Ultrasensitive Magnetic Sensors Research Articles

Ultra-Sensitive Magnetic Sensor Based on Resonator With Asymmetric Wedge Structure

Ultra-Sensitive Magnetic Sensor Based on Resonator With Asymmetric Wedge Structure

Quasi-static modulation of multiferroic properties in flexible magnetoelectric Cr2O3/muscovite heteroepitaxy

Due to the strong coupling between electrical polarization and magnetization, magnetoelectric materials show promising features for low-power spintronics and ultra-sensitive magnetic sensors. Compared to the conventional tunning of magnetoelectricity, this work presents a modulation of magnetic and electric orders in magnetoelectric material through a quasi-static mechanical strain. To acquire this, linear magnetoelectric Cr2O3 film is fabricated epitaxially on muscovite substrates. Taking the natural flexibility of muscovite, applying a strain to the heterostructure is feasible via mechanical bending. In the bending experiment, the magnetization of Cr2O3 film can be enhanced significantly, and the techniques of X-ray absorption dichroism unveil insights with support from theoretical predictions. Besides, the electric polarization and magnetoelectric coupling of Cr2O3 can also be adjusted by mechanical bending. This work offers a comprehensive understanding of the relationship between quasi-static strain and magnetic and electrical behaviors and opens a new aspect of the combination between magnetoelectric materials and flexible substrates for future development.

Magnetic shieldless ultra-low-field MRI with an optically pumped magnetometer

Among magnetic resonance imaging (MRI) techniques, ultra-low field (ULF) MRI has the potential to significantly lower the cost of implementation and maintenance, as well as the size of the scanning system. Due to the small amplitude of the signals produced by ULR-MRI, extremely sensitive magnetic sensors are required. Optically pumped magnetometers (OPMs) have been proposed for use in ULF-MRI as ultra-sensitive magnetic sensors capable of detecting very small signals. However, the cost of a ferromagnetic magnetic shield is often not affordable for many applications. By increasing the Larmor frequency, the influence of low-frequency magnetic noise can be mitigated, allowing OPM to be operated without the use of a magnetic shield chamber. This lowers the cost of the magnetic shield and further raises the signal strength, resulting in benefits such as non-prepolarization. We present a method for implementing the ULF-MRI using low-cost OPMs in this study. The Larmor frequency was adjusted to 300 kHz, and three-dimensional (3D) images of a phantom were acquired with a digital resolution of 3 × 3 × 3 mm3 using a static magnetic field of 7.05 mT without using a magnetic shield room or a prepolarization coil. Additionally, we corrected the frequency response to acquired images to consider the narrow bandwidth, and the SNR of 3D imaging was 18. The experimental results, we believe, establish a new guideline for higher-performance, lower-cost ULF-MRI that does not require expensive magnetic shielding.

Ultrasensitive Magnetic Sensors Enabled by Heterogeneous Integration of Graphene Hall Elements and Silicon Processing Circuits.

Graphene Hall elements (GHEs) have been demonstrated to be promising magnetic field sensors with excellent sensitivity, linearity, temperature stability, and compatibility with complementary-metal-oxide-semiconductor (CMOS)-integrated circuits (ICs). However, the demonstrated GHEs have still not exhibited a comprehensive advantage in performance over commercial integrated Hall sensors which were implemented in integrated Hall element and CMOS processing ICs. In this work, we develop a technology for the three-dimensional (3D) heterogeneous integration of silicon-based CMOS ICs and GHEs, and the fabricated magnetic field sensors outperform commercial high-end integrated Hall sensors. Specifically, the integrated Hall sensors are implemented in a stacked integration on Si based on a chopper programmable-gain amplifier (CPGA), a chopper-stabilized second-order sigma-delta modulator (CSDM), and graphene-based Hall elements on monochips. GHEs with high sensitivity (up to 1000 A/VT) are fabricated with a compatible process on a smoothened silicon nitride passivation layer of silicon-based CMOS ICs, and the two device layers are connected by an interlayer. The heterogeneous integrated Hall ICs exhibit current and voltage magnetic sensitivities up to 64 000 A/VT and 6.12 V/VT, respectively, which are much higher than those in all other reported nanomaterial-based Hall sensors and even in high-end commercial Hall ICs. Furthermore, the 3D heterogeneous integration technology used here can be extended as a universal technology for integrating nanomaterial-based sensors and Si CMOS ICs.

Embedding CoPt magnetic nanoparticles within a phosphate glass matrix

Glasses are materials with highly flexible compositions and high chemical and physical durability. These characteristics make them suitable materials for hosting nanoparticles for different purposes. Hybrid glasses containing magnetic nanoparticles have been highlighted due to their potential for application as ultra-sensitive magnetic sensors and magnetic devices. In this work, phosphate bulk glasses containing 0.5%, 1.0%, and 2.0% in mass of metallic CoPt alloy nanoparticles were prepared by melt-quenching technique. The CoPt nanoparticles were synthesized by reducing metal precursors in a high temperature organic solvent and, in the second step, they were covered with a silica layer in order to protect the nanoparticles for the subsequent melting. The nanoparticles were treated at different temperatures. Heat treatment at 900 °C showed the highest values of saturation magnetization and coercivity, and for that reason these nanoparticles were chosen for incorporation into glass. In the transmission electron microscopy images of the glass containing 2.0% in mass of nanoparticles, the interplanar distance of 0.21 nm was identified and indexed to the 111 plane of CoPt, confirming that the nanoparticles were successfully embedded into the matrix. The UV–Vis spectra presented Co2+ characteristic bands at 528, 578, and 626 nm, indicating that these ions are tetrahedrally coordinated in the matrix. Themagnetic measurements presented behavior close to ferromagnetic, showing that it is possible to prepare a magnetic glass containing bimetallic nanoparticles.

Low-Field MRI: How Low Can We Go? A Fresh View on an Old Debate

For about 30 years, MRI set cruising speed at 1.5 T of magnetic field, with a gentle transition towards 3 T systems. In its first 10 years of existence, there was an open debate on the question of most relevant MRI field strengths considering the gain in T1 contrast, simpler cooling strategies, lower predisposition to generating image artifacts, and naturally cost reduction of small footprint low field systems. At the time, the inherent gain in sensitivity of high field, which would translate in more signal per unit time, quickly ended this debate. The promise of rapid exams or higher image resolution within a reasonable time won over other considerations and set the standards for MR value. Yet, many reasons bring low field MRI in a situation quite different from 40 years ago. From the achieved progress regarding all aspects of MRI technology, an MR scan at 1.5 T in the mid 1980s has very little in common with the equivalent scan in 2020. That clearly indicates that field strength alone is not what drives performance. It is also unlikely that the total number of machines worldwide will grow so to follow the increasing demand considering their overall cost ($1M/T). The natural trend is to better control medical expenses worldwide, and reconsidering low-field MRI could lead to the democratization of dedicated, point-of-care devices to decongest high-field clinical scanners. In the present article, we aim to draw an extensive portrait of most recent MRI developments at low (1-199 mT) and ultra-low field (micro-Tesla range) outside of the commercial sphere, and we propose to discuss their potential relevance in future clinical applications. We will cover a broad spectrum from pre-polarized MRI using ultra-sensitive magnetic sensors up to permanent and resistive magnets in compact designs.

Phosphate glasses containing monodisperse Fe3−δO4@SiO2 stellate nanoparticles obtained by melt-quenching process

The challenge to obtain stable glasses containing monodisperse magnetic nanoparticles with controlled size and shape is an exciting and almost unexplored field of research. Such new materials can be used in magneto-photonics or ultra-sensitive magnetic sensors. In this work, for the first time, colorless and transparent bulk glasses containing monodisperse 20 nm Fe3−δO4 nanoparticles were prepared by a melt-quenching route. Magnetic nanoparticles were synthesized by thermal decomposition and covered with a stellate mesoporous silica shell in order to protect them against dissolution during the glass melting process. The incorporation of nanoparticles into glasses and the ideal parameters obtained for this system are discussed. The new nanocomposite materials were characterized in order to investigate the structure, thermal, and magnetic properties. Such an original approach is a very promising way to incorporate a wide panel of nanoparticles, including metallic, bimetallic and metal oxide nanoparticles, into a variety of glasses providing new properties to these materials.

Enhanced magnetoelectric effects in composite of piezoelectric ceramics, rare-earth iron alloys, and shape-optimized nanocrystalline alloys

An enhancement for magnetoelectric (ME) effects is studied in a three-phase ME architecture consisting of two magnetostrictive Terfenol-D (Tb(0.3)Dy(0.7)Fe(1.92)) plates, a piezoelectric PZT (Pb(Zr,Ti)O3) plate, and a pair of shape-optimized FeCuNbSiB nanocrystalline alloys. By modifying the conventional shape of the magnetic flux concentrator, the shape-optimized flux concentrator has an improved effective permeability (μ(eff)) due to the shape-induced demagnetizing effect at its end surface. The flux concentrator concentrates and amplifies the external magnetic flux into Terfenol-D plate by means of changing its internal flux concentrating manner. Consequently, more flux lines can be uniformly concentrated into Terfenol-D plates. The effective piezomagnetic coefficients (d(33m)) of Terfenol-D plate and the ME voltage coefficients (α(ME)) can be further improved under a lower magnetic bias field. The dynamic magneto-elastic properties and the effective magnetic induction of Terfenol-D are taken into account to derive the enhanced effective ME voltage coefficients (α(ME,eff)), the consistency of experimental results and theoretical analyses verifies this enhancement. The experimental results demonstrate that the maximum d(33m) in our proposed architecture achieves 22.48 nm/A under a bias of 114 Oe. The maximum α(ME) in the bias magnetic range 0-900 Oe reaches 84.73 mV/Oe under the low frequency of 1 kHz, and 2.996 V/Oe under the resonance frequency of 102.3 kHz, respectively. It exhibits a 1.43 times larger piezomagnetic coefficient and a 1.87 times higher ME voltage coefficient under a smaller magnetic bias of 82 Oe than those of a conventional Terfenol-D/PZT/Terfenol-D composite. These shape-induced magnetoelectric behaviors provide the possibility of using this ME architecture in ultra-sensitive magnetic sensors.

Design and Validation of High-Efficiency Chopper for Magnetoresistive Sensors

Low-frequency noise, i.e., 1/f noise severely limits the low-frequency performance of magnetoresistive sensors for applications as ultrasensitive magnetic sensors. The combination of a flux concentrator and chopping system has been proposed to mitigate the effect of 1/f noise and improve the field sensitivity. This paper reports a high-efficiency chopping system for magnetoresistive sensors. A simple reluctance analysis allows us to estimate the chopping efficiency and optimize the design, and finite-element simulations confirm the operation. Experimental results based on the optimized design using an anisotropic magnetoresistance sensor validate the concept. The sensitivity of the sensor with the chopper at OFF and ON positions is determined to be 0.009 and 0.067%/Oe, respectively, which yields a high sensitivity chopping efficiency, namely 72%.

Critical challenges for picoTesla magnetic-tunnel-junction sensors

The extension of small, inexpensive, low-power, low-frequency, ultra-sensitive magnetic sensors to fields between 1 nT and 1 pT, an area currently dominated by fluxgates, optically pumped magnetometers, and SQUIDS, would be a paradigm shift for the field of magnetic sensors. The necessary elements for picoTesla magnetic-tunnel-junction (MTJ) sensors have been identified by modeling the noise characteristics. The results help identify the experimental challenges involved in the integration of these necessary elements into actual sensors, illustrate the trade-offs faced if there are losses in performance upon integration. Scanning electron microscopy with polarization analysis (SEMPA) of the pinned layer provides insights into problems and possible solutions. Issues associated with real-world applications of these sensors to ultra-low field measurements are discussed.

High Tc SQUID Detector for Magnetic Metallic Particles in Products

High-T c superconducting quantum interference device (SQUID) is an ultra-sensitive magnetic sensor. After the discovery of the high-T c superconducting materials, the performance of the high-T c SQUID has been improved and stabilized. One strong candidate for application is a detection system of magnetic foreign matters in industrial products. There is a possibility that ultra-small metallic foreign matter has been accidentally mixed with industrial products such as lithium ion batteries. If this happens, the manufacturer of the product suffers a great loss recalling products. The outer dimension of metallic particles less than 100 micron cannot be detected using X-ray imaging, which is commonly used for the inspection. Therefore a highly sensitive system for small foreign matters is required. We developed detection systems based on high-T c SQUID for industrial products. We could successfully detect small iron particles of less than 50 micron on a belt conveyer. These detection levels were hard to be achieved using conventional X-ray detection or other methods.

SQUID sensor application for small metallic particle detection

High-Tc superconducting quantum interference device (SQUID) is an ultra-sensitive magnetic sensor. Since the performance of the SQUID is improved and stabilized, now it is ready for application. One strong candidate for application is a detection system of magnetic foreign matters in industrial products or beverages. There is a possibility that ultra-small metallic foreign matter has been accidentally mixed with industrial products such as lithium ion batteries. If this happens, the manufacturer of the product suffers a great loss recalling products. The outer dimension of metallic particles less than 100 μm cannot be detected by an X-ray imaging, which is commonly used for the inspection. Ionization of the material is also a big issue for beverages in the case of the X-ray imaging. Therefore a highly sensitive and safety detection system for small foreign matters is required. We developed detection systems based on high-Tc SQUID with a high-performance magnetic shield. We could successfully measure small iron particles of 100 μm on a belt conveyer and stainless steel balls of 300 μm in water. These detection levels were hard to be achieved by a conventional X-ray detection or other methods.

Low frequency picotesla field detection using hybrid MgO based tunnel sensors

Low frequency ultrasensitive magnetic sensors are required for magnetocardiography (1pT at 10Hz) applications. MgO based magnetic tunnel junctions with RA=100–150Ωμm2 and tunnel magnetic resistance=100% were patterned into rectangular pillars with side up to 50μm. Sensors were incorporated in 500nm thick Co94Zr3Nb4 flux guides. Sensor linearization was achieved through internal (Co66Cr16Pt18 pads) and external (3.5mT) longitudinal bias fields resulting in sensitivities of 720%∕mT. Noise levels of 97pT∕Hz0.5 at 10Hz, 51pT∕Hz0.5 at 30Hz, and 2pT∕Hz0.5 at 500kHz were measured in the linear region of the transfer curve.

A study on direction‐of‐arrival estimation methods using cosθ amplitude response antenna

AbstractAt present, when the direction of arrival of a low‐frequency electromagnetic wave such as one in the HF band is measured, loop antennas are used. However, in low‐frequency measurement such as is used in underwater communications, subsurface resource detection, volcanic activity observation, and natural electromagnetic wave observation, the receiving power sensitivity decreases as the frequency is decreased. The purpose of this paper is to use a SQUID (Superconducting QUantum Interference Device), known as an ultrasensitive magnetic sensor, for construction of a small magnetic antenna for estimating the direction of arrival of low‐frequency electromagnetic waves. As a first step, the antenna configuration and direction of arrival estimation method shown below are proposed. As a low‐frequency broadband antenna with an amplitude response type, we consider a SQUID array, which has a cosθ mode vector consisting of SQUID elements on a circle with a small radius, which is negligible in comparison with the wavelength of the incident wave. For an antenna with a real‐valued cosθ mode vector, we consider a direction‐of‐arrival estimation method within the framework of the estimation principle of ESPRIT and MUSIC, known as high‐resolution direction‐of‐arrival estimation methods, and derive an estimation method. The obtained estimation method is shown to be ultimately reducible to a simple estimation equation for deriving the phase difference between the −1st and 1st fundamental waves after beam subspace transformation of the correlation function without eigenfunction expansion. By computer simulation, it is shown that an estimation accuracy of 0.2 to 2° is obtained by this estimation equation for the finitely many elements and snapshots that are actually anticipated. © 2002 Wiley Periodicals, Inc. Electron Comm Jpn Pt 1, 85(10): 8–22, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecja.10009