High-sensitivity Magnetic Sensor Research Articles

Magnetic Flux Concentration Technology Based on Soft Magnets and Superconductors

High-sensitivity magnetic sensors are fundamental components in fields such as biomedicine and non-destructive testing. Flux concentration technology enhances the sensitivity of magnetic sensors by amplifying the magnetic field to be measured, making it the most effective method to improve the magnetic field resolution of magnetic sensors. Superconductors and high-permeability soft magnetic materials exhibit completely different magnetic effects. The former possesses complete diamagnetism, while the latter has extremely high magnetic permeability. Both types of materials can be used to fabricate flux concentrators. This paper compares superconducting and soft magnetic flux concentration technologies through theoretical simulations and experiments, investigating the impact of different structural parameters on the magnetic field amplification performance of superconducting and soft magnetic concentrators. This research is significant for the development of magnetic focusing technology and its applications in weak magnetic detection and other fields.

A high-sensitivity and multi-response magnetic nanofiber-aerogel sensor with directionally aligned porous structure based on triple network for interactive human–machine interfaces

Flexible multimodal sensors with a porous structure have attracted tremendous attentions due to their low density, high specific surface area, a wide detection range and satisfied deformability. However, pores disorder and maldistribution issues of these multimodal sensors are major concerns during their sensing response, which often cause poor linearity and unstable sensing characteristics. Herein, a facile and rapid approach was proposed to fabricate a multifunctional magnetic sensor with directionally aligned porous structure based on triple network by combining electrospun nanofibers, bidisperse magnetic particles and sodium alginate/chitosan foam. The electrospun nanofibers were employed as three-dimensional skeletons for the crystallization and vertical growth of ice crystals. Benefiting from the remarkable structure orientation, the homogenized nanofiber-aerogel scaffold possesses excellent flexibility, superior deformation recoverability and biocompatibility. The assembled sensors not only exhibit a high sensitivity (0.40 T−1), rapid response/recovery time and long stability under magnetic stimuli, but also displays satisfied cross-sensitivity, quick response and reliable durability (8000 cycles) in response to external mechanical functions. Importantly, the respective input stimuli could be clearly discriminated via outputting the electrical signals of opposite or different trends. Furthermore, the sensor could be employed as wearable skins to monitor various physiological signals and realize information translation by Morse mode for individuals with disabilities. Additionally, a wearable gesture recognition system with assistance of the deep learning algorithm was testified, with high average recognition accuracy of 99 %. The simple fabrication process and prominent multifunctional characteristics of the proposed sensor endow it with an extensive application prospect in the field of interactive human–machine interfaces.

A High-Sensitivity Magnetic Tactile Sensor With a Structure-Optimized Hall Sensor and a Flexible Magnetic Film

A High-Sensitivity Magnetic Tactile Sensor With a Structure-Optimized Hall Sensor and a Flexible Magnetic Film

A development of high sensitivity magnetic sensor based on giant magnetoresistance with simple implementation on smartphone

In this paper, the magnetic field-based sensor that can connect with smartphones through Bluetooth connectivity was developed. The design of this high sensitivity magnetic sensor system used Giant Magnetoresistance (GMR) sensors in the form of SOIC8 with AA002-02 series from NVE Corp. The results obtained from the measurements show that the sensor is able to work well when connected to an Android-based smartphone. Measurements were carried out by placing a magnet with a magnetic field strength of 0.4 T in the direction of the sensitivity plane of the GMR sensor. The result showed that the closer the magnet distance relative to the GMR sensor the more voltage signal output from the sensor. Since this developed method is simple but effective for detecting position of magnetic object, the further development of this method will be benefit for many applications.

Design of gradient coils based on the target field method in a high-permeability shield considering sidewall apertures

The combination of high-permeability ferrite and coils is often used to nullify ambient magnetic fields and is widely used in applications such as magnetic resonance imaging and high-sensitivity magnetic sensors. We propose the design method of transverse gradient coils based on the target field method in a high-permeability shield considering sidewall apertures (TFM-HPSCSA), which are installed in the ferrite shield to zero the residual magnetic field gradient. TFM-HPSCSA coils enable an innovative combination of magnetic field gradient linearity and structural luminescence. The coil current density distribution is set to avoid the central sidewall apertures. As a result, the contour distribution of the stream function under the optimized weight coefficients is calculated to obtain the wire alignments of the dBy/dx, dBy/dy and dBy/dz gradient coils. The deviations of the experimentally measured values from the finite element simulations of the three types coils are 0.63%, 1.17% and 0.48%, respectively, which verified the validity and the accuracy of the proposed method. In comparison to the traditional saddle gradient coil designed under the free space condition, the proposed dBy/dx coil produces a 30.5% improvement in the percentage range of regions with 5% magnetic field gradient nonlinearity. The increased linear range of the gradient magnetic field contributes to the improvement of the residual field nulling range in magnetic sensors and magnetic shielding systems in practical applications.

Eddy Current Testing Based Non-Destructive Inspection for Metal 3D Additive Manufacturing Objects With HTS-SQUID

In this work, we applied eddy-current-testing-based HTS-SQUID non-destructive inspection (ECT-based HTS-SQUID NDI) system for inspection of metal 3D additive manufacturing (AM) objects, which were made of AlSi10Mg powder with powder bed fusion (PBF) method. The inspection system combines an ECT probe with an HTS-SQUID gradiometer as high sensitive readout magnetic sensor in low frequency range to overcome disadvantage of conventional ECT. We prepared 5 samples with different densities by changing parameters in the PBF process. Densities of the samples #1-#5 were estimated to be 2.65, 2.64, 2.62, 2.58, and 2.51 g/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> , respectively. The samples were measured by the inspection system with an excitation current of 20 mA at 1.6 kHz, and lift-off of about 1 mm. In inspection results, anomalous magnetic patterns appeared on the sample #3, #4 and #5. The inspection results were verified with X-ray CT scanning. Comparison between the ECT inspection and the X-ray CT results reveals that the positions of the anomalous magnetic patterns show strong correlation with the positions of high porosity existing in the samples.

High-sensitivity optical-fiber magnetic sensor based on diamond and magnetic flux concentrators.

Magnetic field detection exploiting nitrogen-vacancy (NV) centers in diamond has gained increasing attention and development in recent years. Combining diamond NV centers to optical fibers provides a way for achieving magnetic sensors with high integration and portability. Meanwhile, new methods or techniques are urgently desired to improve the detection sensitivity of such sensors. In this paper, we present an optical-fiber magnetic sensor based on the NV ensemble in diamond, and employ the well-designed magnetic flux concentrators to enhance the sensitivity up to 12 pT/Hz1/2, an outstanding level among the diamond-integrated optical-fiber magnetic sensors. The dependence of sensitivity on the key parameters including the size and gap width of the concentrators are investigated by simulations and experiments, based on which the predictions on the further enhancement of sensitivity to fT level are presented.

Sensitivity Enhanced Magnetic Field Sensor Based on Hollow Core Fiber Fabry-Perot Interferometer and Vernier Effect

A simple to fabricate, low cost, and high Sensitivity magnetic sensor is proposed in this paper. The sensor is composed of two parallel Fabry-Perot (FP) interferometers which are fabricated by fusing single mode fiber and hollow core fiber. Two FP interferometers have similar free spectral ranges to achieve vernier effects. The experimental result shows the magnetic field sensitivity of the proposed sensor is 101pm/mT, which is about seven times higher than the single FP cavity.

High sensitivity and large measurement range magnetic field micro-nano fiber sensor based on Mach-Zehnder interference

High sensitivity and large measurement range magnetic field micro-nano fiber (MFM-NF) sensor based on Mach-Zehnder interference has been proposed and demonstrated. This sensor is composed of micro-nano optical fiber (MNF) and magnetic fluid (MF). MNF is fabricated with rapidly changed cone region and small diameter micro-nano region, the core mode will be coupled to the cladding duo to the small enough diameter of the MNF, and the core mode and the cladding mode will be form Mach-Zehnder interference. The change of magnetic field leads to the change of MF refractive index, and the refractive index near the micro-nano region strongly affects the mode interference. Under the magnetic field increased, the magnetic field sensitivity of the sensor is up to 69 pm/Gs in the range of 50 ∼ 200 Gs. The linearity was 0.9952. Under the magnetic field decreased, the linear correlation is 0.9942 in the range of 200 ∼ 0 Gs, and the magnetic field sensitivity is 29 pm/Gs. The sensor has the advantages of easy fabrication, low cost and simple structure, and it would be have a wide application prospect in the field of magnetic field sensing.

A Uniform Magnetic Field Generator Combined with a Thin-Film Magneto-Impedance Sensor Capable of Human Body Scans.

A detection system for magnetic inclusions of large bulk, such as that of a whole human body, is proposed in this paper. The system consists of both a uniform magnetic field generating apparatus capable of the insertion of a whole human body and also of a high-sensitivity magnetic sensor array installed in the strong magnetic field. The system can detect the magnetic inclusion simultaneously through its magnetization, which is advantageous for detecting low-remanence magnetic materials, such as a cluster of nanoparticles. The thin-film magneto-impedance sensor was reported to be capable of tolerating strong magnetic fields of more than 3000 Gauss (0.3 T) in the substrate’s normal direction and can retain its sensitivity even in strong fields. Through a combination of both uniformity of strength and the placement of its directionally aligned, static magnetic field in a particular measurement area and its array of single-dimensional thin-film magneto-impedance sensors, it was reported that it can estimate a magnetic sample’s 3D position by using a simple equation. The aim of the system developed in this study is to nondestructively detect a cluster of magnetic nanoparticles in a human body and also to detect the position and the concentration of the clustered magnetic particles. In this paper, a prototype system consisting of a magnetic field generator with an area of W500 mm × L400 mm × H210 mm and a uniform magnetic field of 370 Gauss (37 mT) is reported. It also reported that the thin-film magneto-impedance sensor installed in the system verified the detection of 2 mm × 1 mm small ellipsoidal magnetic chips at a distance of 27 mm from the sensor element.

Analysis of current-in-plane giant magnetoresistance using Co2FeAl0.5Si0.5 half-metallic Heusler alloy

There has been renewed interest in current-in-plane giant magnetoresistance (CIP-GMR) devices for high-sensitivity magnetic sensors. However, further improvement in magnetoresistance (MR) ratio is necessary to achieve sufficient magnetic field sensitivity. Use of a half-metallic Co-based Heusler alloy ferromagnetic (FM) layer has been demonstrated to be effective in enhancing GMR in the configuration with current perpendicular to the plane; however, only small MR ratios are obtained in the CIP configuration. To understand the origin of the disappointingly low MR in the CIP configuration when using Heusler alloy FM layers, we investigated the magnetotransport properties of CIP-GMR devices using half-metallic Co2FeAl0.5Si0.5 (CFAS) Heusler alloy and conventional CoFe alloy as the FM layers in combination with Ag or Cu as a nonmagnetic (NM) spacer layer. Regardless of the high lattice and electronic band matching at the CFAS/Ag interface, CFAS/Ag CIP spin valves (SVs) show a MR ratio of only 1.2% at room temperature, which is much smaller than those of reference CoFe/Cu and CoFe/Ag SVs (21.6% and 8.4%, respectively). Current density distribution simulations suggest that large current shunting occurs in the Ag layer due to the significant resistivity gap between CFAS and Ag, which limits the generation of highly spin-polarized current from the CFAS layer, resulting in the very small MR ratios. To enhance the MR ratio in CIP-GMR using half-metallic materials, resistivity matching between FM layers and the NM layer is required, in addition to the high electronic band match that has been considered, as a key factor to obtain a high MR ratio in CIP-GMR devices.

DC and AC tunneling magnetoelectric responses of cobalt lateral nanogranular films

We demonstrate a new structural optimization of cobalt–(barium fluoride) lateral nanogranular films, achieving independently variable direct current (DC) and alternating current (AC) tunneling effects, in the film plane and out of the plane, respectively. Lateral nanogranular films were successfully fabricated via programmable tandem sputtering. Flat-shaped magnetic Co nanogranules were periodically inserted between dielectric barium fluoride layers of different thicknesses. All the lateral nanogranular films were insulating, with electrical resistivities ρ exceeding 1013 μΩ·cm, even with a Co content as high as 30 at.%. A lateral tunneling magnetoresistance (TMR) effect and vertical tunneling magnetodielectric (TMD) effect were found in the same lateral nanogranular film with 30 at.% Co. For a nanogranular film with lateral Co nanoparticles, the sensitivity of both the TMR and TMD ratio were improved at small magnetic fields compared with conventional films with spherical Co nanoparticles, reflecting its small saturation magnetic field. This layer-ordered metal–insulator nanogranular structure has potential applications in AC/DC hybrid elements as a high-sensitivity magnetic sensor.

Ferromagnetic, ferroelectric, and magnetoelectric properties in individual nanotube-based magnetoelectric films of CoFe2O4/BaTiO3 using electrically resistive core-shell magnetostrictive nanoparticles

Since 2010, two-phase magnetoelectric (ME) films consisting of magnetostrictive and piezoelectric phases have been continuously developed with regard to their promising ME responses, for potential applications such as ultra-sensitive magnetic sensing, magnetic-field induced energy harvesting, and nano-transduction. Generally, one-dimensional piezoelectric nanowires, nanorods, and nanotubes are used to enlarge the active area for strain transfer between the piezoelectric and magnetostrictive phases of ME films. In this study, we developed 1–2 type individual nanotube-based ME films of CoFe2O4/BaTiO3 with main considerations in the optimization of magnetostrictive phase, piezoelectric phase, and interface between the phases, respectively. First, core-shell magnetostrictive nanoparticles were improved with a high resistivity of 3.89 MΩ·cm and saturation magnetization of 20.8 emu/g via a sufficient TiO2 coating on the CoFe2O4 nanoparticles. Then, individually free-standing TiOx nanotubes were prepared with an optimal structure exhibiting a large pore size of 220 nm and high interspace distance of 80 nm. Final 1–2 type ME films of CoFe2O4/BaTiO3 were successfully prepared by magnetostrictive particle electrodeposition and perovskite conversion in sequence. Enhanced dipole moments through high amounts of BaTiO3 phase were achieved with a maximal remnant polarization (Pr) of 0.192 µC/cm2 from the AC-electrodeposited (AC-) ME films exhibiting a dense film structure. Maximal ME coefficient of the AC-ME films was achieved with magnitude of 27.39 mV/cm∙Oe at Hbias = 1000 Oe. As a result, considering the investigations, the 1–2 type ME films could be expected for possible applications such as high-sensitive magnetic sensors or low-power energy harvesters.

Development of Magnetic Particle Distribution Imaging Using Magnetic Field Reconstruction for Biopsy of the Sentinel Lymph Node

The sentinel lymph node is the first lymph-node-draining cancer metastasis. The identification of the sentinel lymph node using magnetic particles and a magnetic sensor has attracted attention in recent years, as this method is less invasive than the conventional method of radiotracer injection. However, the development of a two-dimensional measurement method for sentinel lymph nodes using magnetic nanoparticles remains an issue. In the present study, a method and apparatus for the two-dimensional imaging of magnetic particle distribution were developed to detect a lymph node with magnetic particles concentrated within lymphoid tissues. The method comprises the reconstruction of the magnetic field measured with a high-sensitivity magnetic sensor and with a magnetic detection ability of 2 nT/√Hz at 100 Hz (5 nT/√Hz at 1 Hz). The proposed system measures the two-dimensional magnetic field distribution in an area of up to 25 × 25 mm2 using a coil generating a 0.77 mT external magnetic field applied to the measurement target. The improved spatial resolution of the images makes it possible to use two-dimensional imaging for diagnostics of breast cancer metastases.

Poly acrylic acid stabilized magnetic nanoemulsions for visual defect detection: Effect of pH on detection sensitivity and colloidal stability

Magnetic field assisted tunable colour contrast in magnetic nanoemulsions is exploited for visual detection of defects in ferromagnetic components. We demonstrate an enhanced defect detection sensitivity by stabilizing oil-in-water magnetic nanoemulsion with poly acrylic acid (PAA) at varying pH. In this detection technique, the variation of magnetic permeability, in the vicinity of the defects, causes magnetic flux leakage, which results in a distinct colour contrast on the magnetic nanoemulsion sensor due to the formation of anisotropic periodic structures, parallel to the external field direction, where the inter-droplet spacing within the linear chains depends on the effective magnetic flux. By exploiting this colour and magnetic flux dependence, severity of the defects is assessed by observing the colour contrast. Our studies indicate that defect detection sensitivity decreases with increasing pH due to globule-to-coil conformational transition of the interfacial PAA molecules. With increasing pH, the PAA network swells due to mobile counter-ions induced osmotic pressure and electrostatic repulsion between the charged monomer units, leading to a solvated coil-like configuration, which is confirmed from the increase in hydrodynamic diameter and zeta potential. The inter-droplet force measurements show an increase in decay length by ~3.5 times, when pH is increased from ~1.5 to ~4.0. The lower decay length at pH ~ 1.5 indicates the formation of linear chain-like structures with smaller inter-droplet spacing near the defect edges, where the magnetic flux leakage is maximum. This results in a fewer number of randomly oriented droplets at the defect centre, thereby enhancing the defect detection sensitivity at lower pH. However, nanoemulsions stabilized at pH > 3 show increased colloidal stability, as compared to pH ~ 1.5. Our studies indicate that nanoemulsions at pH ~ 3.5 exhibit an optimum colloidal stability with good defect detection sensitivity. The obtained results are beneficial for developing high sensitivity, reliable and reusable magnetic nanoemulsion sensors for naked eye visualization of defects in ferromagnetic components.

Enhanced magnetoelectric response of cofired ceramic layered composites by adjusting the grain boundary conductivity of the magnetostrictive component

Magnetoelectric composites provide large enough room-temperature functional responses to enable a range of novel technologies, like uncooled high-sensitivity magnetic sensors. Magnetoelectricity in these composites appears as a product property of the piezoresponses of two elastically coupled ferroic phases. Coefficients then depend not only on the magnetostriction and piezoelectricity of the components, but also on interface quality. Different composite approaches are under development, among which cofired ceramic layered structures of ferrimagnetic spinel and ferroelectric perovskite oxides stand out because direct bonding between phases is attained, which facilitates miniaturization. An aspect rarely investigated is the role of the electrical characteristics of the magnetostrictive material beyond their effect on ferroelectric poling. We report here a three-fold enhancement of magnetoelectric coefficients by the only adjustment of the grain boundary conductivity of the spinel oxide. Significant differences in poling level are ruled out, and the effect is related with changes in the composite effective permittivity.

High sensitive magnetic sensor using symmetric response GMR

High sensitive magnetic sensor using symmetric response GMR

Enhancement of magnetic detection by ensemble NV color center based on magnetic flux concentration effect

The high-sensitivity magnetic sensor is the key to the weak magnetic and extremely weak magnetic detection imaging. In this paper, based on ensemble nitrogen-vacancy (NV) color center in diamond, a wide-field magnetic field distribution imaging system combined with the magnetic flux concentrator (MFC) is built for enhancing the magnetic detection. The paired T-shape chip MFC structures are designed and prepared based on the simulation of magnetic flux concentration effect, and the enhancement of magnetic field of MFC is verified by continuous wave optical detection magnetic resonance (CW-ODMR) imaging technology. When the gap width between the MFCs is 1.0 mm, the magnetic enhancement factor is about 10.35. To verify the effectiveness of the magnetic enhancement effect of the MFC, The magnetic enhancement effects are also measured under different magnetic field strengths and different gap widths. The magnetic sensitivity of the system increases from 1.10 nT/Hz&lt;sup&gt;1/2&lt;/sup&gt; to 0.30 nT/Hz&lt;sup&gt;1/2&lt;/sup&gt;. By comparing the simulations with the measurements, the relationship between the measured magnetic enhancement multiple and the gap width can be obtained, and the better magnetic enhancement capability and sensitivity of the experimental system are also estimated. When the MFC’s gap width is 0.5 mm, the corresponding magnetic enhancement factor is increased to 18.21, and the corresponding magnetic sensitivity is 0.25 nT/Hz&lt;sup&gt;1/2&lt;/sup&gt;. These results show that the magnetic detection sensitivity of the ensemble NV in diamond can be effectively improved based on magnetic flux concentration effect, which provides a reference for the applications of precision quantum measurement technology in weak magnetic and extremely weak magnetic detection.

Developing Gradiometer to Reduce the Low Frequency Noise of Magnetic Sensor with Amorphous Wire

Pico-Tesla level high sensitivity magnetic sensor has been developed using FeCoSiB amorphous wire as the core of a coil, but the low frequency noise of the magnetic sensor is somewhat high, and some applications of magnetic sensors, such as magnetic microscope or deep defect detection, require that the sensors have low noise at low frequency. To reduce the noise, it is necessary to know the noise sources of the magnetic sensor. By measuring the noise spectrum of each part of the magnetic sensor with FeCoSiB amorphous wire was measured, it was found that the white noise of the magnetic sensor was mainly determined by the noise of the preamplifier and the low frequency noise was mainly determined by the low frequency noises of the DC bias current and AC bias current of the FeCoSiB magnetic sensor. To reduce the low frequency noise of the magnetic sensor caused by the bias current noise, an electrical gradiometer composed of two magnetic sensors sharing the same DC bias current and AC bias current was developed. The results proved that the gradiometer was not only effective to reduce the low frequency noise caused the bias current, but also effective to reduce the environmental noise caused by the 50 Hz line interference. Using the gradiometer, an eddy current testing (ECT) system were constructed, and the deep defect with the depth of about 8 mm in an aluminum plate was successfully detected by the ECT system.

The influence of magnetic head’s pose on magnetic flux leakage detection

As a high-sensitivity magnetic sensor, the magnetic head has been gradually applied to non-destructive testing. Compared with the single coil and the coil with an iron core, the magnetic core can converge the weak leakage magnetic field because it can form a magnetic circuit to transmit the leakage field to the coil wound thereon. So that it has a higher sensitivity. However, in practical applications, it’s found that the pose error of the head affects its sensitivity. Therefore, this paper analyzes the influence of the tilt of the head on the magnetic core’s ability to sense leakage magnetic field, and then the influence is verified by simulations and experiments. The study finds that, when tilted around two horizontal axes, the larger the tilt angle is, the larger the signal is. When tilted around the vertical axis, the larger the tilt angle is, the smaller the signal is.