exposed to parallel-to-the-tape magnetic field in the frequency range of 10−4 to 1 Hz at 77 K and 4.2 K, while carrying a DC transport current. AC loss is measured magnetically and electrically. The main goal of these experiments is to validate our 2D numerical model, which provides more insight into the origin of the AC loss. At low frequencies, inter-turn coupling currents are spread more or less homogeneously throughout the winding pack. Whereas at high frequencies, the skin effect causes shielding of the interior of the coil and large induced currents only occupy the coil’s outer surface.

Magnetoelectric Properties of Lead-Free Three-Layer Structure Barium–Titanate–Piezoceramic Nickel

The results of the comprehensive magnetoelectric interaction research in three-layer structure Ni–piezoceramic BaTiO3–Ni are presented. It has been theoretically shown and experimentally confirmed that, in the general case, the dependence of the magnetoelectric response has non-linear character. At low bias magnetic field, a quadratic dependence magnetoelectric response from an AC magnetic field is observed then there is a linear section, as well as at high values of the field magnetoelectric response has saturation. The obtained values of the magnetoelectric characteristics (αEmax = 32 V(cmOe) for resonance and 437 mV/(cmOe) for field dependence) for lead-free three-layer structure barium–titanate–piezoceramic nickel are comparable with the magnetoelectric characteristics for similar structures, based on lead-containing ceramics.

Interplay between thermal and magnetic properties of polymer nanocomposites with superparamagnetic Fe3O4 nanoparticles

Magnetic nanoparticles embedded in polymer matrices have excellent potential for multifunctional applications like magnetic remote heating, controlled drug delivery, hyperthermia, and thermally functionalized biomedical devices. A solvent-based processing method was developed to produce magnetic composites consisting of magnetite (Fe3O4) superparamagnetic nanoparticles embedded in a biomedical-grade polyurethane (ChronoFlex® C). The particles had a log-normal size distribution spanning from 4−16 nm, with a mean-size of 9.5 ± 2 nm. X-ray diffraction, transmission electron microscopy, and scanning electron microscopy with elemental mapping were used to assess the phase purity, surface morphology, particle size, and homogeneity of the resulting nanocomposite. The magnetic properties of composites with 7–13 wt% of Fe3O4 were studied between 5 and 300 K using vibrating sample magnetometry. Room temperature magnetic attraction was observed, with a saturation magnetization of up to 5 emu/g and a low coercive field (Hc < 50 Oe), where the non-zero coercive field was attributed to a small fraction of larger particles that are ferromagnetic at room temperature. Field-cooled and zero-field-cooled magnetometry data were fitted to a numerical model to determine the superparamagnetic mean blocking temperature (TB = 90 K) of the embedded magnetite particles, and an effective magnetic anisotropy of 6×105 erg/cm3. Using an AC magnetic field operating at 85 kHz, we demonstrate that remote heating of the base polyurethane material is greatly enhanced by compositing with Fe3O4 nanoparticles, leading to temperatures up to 45 °C within 18 min for composites submerged in water. This work demonstrates the fundamental principles of a custom-designed thermomagnetic polymer composite that could be used in applications, including medical and heat management.

Simultaneous determination of AC and DC magnetic field by a whispering gallery mode optical resonator

Alternating current (AC) and direct current (DC) magnetic fields are monitored by an optical whispering gallery mode (WGM) microcapillary resonator filled with magnetostrictive material. The DC magnetic field is demodulated with the signal-to-noise ratio of an AC-modulated magnetic field at a particular frequency and a sensitivity of 2.1 dB/mT is obtained across a detection range of 0.9 mT. An AC field sensitivity of 2.08 μT/ H z at 346 kHz is obtained with the same resonator. Introducing a biased DC field of 90Gs provides a four-fold improvement in AC field sensitivity. The AC and DC fields may be used to improve the corresponding DC and AC sensing. Therefore, this scheme may play a role monitoring magnetic fields in defence, biomedical analysis, and consumer electronics.

Physicochemical properties and AC magnetic field induced heating properties of solvothermally prepared thiospinel: Fe3S4 (greigite) nanoparticles

The presence of greigite (Fe3S4) nanoparticles in bacterial magnetosomes, and its lower toxicity have emerged as favourable aspects for its potential applications in various bio-medical applications, including magnetic hyperthermia. Despite having a number of intriguing features, systematic research on the heating efficiency of Fe3S4 nanoparticles (MNPs) in an AC magnetic field is scarce, which is primarily due to the difficulties in preparing phase pure greigite MNPs. In this study, greigite MNPs are prepared using a solvothermal approach, utilizing ethylene glycol as a solvent, and surface functionalized with varied concentrations of poly vinyl alcohol (PVA). Studies using powder x-ray diffraction and electron microscopy demonstrate the development of crystalline Fe3S4 MNPs (average crystallite size: 19–23 nm) with flaky or flower-like morphology. X-ray photoelectron spectroscopy indicates that the lattice is composed primarily of iron and sulphur. The existence of bio-compatible PVA polymer on the surface of the coated MNPs is confirmed using Fourier transform infrared spectroscopy. For the uncoated MNPs, the magnetization at 90 kOe and the effective anisotropy energy density values are found to be ∼ 15.2 emu g−1 and ∼ 22.3 kJ m−3, respectively. Due to the improved colloidal stability, magneto-calorimetric experiments reveal higher AC magnetic field induced heating efficiency for the PVA-coated MNPs. The highest specific absorption rate (SAR) is obtained as ∼ 67.8 ± 2.6 W/gFe in the current study, which is several times higher than the previously published values for synthetic Fe3S4 MNPs. Furthermore, for samples with comparable saturation magnetization and crystallite size, SAR is found to increase with initial susceptibility. The in vitro cytotoxicity studies show good bio-compatibility for the prepared greigite MNPs. The experimental findings provide deeper insights into the preparation of Fe3S4 MNPs using a simple solvothermal technique, and its AC magnetic field induced heating efficiency.

Photo-assisted spin transport in double quantum dots with spin–orbit interaction

We investigate the effect of spin–orbit interaction on the intra- and interdot particle dynamics of a double quantum dot (QD) under ac electric fields. The former is modeled as an effective ac magnetic field that produces electric-dipole spin resonance transitions, while the latter is introduced via spin-flip tunneling amplitudes. We observe the appearance of non-trivial spin-polarized dark states (DSs), arising from an ac-induced interference between photo-assisted spin-conserving and spin-flip tunneling processes. These DSs can be employed to precisely measure the spin–orbit coupling in QD systems. Furthermore, we show that the interplay between photo-assisted transitions and spin-flip tunneling enables the system to operate as a highly tunable spin filter. Finally, we investigate the operation of the system as a resonant flopping-mode qubit for arbitrary ac voltage amplitudes, allowing for high tunability and enhanced qubit control possibilities.

Single-Beam Room-Temperature Atomic Magnetometer with Large Bandwidth and Dynamic Range

We present a single-beam atomic magnetometer operating at room temperature for the measurement of ac magnetic fields. The magnetometer functions in the nonlinear regime of magneto-optical rotation of ${}^{85}\mathrm{Rb}$ atomic vapor without any buffer gases. We demonstrate a sensitivity of approximately $0.9\phantom{\rule{0.2em}{0ex}}\mathrm{pT}/\sqrt{\mathrm{Hz}}$ at 2 kHz and a large bandwidth of 24 kHz. The dynamic range of measurement is ${10}^{6}$, making the sensor effective even in Earth's field. We present the signal-to-noise and bandwidth characteristics of the system for both shielded and unshielded modes of operation. Moreover, we perform theoretical analysis for the atom-light system for the single-laser-beam configuration. The effect of light intensity and detuning on the magnetometer is studied theoretically as well as experimentally to understand the strengths and limitations of the technique.

Multi-physical field simulations of directional seawater freeze-crystallisation under a magnetic field

Ice crystals produced through freeze-desalination of seawater often contain salt pockets that impede the desalination rate. The researchers used phase-field and lattice Boltzmann methods to analyse the influence of magnetic fields on crystal subcooling and the growth law of seawater crystallisation. Results showed that using a DC magnetic field increased the directional growth of ice crystals, leading to a 103.9 % increase in ice crystal area and a 13.05 % reduction in salt pockets compared to no magnetic field. Increasing the strength of the magnetic field improved the overall desalination rate by 25.8 %. The application of an AC magnetic field enhanced the ice crystal area by 37.8 % and the overall desalination rate by 17.0 %, but also increased salt pockets by 11.6 %. The step magnetic field did not affect the crystal area but enhanced the overall desalination rate by 14.3 %. Overall, this study demonstrates the potential for improving the efficiency of freeze desalination by controlling magnetic fields during the sea-ice growth process.

Magnetoelectric Effect in Amorphous Ferromagnetic FeCoSiB/Langatate Monolithic Heterostructure for Magnetic Field Sensing.

This paper investigates the possibilities of creating magnetic field sensors using the direct magnetoelectric (ME) effect in a monolithic heterostructure of amorphous ferromagnetic material/langatate. Layers of 1.5 μm-thick FeCoSiB amorphous ferromagnetic material were deposited on the surface of the langatate single crystal using magnetron sputtering. At the resonance frequency of the structure, 107 kHz, the ME coefficient of linear conversion of 76.6 V/(Oe∙cm) was obtained. Furthermore, the nonlinear ME effect of voltage harmonic generation was observed with an increasing excitation magnetic field. The efficiency of generating the second and third harmonics was about 6.3 V/(Oe2∙cm) and 1.8 V/(Oe3∙cm), respectively. A hysteresis dependence of ME voltage on a permanent magnetic field was observed due to the presence of α-Fe iron crystalline phases in the magnetic layer. At the resonance frequency, the monolithic heterostructure had a sensitivity to the AC magnetic field of 4.6 V/Oe, a minimum detectable magnetic field of ~70 pT, and a low level of magnetic noise of 0.36 pT/Hz1/2, which allows it to be used in ME magnetic field sensors.

In-plane spin excitation of skyrmion bags

Skyrmion bags are spin structures with arbitrary topological charges, each of which is composed of a big skyrmion and several small skyrmions. In this work, by using an in-plane alternating current (AC) magnetic field, we investigate the spin-wave modes of skyrmion bags, which behave differently from the clockwise (CW) rotation mode and the counterclockwise (CCW) rotation mode of skyrmions because of their complex spin topological structures. The in-plane excitation power spectral density shows that each skyrmion bag possesses four resonance frequencies. By further studying the spin dynamics of a skyrmion bag at each resonance frequency, the four spin-wave modes, i.e., a CCW-CW mode, two CW-breathing modes with different resonance strengths, and an inner CCW mode, appear as a composition mode of outer skyrmion–inner skyrmions. Our results are helpful in understanding the in-plane spin excitation of skyrmion bags, which may contribute to the characterization and detection of skyrmion bags, as well as the applications in logic devices.

Significant magnetoelectric enhancement of composite films of CoZnxFe2-xO4 particles and poly (vinylidene fluoride-trifluoroethylene) for AC magnetic sensors

Polymer-based magnetoelectric (ME) nanocomposites exhibit a low ME effect and high bias field at room temperature, which severely limits their application in flexible wearable sensors. To overcome these obstacles, Zn-doped magnetic nanoparticles of CoZnxFe2-xO4 (x = 0, 0.1, 0.2, and 0.3) were prepared by an auto-combustion method in this work. The obtained magnetostrictive curves imply that Zn element doping can improve greatly the piezomagnetic coefficient of magnetic nanoparticles, and among the tested materials, CoZn0.1Fe1.9O4 has the largest piezomagnetic coefficient. Composite films of poly(vinylidene fluoride-trifluoroethylene) and CoZn0.1Fe1.9O4 (P(VDF-TrFE)/CoZn0.1Fe1.9O4) were prepared by spin coating. The maximum ME voltage coefficient of P(VDF-TrFE)/CoZn0.1Fe1.9O4 composite film with a filler weight concentration of 10 wt% is 87.9 mV cm−1 Oe−1 with a bias field of 1050 Oe and a resonance frequency of 46 kHz, which is the highest value reported in the literature of 0–3 type polymer-based ME composite films at present. To evaluate the composite films for application in magnetic sensors, the ME output voltage was measured at a bias field of 1050 Oe with increasing AC magnetic field, demonstrating a good linear relationship with linearity of 0.999. These results indicate that the piezomagnetic coefficient of magnetic materials is an the important factor for the great enhancement of the ME voltage coefficient. This work provides a new approach for the synthesis of more effective polymer-based ME nanocomposites for potential applications in smart wearables.

Development of Capacitive-coupled Hall-type MHD Generator

We have demonstrated a capacitively coupled Hall-type MHD generator using ECR plasma. To clarify the characteristics of the fabricated MHD generator, we measured the power generation characteristics as a function of magnetic field strength using a DC Hall-type MHD power generation experiment. The results showed that the output power decreased due to magnetic pressure at the higher magnetic field. However, the output power corresponded to the theoretical value at the lower magnetic field. An AC Hall-type MHD power generation experiment was conducted using an AC magnetic field. As a result, full-wave rectification voltage was observed as per theory. Finally, capacitively coupled Hall-type MHD power generation experiments were conducted, and full-wave rectified waveforms were observed as in AC Hall-type MHD power generation. These waveforms were similar to the output waveforms predicted from theory. These results show that the capacitively coupled Hall-type MHD generator is feasible.