External AC Magnetic Field Research Articles

Magnetic domain imaging of a very rough fractured surface of Sr ferrite magnet without topographic crosstalk by alternating magnetic force microscopy with a sensitive FeCo-GdOx superparamagnetic tip

We imaged the magnetic domain of an extremely rough surface (with a roughness of ∼1 μm) of the anisotropic Sr ferrite sintered magnet without any topographic crosstalk by alternating magnetic force microscopy (A-MFM) using a sensitive FeCo-GdOx superparamagnetic tip. The magnetic moment of the FeCo-GdOx superparamagnetic tip is driven by an external AC magnetic field applied out of the plane direction to the magnetic sample. The static magnetic field is from the rough fractured ferrite sample parallel to the direction of the external AC magnetic field and is imaged by modulating the magnetic moment of the superparamagnetic tip. By using the frequency demodulation phenomena, A-MFM can extract the magnetic signal without any topography crosstalk versus the conventional MFM method. The intensity and the polarity of the static magnetic field originate from highly rough fractured hard magnetic Sr ferrite samples, and these were successfully detected and identified. This technique with the as-fabricated FeCo-GdOx superparamagnetic tips gives information about the intensity as well as polarity of magnetic fields from the magnetic domain structure of very rough fractured magnetic materials without any topographic crosstalk. This is crucial for the development of high performance hard magnets and magnetic devices.

Active magnetic force microscopy of Sr-ferrite magnet by stimulating magnetization under an AC magnetic field: Direct observation of reversible and irreversible magnetization processes

Understanding the dynamic magnetization process of magnetic materials is crucial to improving their fundamental properties and technological applications. Here, we propose active magnetic force microscopy for observing reversible and irreversible magnetization processes by stimulating magnetization with an AC magnetic field based on alternating magnetic force microscopy with a sensitive superparamagnetic tip. This approach simultaneously measures sample's DC and AC magnetic fields. We used this microscopy approach to an anisotropic Sr-ferrite (SrF) sintered magnet. This is a single domain type magnet where magnetization mainly changes via magnetic rotation. The proposed method can directly observe the reversible and irreversible magnetization processes of SrF and clearly reveal magnetic domain evolution of SrF (without stimulating magnetization—stimulating reversible magnetization—stimulating irreversible magnetization switching) by slowly increasing the amplitude of the external AC magnetic field. This microscopy approach can evaluate magnetic inhomogeneity and explain the local magnetic process within the permanent magnet.

Magnetic domain structure imaging near sample surface with alternating magnetic force microscopy by using AC magnetic field modulated superparamagnetic tip

For magnetic domain imaging, with a very high spatial resolution magnetic force microscope, the tip-sample distance should be as small as possible. However, magnetic imaging near the sample surface is very difficult with conventional magnetic force microscopy (MFM) because the interactive forces between the tip and sample include van der Waals and electrostatic forces along with a magnetic force. In this study, we proposed alternating MFM which only extracts a magnetic force near the sample surface without any topographic and electrical crosstalk. In the present method, the magnetization of an FeCo–GdOx superparamagnetic tip is modulated by an external AC magnetic field in order to measure the magnetic domain structure without any perturbation from the other forces near the sample surface. Moreover, it is demonstrated that the proposed method can also measure the strength and identify the polarities of the second derivative of the perpendicular stray field from a thin film permanent magnet with a DC demagnetized state and remanent state.

Smart Surfaces: Magnetically Switchable Light Diffraction through Actuation of Superparamagnetic Plate‐Like Microrods by Dynamic Magnetic Stray Field Landscapes

AbstractThis work reports on a remotely controllable, all‐magnetic reflective diffraction grating‐like optical element for electromagnetic waves in the visible spectrum. The switchable grating is realized by the unique interplay between nanostructured superparamagnetic plate‐like microrods and a magnetic stray field landscape generated by an engineered magnetic stripe domain pattern superimposed by a small external magnetic field. It is shown that the purposeful design of local magnetic field sources in such a continuous thin‐film system enables a precise manipulation of the microrod alignment and, hence, dynamic control of the corresponding grating constant. It is demonstrated that the magnetic grating can be turned on and off due to disappearance of the engineered domain pattern when magnetically saturated. Moreover, the grating constant can be dynamically changed between two states when applying an AC external magnetic field. The experimental findings are corroborated by a theoretical model based on a quantitative description of the involved forces among the microrods and between microrods and substrate, respectively. These results therefore serve as a basis for smart surfaces with switchable diffraction properties on demand upon remote control.

Numerical Modeling of Dynamic Loss in HTS-Coated Conductors Under Perpendicular Magnetic Fields

High-Tc superconducting (HTS)-coated conductors are a promising option for the next-generation power devices. However, their thin-film geometry incurs dynamic loss when exposed to a perpendicular external ac magnetic field, which is difficult to predicate and estimate. In this paper, we propose and verify a numerical simulation model to predict the dynamic loss in HTS-thin-coated conductors by taking into account their Jc-B dependence and I-V characteristics. The model has been tested on a SuperPower YBCO-coated conductor, and we observed a linear increase of dynamic loss along the increasing field amplitude after the threshold field. Our simulation results agree closely with experimental measurements as well as an analytical model. Furthermore, the model can predict the nonlinear increase of dynamic loss at high current, while the analytical model deviates from the measurement results and still shows a linear correlation between the dynamic loss and the external magnetic field. In addition, we have used this model to simulate the distributions of magnetic field and current density when dynamic loss occurs. Results clearly show the flux traversing the coated conductor, which causes dynamic loss. These distributions have also been used to analyze the change of dynamic loss when either the transport current or the magnetic field increase individually, while the other factor remains constant. The simulation analysis on dynamic loss is done for the first time in this paper, and our results clearly demonstrate how dynamic loss changes as well as its dependence on transport current and magnetic field.

Vibrational energy harvesting device with magnetic tip mass

In this paper, the energy-harvesting device based on vibrational piezoelectric transducer for AC magnetic field conversion is presented. The harvester consists of the transducer and an AC/DC-converter. The load dependencies of the transducer showed that the maximal rms power reached a peak of Prms= 37 μW at f = 50 Hz and Prms= 200 μW at f = 48 Hz. If the harvester is placed into an external AC magnetic field h(t) of the frequency f = 48 Hz and constant amplitude of h = 5 Oe, DC voltage of fixed level (1.8 V, 2.5 V, 3.3 V or 3.6 V) is available on the output. Charging time depends on the voltage level and has a maximum value of t ≈ 11 s for 3.6 V and minimum value t ≈ 6 s for 1.8 V.

High-Resolution Quantum Sensing with Shaped Control Pulses.

We investigate the application of amplitude-shaped control pulses for enhancing the time and frequency resolution of multipulse quantum sensing sequences. Using the electronic spin of a single nitrogen-vacancy center in diamond and up to 10 000 coherent microwave pulses with a cosine square envelope, we demonstrate 0.6-ps timing resolution for the interpulse delay. This represents a refinement by over 3 orders of magnitude compared to the 2-ns hardware sampling. We apply the method for the detection of external ac magnetic fields and nuclear magnetic resonance signals of ^{13}C spins with high spectral resolution. Our method is simple to implement and especially useful for quantum applications that require fast phase gates, many control pulses, and high fidelity.

PEG mediated shape-selective synthesis of cubic Fe3O4 nanoparticles for cancer therapeutics

A facile strategy for shape-selective synthesis of PEGylated Fe3O4 cubic magnetic nanoparticles (PCMN) by thermal decomposition of Fe (III) acetylacetonate was developed and explored their applications in drug delivery and hyperthermia. The structural analysis by XRD and TEM showed the formation of spinel-structured Fe3O4 nanoparticles of good crystallinity. The presence of carboxyl PEG group on the surface of PCMN provides colloidal stability, non-toxicity and protein resistance characteristics to them. These negatively charged PCMN have high electrostatic binding affinity for positively charged anticancer drug, doxorubicin hydrochloride (DOX) and followed pH responsive release behaviour. The in-vitro cytotoxicity studies using normal human fibroblast (NIH 3T3) cells did not show any significant toxicity when cells were treated with PCMN. However, DOX loaded PCMN (PCMN-DOX) exhibit good cellular internalization and substantial toxicity to mouse skin fibrosarcoma (WEHI-164) cells. In addition, the superparamagnetic nature of these particles with optimal magnetization and excellent specific absorption rate (SAR) under external AC magnetic field makes it a valuable system which can be further used as an effective heating agent for hyperthermia treatment of cancer.

A Fast AC Field Controlled Impedance in HTS Coated Conductors: Response Speed and Electric Field Value

Direct current carrying Type II superconductors present a dc electric field under external perpendicular ac magnetic field. This effect can also be considered as an equivalent dynamic resistance. The minimum field magnitude required to generate the dynamic resistance is less than the fully penetration field of the superconductor, which is much less than its upper critical field. Thus, the superconductor can remain superconducting, leading to fast recovery times and meaning that it is feasible to use dynamic resistance to add a controlled impedance into the system. In this study, we built a demonstrator using a section of YBCO coated conductor controlled by an external ac field of a wide frequency range. We tested the electric field value and response time of the system. The work may be indicative for designing impedances for flux pumps or superconducting magnetic energy storage systems.

Surface modified electrospun porous magnetic hollow fibers using secondary downstream collection solvent contouring

In this study, a relatively facile porous magnetic hollow fiber engineering electrospinning method is demonstrated, which modulates fiber morphology based on secondary solvents (at variable temperatures). To demonstrate this, polycaprolactone (PCL) polymer and iron oxide nanoparticles (NPs) were used as the fibrous composite matrix. Fiber pore size increased with increasing immersed secondary solvent temperature. By contouring the surface morphology (via modulation of secondary collection solvents) of hollow magnetic fibers, drug (ketoconazole) release kinetics from spun mats were tuned. Furthermore, applying an external AC magnetic field to NP embedded porous fibers enhanced drug release. These findings are promising for alternative engineering, tuning and controlling fiber morphology and drug release behavior.

Simulation of Screening Current Reduction Effect in REBCO Coils by External AC Magnetic Field

Second-generation high-temperature superconducting (HTS) tapes have been examined for applications, such as nuclear magnetic resonance, MRI, and accelerators. Each of these applications requires a precise magnetic field profile. However, screening currents induced while charging an HTS magnet degrade its magnetic field quality. Techniques to reduce the screening current effect have been proposed in the literature. One of the means to reduce screening currents is to apply an ac magnetic field using a “shaking magnet.” The shaking effect enhances the quality of magnetic field by reallocating the screening currents inside HTS tapes. Although some experiments to study the shaking field effect were reported, the current distribution inside HTS tapes has not yet been clarified by simulation. This paper presents the simulation results for an ac magnetic field applied to a REBCO tape to reduce the influence of screening currents. In addition, we investigated the influence of the angle of applied ac magnetic field at the magnet center. The area of negative current density is also shown. From the simulation results, we conclude that a shaking field applied at an angle between 10° and 30° is effective to reduce the screening current effect.

Angular dependence of direct current decay in a closed YBCO double-pancake coil under external AC magnetic field and reduction by magnetic shielding

High Tc superconducting (HTS) coils are ideal candidates in the use of high field magnets. HTS coils carrying a direct current, however, suffer a non-negligible loss when they are exposed to an external AC magnetic field. Although this phenomenon is well known, no study concerning AC magnetic field angular dependence of direct current decay has ever been shown. In this work, we experimentally investigate the direct current decay characteristics in a closed double pancake coil made of a YBCO coated conductor under external AC field. AC field of different angles with respect to the coil plane is applied. Results show that the current decay rate presents a strong angular dependence. The fastest decay occurs when the field is parallel to the coil plane, in which case the surface of the tape in the outermost layer experiences most flux variation. To reduce the decay rate, we propose wrapping superconducting tapes around the outermost layer of the coil to shield external AC field. This method significantly reduces direct current decay rate under parallel field, without affecting the perpendicular self-field of the coil.

In-situ particles reorientation during magnetic hyperthermia application: Shape matters twice.

Promising advances in nanomedicine such as magnetic hyperthermia rely on a precise control of the nanoparticle performance in the cellular environment. This constitutes a huge research challenge due to difficulties for achieving a remote control within the human body. Here we report on the significant double role of the shape of ellipsoidal magnetic nanoparticles (nanorods) subjected to an external AC magnetic field: first, the heat release is increased due to the additional shape anisotropy; second, the rods dynamically reorientate in the orthogonal direction to the AC field direction. Importantly, the heating performance and the directional orientation occur in synergy and can be easily controlled by changing the AC field treatment duration, thus opening the pathway to combined hyperthermic/mechanical nanoactuators for biomedicine. Preliminary studies demonstrate the high accumulation of nanorods into HeLa cells whereas viability analysis supports their low toxicity and the absence of apoptotic or necrotic cell death after 24 or 48 h of incubation.

Improvement of drug delivery by hyperthermia treatment using magnetic cubic cobalt ferrite nanoparticles

In this study, we report a novel synthesis method, characterization and application of a new class of ferromagnetic cubic cobalt ferrite magnetic nanoparticles (MNPs) for hyperthermia therapy and temperature triggered drug release. The MNPs are characterized by XRD, TEM, FESEM, AC magnetic hysteresis and VSM. These MNPs were coated with folic acid and loaded with an anticancer drug. The drug release studies were done at two different temperatures (37°C and 44°C) with progress of time. It was found that higher release of drug took place at elevated temperature (44°C). We have developed a temperature sensitive drug delivery system which releases the heat sensitive drug selectively as the particles are heated up under AC magnetic field and controlled release is possible by changing the external AC magnetic field.

In vitro study on apoptotic cell death by effective magnetic hyperthermia with chitosan-coated MnFe2O4

Magnetic nanoparticles (MNPs) have been widely investigated as a hyperthermic agent for cancer treatment. In this study, thermally responsive Chitosan-coated MnFe2O4 (Chitosan-MnFe2O4) nanoparticles were developed to conduct localized magnetic hyperthermia for cancer treatment. Hydrophobic MnFe2O4 nanoparticles were synthesized via thermal decomposition and modified with 2,3-dimercaptosuccinic acid (DMSA) for further conjugation of chitosan. Chitosan-MnFe2O4 nanoparticles exhibited high magnetization and excellent biocompatibility along with low cell cytotoxicity. During magnetic hyperthermia treatment (MHT) with Chitosan-MnFe2O4 on MDA-MB 231 cancer cells, the targeted therapeutic temperature was achieved by directly controlling the strength of the external AC magnetic fields. In vitro Chitosan-MnFe2O4-assisted MHT at 42 °C led to drastic and irreversible changes in cell morphology and eventual cellular death in association with the induction of apoptosis through heat dissipation from the excited magnetic nanoparticles. Therefore, the Chitosan-MnFe2O4 nanoparticles with high biocompatibility and thermal capability can be an effective nano-mediated agent for MHT on cancer.

Study on elimination of screening-current-induced field in pancake-type non-insulated HTS coil

This paper presents the details of a recent study on the removal of the screening-current-induced field (SCIF) in a pancake-type non-insulated high-temperature superconductor coil (NI coil). To determine the SCIF in the NI coil, the magnetic flux density (Bz) was calculated using the equivalent circuit model of the coil and compared to the Bz obtained empirically. The experimental results indicate that the SCIF elimination in the NI coil was enhanced upon increasing the amplitude and frequency of the AC current being supplied to the background coil. Moreover, the SCIF in the NI coil was successfully removed by applying the appropriate external AC magnetic field intensity. This is because the magnetization direction of the SCIF changed completely from radial to spiral, a phenomenon termed the ‘vortex shaking effect.’ Overall, this study confirmed that the SCIF in a pancake-type NI coil can be effectively removed by exposing the coil to an external AC magnetic field.

Assessing Therapeutic Potential of Magnetic Mesoporous Nanoassemblies for Chemo-Resistant Tumors.

Smart drug delivery system with strategic drug distribution is the future state-of-the-art treatment for any malignancy. To investigate therapeutic potential of such nanoparticle mediated delivery system, we examined the efficacy of dual drug-loaded, pH and thermo liable lipid coated mesoporous iron oxide-based magnetic nanoassemblies (DOX:TXL-LMMNA) in mice bearing both drug sensitive (A2780S) and drug resistant (A2780-CisR) ovarian cancer tumor xenografts. In presence of an external AC magnetic field (ACMF), DOX:TXL-LMMNA particles disintegrate to release encapsulated drug due to hyperthermic temperatures (41-45 ºC). In vivo bio distribution study utilizing the optical and magnetic properties of DOX:TXL-LMMNA particles demonstrated minimum organ specific toxicity. Noninvasive bioluminescence imaging of mice bearing A2780S tumors and administered with DOX-TXL-LMMNA followed by the application of ACMF revealed 65% less luminescence signal and 80% mice showed complete tumor regression within eight days. A six months follow-up study revealed absence of relapse in 70% of the mice. Interestingly, the A2780-CisR tumors which did not respond to drug alone (DOX:TXL) showed 80% reduction in luminescence and tumor volume with DOX:TXL-LMMNA after thermo-chemotherapy within eight days. Cytotoxic effect of DOX:TXL-LMMNA particles was more pronounced in A2780-CisR cells than in their sensitive counterpart. Thus these novel stimuli sensitive nanoassemblies hold great promise for therapy resistant malignancies and future clinical applications.

On-demand, magnetic hyperthermia-triggered drug delivery: optimisation for the GI tract.

An orally-administered vehicle for targeted, on-demand drug delivery to the gastrointestinal (GI) tract is highly desirable due to the high incidence of diseases of that organ system and harsh mechanical and physical conditions any such drug delivery vehicle has to endure. To that end, we present an iron oxide nanoparticle/wax composite capsule coating that protects the capsule contents from the highly variable chemical conditions of the GI tract. It can be triggered using magnetic hyperthermia initiated from an external AC magnetic field. The coating is produced from pharmaceutically approved materials and is applied using a simple dip-coating process using a gelatin drug capsule as a template. We show that the coating is impervious to chemical conditions found within the GI tract, but is completely melted within two minutes of magnetically-induced heating under biologically-relevant conditions of temperature, pH, buffer and external field strength, allowing the delivery and dispersal of the capsule contents. The overall simplicity of action, durability and non-toxic and inexpensive nature of our drug delivery vehicle demonstrated herein are key for successful drug delivery systems for the kinds of focal therapy being sought for modern precision medicine.

Imaging of Magnetic Nanoparticles Using Second-Harmonic Signals

Magnetic nanoparticle imaging (MPI) utilizes the non-linear magnetization response $M$ for the detection of superparamagnetic iron oxide magnetic nanoparticles (MNPs). When an external ac magnetic field is applied to the MNPs, some harmonic responses such as second, third, and higher ones arise. We propose and demonstrate that the use of the second-harmonic response is the most effective technique to enhance the signal. The advantage of using the second-harmonic response is that the response can be measured even in a small ac field. A 2-D MPI system using this technique was realized and its performance was evaluated. The MPI system has no mechanical parts but instead magnetically scans an MNP phantom by changing the dc bias field in two directions. The cosine component of the signal is detected by a lock-in-amplifier and differentiated in one direction. The image of the phantom is then reconstructed using the $d(\cos\theta)/dz$ data. As a result, the position of the phantom may be identified down to the iron content of 9 $\mu \text{g}$ .

Total AC loss study of 2G HTS coils for fully HTS machine applications

The application of HTS coils for fully HTS machines has become a new research focus. In the stator of an electrical machine, HTS coils are subjected to a combination of an AC applied current and AC external magnetic field. There is a phase shift between the AC current and AC magnetic field. In order to understand and estimate the total AC loss of HTS coils for electrical machines, we designed and performed a calorimetric measurement for a 2G HTS racetrack coil. Our measurement indicates that the total AC loss is greatly influenced by the phase shift between the applied current and the external magnetic field when the magnetic field is perpendicular to the tape surface. When the applied current and the external magnetic field are in phase, the total AC loss is the highest. When there is a 90 degree phase difference, the total AC loss is the lowest. In order to explain this phenomenon, we employ H formulation and finite element method to model the 2G HTS racetrack coil. Our calculation agrees well with experimental measurements. Two parameters are defined to describe the modulation of the total AC loss in terms of phase difference. The calculation further reveals that the influence of phase difference varies with magnetic field direction. The greatest influence of phase difference is in the perpendicular direction. The study provides key information for large-scale 2G HTS applications, e.g. fully HTS machines and superconducting magnetic energy storage, where the total AC loss subjected to both applied currents and external magnetic fields is a critical parameter for the design.