Bias Magnetic Field Research Articles

Coil-Only High-Frequency Lamb Wave Generation in Nickel Sheets

This study presents a novel, coil-only magnetostrictive ultrasonic detection method that operates effectively without permanent magnets, introducing a simpler alternative to conventional designs. The system configuration is streamlined, consisting of a single meander coil, an excitation source, and a nickel sheet, with both the bias magnetic field and ultrasonic excitation achieved by a composite excitation containing both DC and AC components. This design offers significant advantages, enabling high-frequency Lamb wave generation in nickel sheets for ultrasonic detection while reducing device complexity. Experimental validation demonstrated that an S0-mode Lamb wave at a frequency of 2.625 MHz could be effectively excited in a 0.2 mm nickel sheet using a double-layer meander coil. The experimentally measured wave velocity was 4.9946 m/s, with a deviation of only 0.4985% from the theoretical value, confirming the accuracy of the method. Additionally, this work provides a theoretical basis for future development of flexible MEMS-based magnetostrictive ultrasonic transducers, expanding the potential for miniaturized magnetostrictive patch transducers.

Cumulative optimization of magnetoelectric composite-based wireless energy transfer

Abstract Magnetoelectric composite-based wireless energy transfer (WET) comprising omnidirectional ring-shaped transmitter and laminated plate receiver elements have been studied experimentally and theoretically. The objective is to reveal the conditions conducive to achieving optimal power based on the device geometries, the relative orientations, and the operating conditions, including the vibrational frequency, applied electric field, bias magnetic field, and the corresponding resistive load. The reformulated physics-based model establishes the dependence of the output power on the load resistance, elucidating a strong interrelationship between the transferred power, vibrational displacement, and strain transduction coefficient based on relaxing all previous simplifying assumptions. The model ascertained the non-monotonic variation of the output power as a function of the interface coupling factor, emphasizing the crucial influence of gradual change in material properties within the strain-mediated magnetoelectric transmitter and receiver devices, i.e., an agile design framework for multiferroics-based power transfer devices.

Combining Magnetostriction with Variable Reluctance for Energy Harvesting at Low Frequency Vibrations

In this paper, we explore the benefits of using a magnetostrictive component in a variable reluctance energy harvester. The intrinsic magnetic field bias and the possibility to utilize magnetic force to achieve pre-stress leads to a synergetic combination between this type of energy harvester and magnetostriction. The proposed energy harvester system, to evaluate the concept, consists of a magnetostrictive cantilever beam with a cubic magnet as proof mass. Galfenol, Fe81.6Ga18.4, is used to implement magnetostriction. Variable reluctance is achieved by fixing the beam parallel to an iron core, with some margin to create an air gap between the tip magnet and core. The mechanical forces of the beam and the magnetic forces lead to a displaced equilibrium position of the beam and thus a pre-stress. Two configurations of the energy harvester were evaluated and compared. The initial configuration uses a simple beam of aluminum substrate and a layer of galfenol with an additional magnet fixing the beam to the core. The modified design reduces the magnetic field bias in the galfenol by replacing approximately half of the length of galfenol with aluminum and adds a layer of soft magnetic material above the galfenol to further reduce the magnetic field bias. The initial system was found to magnetically saturate the galfenol at equilibrium. This provided the opportunity to compare two equivalent systems, with and without a significant magnetostrictive effect on the output voltage. The resonance frequency tuning capability, from modifying the initial distance of the air gap, is shown to be maintained for the modified configuration (140 Hz/mm), while achieving RMS open-circuit coil voltages larger by a factor of two (2.4 V compared to 1.1 V). For a theoretically optimal load, the RMS power was simulated to be 5.1 mW. Given the size of the energy harvester (18.5 cm3) and the excitation acceleration (0.5 g), this results in a performance metric of 1.1 mW/cm3g2.

Study on dynamic and static magnetic properties of polycrystalline cobalt ferrite under bias magnetic field

Owing to their low resistivity, metal-based magnetostrictive materials are prone to eddy current loss under high-frequency magnetic field excitation, leading to a significant decline in output characteristics. Cobalt ferrite, a new non-metal-based magnetostrictive material, possesses both high magnetostriction strain and high resistivity, making it highly promising for high-frequency driving applications. However, reports on cobalt ferrite as the core driving material for ultrasonic transducers remain limited, and research on its high-frequency vibration characteristics is insufficient. In this study, magnetostrictive samples of cobalt ferrite were prepared using a solid-state reaction method, and their morphologies and magnetic properties were characterized. Under a bias magnetic field, the dynamic magnetic properties of cobalt ferrite and temperature variations with the driving magnetic field amplitude and prestress were analyzed. The results indicated that the density of the sample prepared by solid-state reaction can reach 88 % and the saturation magnetostriction coefficient λs can reach 186 × 10−6, with an optimal bias range of 400–600 Oe for the cobalt ferrite. When the driving magnetic field amplitude is 100 Oe in the optimal bias range, the output amplitude of the cobalt ferrite is 2160 nm, meeting the output requirement of high-frequency magnetostrictive ultrasonic transducers.

A tunable self-bias effect in rubbery magnetoelectric materials

Magnetoelectric (ME) composites have recently received extensive attention due to their much higher ME coefficients and relatively high operating temperatures compared to single-phase ME materials. However, the ME coefficients of ME composites depend on the external magnetic field, and high ME coefficients usually require the presence of a biased external DC magnetic field. In this work, we propose a hybrid magnetoactive elastomer, which is a rubber matrix embedded with both soft iron particles and hard NdFeB particles. It is found that such a hybrid MAE shows a nonzero piezomagnetic coefficient even as the applied magnetic field approaches zero. Based on this phenomenon, we further propose a soft ME material with a self-bias effect and experimentally demonstrate that the self-bias effect can be tailored by changing the residual magnetization of the hybrid MAE and the charge density of the electret layer. This work successfully demonstrates a new mechanism of the self-bias effect for magnetoelectric materials and introduces a new member to the family of ME materials.

Development of a flexible phased array electromagnetic acoustic testing system with array pickups

The thermal barrier coatings (TBCs) used to protect blades of heavy-duty gas turbines is prone to debonding defects during fabrication and service. It is difficult to detect the debonding defects in the TBC system non-destructively and in situ due to the curved shape of blade and the narrow space between the blades setting in the gas turbine. In this paper, a flexible phased array electromagnetic acoustic (FPA-EMA) testing system with array pickups is developed, which is capable to drive a flexible phased array electromagnetic acoustic transducer (FPA-EMAT) to generate and receive ultrasonic wave directly in the metallic substrate of TBCs, and is potential to be applied to the inspection of debonding defects in the TBC system of gas turbine blades. A phased array excitation unit, a multi-channel signal receiving unit and a new signal post-processing algorithm are developed for the new EMA testing system to enhance the surface acoustic wave in the layered structure and to improve the low conversion efficiency of the conventional EMA testing system. In addition, a bias magnetic field coil fed with long pulse current of large amplitude is adopted to make the new EMAT thin and flexible in order to be applied in a narrow space. The interfering noise and geometric size problems brought by the multiple excitation and pickup channels are well addressed in the new system. The good performances of the developed FPA-EMA testing system were verified experimentally and show the system of good potential to be applied to NDT of practical curved structures.

Phase-error-free atomic magnetometer and vector measurement method based on demodulated signal phase in geomagnetic environment

The performance of double-beam atomic magnetometer in the geomagnetic environment is greatly affected by the vectorial direction of magnetic field, which leads to the shift of the phase of the demodulation signal. We propose a phase-error-free scalar double-beam atomic magnetometer. When the RF field is along the direction of probe beam, the fluctuation of the phase of the demodulation signal can be avoided. For the practical measurement environment, we propose a phase extraction method for atomic magnetometer. Additionally, Based on the phase of the demodulated signal when the RF field is applied in the directions of pumping, probing, and perpendicular to the pumping-detection plane, we propose a method for vector magnetic field measurement. The measurement results indicate that under a 10 μT bias magnetic field, the deviations θ and φ are within 0.1°. The theory and method proposed are of great significance to the application of magnetometer in geomagnetic magnetic field.

Simultaneous vector magnetometry based on fluorescence polarization of NV centers ensemble in diamond

The nitrogen-vacancy (NV) centers ensemble has extensive application prospects in vector-magnetic-field measurement due to its accurate and fixed spatial orientations along the crystallographic axes of diamonds. However, to address signals of NV centers along all four axes, a large bias magnetic field sufficient to spectrally separate their resonances is typically inevitable, which may affect the magnetic substance under test and require multiple-frequency microwaves to interrogate signals of the four axes. Here, we demonstrate an NV-based simultaneous vector magnetometer that works at a bias field as low as just separating the resonant peaks of |ms=±1 states and utilizes a single-frequency microwave. By simultaneously detecting the fluorescence at specific optical polarization angles in three orthogonal directions and determining the transformation matrix in advance, all the Cartesian components of the magnetic field under test are distinguished. The experimentally achieved magnetic-field sensitivity is 63 nT/Hz, and the bias field is reduced to around 11 Gauss (still reducible by narrowing the linewidth) in ambient conditions. The proposed methods dramatically reduce the bias field for NV-based simultaneous vector magnetometers and potentially expand their applications in biological science, materials science, and industrial noninvasive detection.

Picotesla-sensitivity microcavity optomechanical magnetometry.

Cavity optomechanical systems have enabled precision sensing of magnetic fields, by leveraging the optical resonance-enhanced readout and mechanical resonance-enhanced response. Previous studies have successfully achieved mass-produced and reproducible microcavity optomechanical magnetometry (MCOM) by incorporating Terfenol-D thin films into high-quality (Q) factor whispering gallery mode (WGM) microcavities. However, the sensitivity was limited to 585 pT Hz-1/2, over 20 times inferior to those using Terfenol-D particles. In this work, we propose and demonstrate a high-sensitivity and mass-produced MCOM approach by sputtering a FeGaB thin film onto a high-Q SiO2 WGM microdisk. Theoretical studies are conducted to explore the magnetic actuation constant and noise-limited sensitivity by varying the parameters of the FeGaB film and SiO2 microdisk. Multiple magnetometers with different radii are fabricated and characterized. By utilizing a microdisk with a radius of 355 μm and a thickness of 1 μm, along with a FeGaB film with a radius of 330 μm and a thickness of 1.3 μm, we have achieved a remarkable peak sensitivity of 1.68 pT Hz-1/2 at 9.52 MHz. This represents a significant improvement of over two orders of magnitude compared with previous studies employing sputtered Terfenol-D film. Notably, the magnetometer operates without a bias magnetic field, thanks to the remarkable soft magnetic properties of the FeGaB film. Furthermore, as a proof of concept, we have demonstrated the real-time measurement of a pulsed magnetic field simulating the corona current in a high-voltage transmission line using our developed magnetometer. These high-sensitivity magnetometers hold great potential for various applications, such as magnetic induction tomography and corona current monitoring.

On the Piezomagnetism of Magnetoactive Elastomeric Cylinders in Uniform Magnetic Fields: Height Modulation in the Vicinity of an Operating Point by Time-Harmonic Fields.

Soft magnetoactive elastomers (MAEs) are currently considered to be promising materials for actuators in soft robotics. Magnetically controlled actuators often operate in the vicinity of a bias point. Their dynamic properties can be characterized by the piezomagnetic strain coefficient, which is a ratio of the time-harmonic strain amplitude to the corresponding magnetic field strength. Herein, the dynamic strain response of a family of MAE cylinders to the time-harmonic (frequency of 0.1-2.5 Hz) magnetic fields of varying amplitude (12.5 kA/m-62.5 kA/m), superimposed on different bias magnetic fields (25-127 kA/m), is systematically investigated for the first time. Strain measurements are based on optical imaging with sub-pixel resolution. It is found that the dynamic strain response of MAEs is considerably different from that in conventional magnetostrictive polymer composites (MPCs), and it cannot be described by the effective piezomagnetic constant from the quasi-static measurements. The obtained maximum values of the piezomagnetic strain coefficient (∼102 nm/A) are one to two orders of magnitude higher than in conventional MPCs, but there is a significant phase lag (35-60°) in the magnetostrictive response with respect to an alternating magnetic field. The experimental dependencies of the characteristics of the alternating strain on the amplitude of the alternating field, bias field, oscillation frequency, and aspect ratio of cylinders are given for several representative examples. It is hypothesized that the main cause of observed peculiarities is the non-linear viscoelasticity of these composite materials.

Current induced hidden states in Josephson junctions

Josephson junctions enable dissipation-less electrical current through metals and insulators below a critical current. Despite being central to quantum technology based on superconducting quantum bits and fundamental research into self-conjugate quasiparticles, the spatial distribution of super current flow at the junction and its predicted evolution with current bias and external magnetic field remain experimentally elusive. Revealing the hidden current flow, featureless in electrical resistance, helps understanding unconventional phenomena such as the nonreciprocal critical current, i.e., Josephson diode effect. Here we introduce a platform to visualize super current flow at the nanoscale. Utilizing a scanning magnetometer based on nitrogen vacancy centers in diamond, we uncover competing ground states electrically switchable within the zero-resistance regime. The competition results from the superconducting phase re-configuration induced by the Josephson current and kinetic inductance of thin-film superconductors. We further identify a new mechanism for the Josephson diode effect involving the Josephson current-induced phase. The nanoscale super current flow emerges as a new experimental observable for elucidating unconventional superconductivity, and optimizing quantum computation and energy-efficient devices.

Whole-Head Noninvasive Brain Signal Measurement System with High Temporal and Spatial Resolution Using Static Magnetic Field Bias to the Brain.

Noninvasive brain signal measurement techniques are crucial for understanding human brain function and brain-machine interface applications. Conventionally, noninvasive brain signal measurement techniques, such as electroencephalography, magnetoencephalography, functional magnetic resonance imaging, and near-infrared spectroscopy, have been developed. However, currently, there is no practical noninvasive technique to measure brain function with high temporal and spatial resolution using one instrument. We developed a novel noninvasive brain signal measurement technique with high temporal and spatial resolution by biasing a static magnetic field emitted from a coil on the head to the brain. In this study, we applied this technique to develop a groundbreaking system for noninvasive whole-head brain function measurement with high spatiotemporal resolution across the entire head. We validated this system by measuring movement-related brain signals evoked by a right index finger extension movement and demonstrated that the proposed system can measure the dynamic activity of brain regions involved in finger movement with high spatiotemporal accuracy over the whole brain.

Bipolar topological spin diode and topological spin transistor in finite size quantum spin Hall insulators

Abstract The miniaturization and stability of electronic devices are becoming increasingly important today. We attempt to provide the theoretical support for designing spintronic devices by numerically investigating spin transport in finite size quantum spin Hall insulators (QSHI) under a perpendicular weak magnetic field. By modifying magnetic field strength, we find the gapped spin up and spin down bands are split to realize a half-metal phase which is a promising candidate for designing an efficient spin filter. Moreover, one of the two energy gaps becomes larger and the other smaller due to the weakened or enhanced coupling between two edge states. Here we propose and demonstrate the spin filter based on a finite size QSHI junction under a magnetic field and the polarity can be inverted by a bias voltage or magnetic field. Interestingly, we find a bipolar spin diode effect that only one spin channel is opened and the other spin channel is closed at positive bias, and the opposite spin electron can be transmitted with negative bias. Two spin filters in series can be a spin transistor, the on and off states can be controlled by spin polarization of one spin filter. We show that the topological spin transistor can be controlled by the gate voltage, and it survives in moderate disorder.

Vector magnetometry in zero bias magnetic field using nitrogen-vacancy ensembles

Abstract The application of the vector magnetometry based on Nitrogen-Vacancy (NV) ensembles has been widely investigated in multiple areas. It has the superiority of high sensitivity and high stability in ambient conditions with microscale spatial resolution. However, a bias magnetic field is necessary to fully separate the resonance lines of optically detected magnetic resonance (ODMR) spectrum of NV ensembles. This brings disturbances in samples being detected and limits the range of application. Here, we demonstrate a method of vector magnetometry in zero bias magnetic field using NV ensembles. By utilizing the anisotropy property of fluorescence excited from NV centers, we analyzed the ODMR spectrum of NV ensembles under various polarized angles of excitation laser in zero bias magnetic field with a quantitative numerical model and reconstructed the magnetic field vector. The minimum magnetic field modulus that can be resolved accurately is down to ~0.64 G theoretically depending on the ODMR spectral line width (1.8MHz), and ~2 G experimentally due to noises in fluorescence signals and errors in calibration. By using 13C purified and low Nitrogen concentration diamond combined with improving calibration of unknown parameters, the ODMR spectral line width can be further decreased below 0.5 MHz corresponding to ~0.18 G minimum resolvable magnetic field modulus.

A one-step fabrication method of flexible magnetostrictive fiber ribbon based on Fe50Co50 alloy and its characteristics study

Magnetostrictive materials are widely used in sensors, actuators and micro-nano electronics because of their excellent performance. However, traditional rigid magnetostrictive materials are difficult to adapt to the flexible and deformable structure of flexible electronic devices. In order to solve the deformation, sensing and control problems in the field of flexible electronics, a one-step new method of is proposed to deposit Fe50Co50 fiber ribbons with reciprocated and loop structures on flexible substrate. Helmholtz coil is designed to test the magnetostrictive positive effect ability of Fe50Co50 ribbon, and its magnetostrictive strain curve is analyzed. Furthermore, frequency domain characteristics of Fe50Co50 ribbon are tested, and the influence of externally driving magnetic field on elastic layer materials and resonance output characteristics is analyzed. The experimental results show that Fe50Co50 ribbon exhibits good low-frequency magnetic field characteristics. And free end displacement of Fe50Co50 ribbon based on loop structure, under 12 mT alternating magnetic field and 134 mT bias magnetic field, reaches a peak-to-peak value of 870 μm for first-order resonance, and 320 μm for second-order resonance. This verifies that Fe50Co50 ribbon possesses the expected properties that magnetostrictive materials should have, and innovatively breaks through the technical bottleneck of manufacturing flexible micro-nano magnetostrictive energy exchange components.

The effects of bias voltage and magnetic field on optical properties of boron phosphide monolayer

The effects of bias voltage and magnetic field on optical properties of boron phosphide monolayer

Development of Reconfigurable High-Frequency Devices Using Liquid Crystal in Substrate-Integrated Gap Waveguide Technology

This article presents the theoretical study, numerical simulation and fabrication of a phase shifter and a stub resonator for use in microstrip ridge gap waveguide (MRGW) technology, using a liquid crystal (LC) in the substrate as a reconfigurable material. The phase shifter and the stub resonator are filled with LC, and thanks to the LC’s dielectric anisotropy properties, the phase shift and the resonance response can be easily controlled using an external electric or magnetic bias field. The phase shifter was designed to operate in the range of 10 to 20 GHz, and the resonator was designed to operate in the range of 7.8 to 8.8 GHz. The phase shifter’s responses (including both phase shift and insertion losses), associated with both the parallel and perpendicular permittivity values of the LC, were computed and measured, and then the corresponding figure of merit (FoM) was extracted. The resonator’s frequency responses, associated with both the LC’s parallel and perpendicular permittivity, were computed. The resonator’s frequency responses, which provided different polarization voltages, were measured and compared to the simulation results. All technological issues related to both prototypes are also discussed here. The good agreement between the simulation and measurement results confirm this technology as a viable approach to the practical implementation of these microwave reconfigurable devices.

Microwave magnetic excitations in U-type hexaferrite Sr4CoZnFe36O60 ceramics

Microwave (MW) transmission, absorption, and reflection loss spectra of the ferrimagnetic U-type hexaferrite Sr4CoZnFe36O60 ceramics were studied from 100 MHz to 35 GHz at temperatures between 10 and 390 K. Nine MW magnetic excitations with anomalous behavior near ferrimagnetic phase transitions were revealed. They also change under the application of the weak bias magnetic field (0–700 Oe) at room temperature. Six pure magnetic modes are assigned to dynamics of the magnetic domain walls and inhomogeneous magnetic structure of the ceramics, to the natural ferromagnetic resonance (FMR), and to the higher-frequency magnons. Three modes are considered the magnetodielectric ones with the dominating influence of the magnetic properties on their temperature and field dependences. The presence of the natural FMR in all ferrimagnetic phases proves the existence of the non-zero internal magnetization and magnetocrystalline anisotropy. Splitting of the FMR into two components without magnetic bias was observed in the collinear phase and is attributed to a change in the magnetocrystalline anisotropy during phase transition. The high-frequency FMR component critically slows down to phase transition. At room temperature, FMR splitting and essential suppression of the higher-frequency modes were revealed under the weak bias field (300–700 Oe). The highly nonlinear MW response and FMR splitting are caused by the gradual evolution of the polydomain magnetic structure to a monodomain one. The high number of magnetic excitations observed in the MW region confirms the suitability of using hexaferrite Sr4CoZnFe36O60 ceramics as MW absorbers, shielding materials and highly tunable filters.

Enhanced sensitivity of magnetic field sensing using L-shaped dual Terfenol-D-FBGs based on the Vernier effect and a dual-loop optoelectronic oscillator

We proposed and successfully validated improved the sensitivity of magnetic field sensing using the Vernier effect in a dual-loop optoelectronic oscillator (OEO) incorporating cascaded L-shaped Terfenol-D-FBGs. Thanks to the time delay introduced by the dispersion compensating fiber (DCF) in the OEO cavity, precise conversion between the sensing FBG wavelength changes and the OEO oscillation frequency can be achieved. The central wavelengths of the sensing Terfenol-D-FBG change along with the magnetic field. Therefore, the magnetic field can be monitored by measuring the oscillating frequency of the OEO. In the proposed scheme, two reflection signals from the cascaded FBGs pass through two fiber paths with slightly different lengths, inducing the Vernier effect in the OEO, which significantly enhances the sensitivity of the sensor. Additionally, a bias magnetic field is applied to the sensing Terfenol-D alloy to further improve the sensitivity. Experimental results demonstrate that by applying a bias magnetic field on both sides of the sensing Terfenol-D, the sensitivity of the single-loop OEO is increased from 421 Hz/mT to 560 Hz/mT. Utilizing the Vernier effect, the magnetic field sensitivity of the dual-loop OEO reaches up to −16.54 kHz/mT, with a 29-times improvement over the single-loop OEO's 560 Hz/mT. By finely adjusting the path length difference, we further enhance the performance of the sensor, achieving sensitivities of −24.58 kHz/mT, −28.49 kHz/mT and −35.94 kHz/mT, respectively, which are 44 times, 51 times and 64 times higher than the single-loop OEO structure. Besides, to overcome the influence of temperature, a temperature compensation sensor is designed using a pair of L-shaped Terfenol-D alloys to host the cascaded sensing FBG and reference FBG. The experimental results show that the proposed L-shaped Terfenol-D with cascaded FBG can reduce the magnetic field error caused by temperature crosstalk to 0.03 mT.

Contributions of the extended ELISE and BATMAN Upgrade test facilities to the roadmap towards ITER NBI

ITER’s NBI systems are a first of its kind system with very challenging targets for the RF-driven ion source and the acceleration stage. In a step ladder approach, the ion source test facilities BATMAN Upgrade (BUG) and ELISE support the activities carried out at the Neutral Beam Test Facility, Padua, which is equipped with the ion source facility SPIDER and with MITICA being equivalent to the ITER Heating Neutral Beam injector (HNB), capable of operating at the full power and pulse length of the ITER HNBs. The contributions of the prototype ion source at BUG (1/8 scale) and the size scaling experiment ELISE (1/2 size ITER source) to the roadmap are manifold: for hydrogen operation the ion source performance is demonstrated in several sequential 1000 s pulses, whereas long pulse deuterium operation is limited by the heat load of the co-extracted electrons on the extraction grid. Measures like special magnetic filter field configurations or biasing of surfaces and improved Cs management are identified. Both facilities have recently been extended to full steady state compatibility and very first insights of the ion source performance with steady state extraction compared to the previously used beam blips (10 s extraction every 150 s) are already gained. A pulse length of 400 s, as required for the first deuterium campaigns at ITER, seems to be feasible soon, whereas the one hour pulse imposes the highest challenge to overcome. Investigations on beam divergence revealed a divergence at the upper limit of the acceptable value for the HNB. Measurements on the beam uniformity on the scale of beamlet groups and grid segments at ELISE demonstrated a uniformity of better than the required 90%. BUG and ELISE gave input to recent implementations at SPIDER; MITICA and ITER’s NBI. Still open points and challenges are addressed, for which a continuation of the step ladder approach is essential.