Anisotropic Magnetoresistive Sensors Research Articles

On the influence of magnetic flux guide location to the out-of-plane magnetic field sensing of chip-scale AMR sensor

The anisotropic magneto-resistive (AMR) sensors could detect external magnetic fields through the change of magnetization and resistance. Typically made of thin films, the AMR sensors are generally only sensitive to the in-plane magnetic field. The AMR sensors with Ni pillar array structures as magnetic flux guides have been presented to enhance the detection of out-of-plane magnetic fields. However, the magnetic flux lines transmitted from adjacent pillars may interact with each other, so that their directions will be changed accordingly. Thus, the locations of pillar guides may influence the distribution of magnetic flux lines, and further change the out-of-plane magnetic field sensing performance of the AMR sensor. To gain insight on this design consideration, this study introduces a serpentine routing AMR film with flux guides distributed along each wire gaps. By varying the gap of the serpentine AMR wire and the distance between the pillar and the AMR wire, the influence of flux guide location on the out-of-plane magnetic field sensing is investigated. Through microfabrication processes, the flux guide array can be batch fabricated and defined at appropriate locations, and further monolithically integrated with AMR sensor. Measurements show that for a serpentine AMR sensor with a gap of 70μm, the sensitivity can be increased by 4.6-fold as the distance between flux guide and AMR film decreases from 20 μm to 0 μm. Moreover, when the flux guide fixed in the middle of gap of the serpentine AMR sensor, the sensitivity can be improved by 5.6-fold as the gap decreases from 70 μm to 50 μm.

A high-sensitive anisotropic magnetoresistive sensor based on hybrid Ta/NiFe/Ta/Al multilayer structure

High sensitivity is crucial for anisotropic magnetoresistive (AMR) sensors in industrial applications. In this paper, a high- sensitive AMR sensor based on magnetoresistive thin films with Ta/NiFe/Ta/Al four-layer structure is proposed and fabricated. Firstly, the structural parameters were optimized by finite element analysis. Secondly, thin film samples and AMR sensors were prepared. Through the analysis and characterization of reluctance change rate, hysteresis loop, x-ray diffraction and surface morphology and structure, the process parameters were optimized. Finally, the sensor was connected to the designed external circuit, and its technical parameters were tested in a magnetic field test system. The results show that the prepared AMR sensor performs well. It has a high sensitivity of 1.27 mV/V/Oe, a low bridge offset voltage of ±1.64 mV V−1, and a low temperature coefficient of sensitivity of −0.102%/°C. The results contribute to the future development of AMR magnetic field sensor chips.

Anisotropic magnetoresistance probed by non-resonant microwave absorption in exchange-biased NiFe/IrMn structures

Sharp change of non-resonant microwave absorption was found in exchange-biased NiFe/IrMn and NiFe/Cu/IrMn structures possessing anisotropic magnetoresistance (AMR) and planar Hall effect (PHE). Angular dependence of the microwave detected AMR and contribution of the domain expansion to AMR microwave signal were analyzed. Decrease in the microwave absorption signal caused by Cu spacer between ferromagnetic NiFe and antiferromagnetic IrMn layers was observed, when NiFe and IrMn layers become exchange decoupled. Additional characterization of the electric transport was proved by asymmetry of resonant line. Sensitivity of the microwave AMR method is enough high to detect magnetic fields of microTesla level, that provides remote detection with a sensitivity corresponding to standard galvanic sensors. Developed technique is promising for wireless AMR sensor technology providing coordinate and angular field measurements through remote microwave absorption.

3D Anisotropic Magnetoresistance sensor for steel health monitoring

Electromagnetic sensors are widely used for safety and maintenance inspection purposes of ferromagnetic materials used in aerospace, construction and transport industry. In this paper, a portable device based on an Anisotropic Magnetoresistance (AMR) sensor for steel health monitoring is presented. The principle of operation of this device is the detection of magnetic anomalies of ferromagnetic materials, such as steel, due to stresses, corrosion, or other kind of defections. For this purpose, a cost-effective, low-consumption and high-sensitivity device has been developed, consisting of a sensor, a magnetic yoke and a microcontroller. The AMR sensor, is placed above the material under test, giving the ability of contactless measurement of the magnetic flux density on 3 axes. Additionally, a yoke, consisting of two iron poles and a permanent magnet between them, is used in this implementation, in order to create a closed magnetic circuit with the steel sample, enhancing the measurements. Then, a microcontroller with the appropriate electronic components receives the acquired data, enabling their real-time analysis and visualization of the measurements through a personal computer, smartphone, etc. Moreover, a 3D-printed enclosure has been designed and constructed, which both protects the sensor and maintains the desired position and alignment with the samples. The device can be portable due to an integrated Li-Po battery and its Wi-Fi capabilities. It can be used to determine the location of the defective spots or zones of the material under inspection, facilitating its treatment through rehabilitation methods that can be applied on the affected area. Both single-point and scanning mode monitoring are possible, making it suitable for many testing scenarios. Furthermore, its ease of use and portability render it accessible to a wide range of users, and appropriate for on-site measurements. The initial measurements using the device on test samples, demonstrate its reliability and accuracy in steel health monitoring.

Combining annealing temperature and interface engineering for improving anisotropic magnetoresistance in Ta/NiFe/Ta films

Anisotropic magnetoresistance (AMR) sensors are pivotal in various applications due to their low power consumption, scalability, and cost-effectiveness due to the simple sensor structure, comprising one NiFe film, usually encased in a buffer and cap layer. In this work, we explore the effects of inserting MgO and Pt dusting layers between the NiFe sensing layer and adjacent capping and buffer layers, on the electric, magnetic and structural properties of AMR sensors. We describe results on sensors based on Ta/NiFe/Ta, with an as-processed AMR value of 2.0 %. The insertion of Pt thin films had a positive impact, with AMR values increasing to 2.2 %, contrary to the observed with MgO dusting films. Magnetic annealing up to 370 °C caused an increase of the resistivity and reduction in AMR (with Pt dusting layers), on the contrary, MgO dusting layers improved the sensor performance upon annealing, with AMR increasing to 2.5 % (5 h at 370 °C). In light of the findings, the incorporation of Pt and MgO dusting layers enables tailoring the grain size and resistance of Ta/NiFe/Ta films, while combined with proper annealing, which is relevant for applications where Ta and NiFe are available for AMR sensor fabrication.

Development of a steel health monitoring device based on anisotropic magnetoresistance sensors

Development of a steel health monitoring device based on anisotropic magnetoresistance sensors

A nonuniform demagnetizing field model for both static and dynamic responses of anisotropic magnetoresistive thin film sensors

In this paper, an anisotropic magnetoresistive (AMR) thin film sensor which can be used for magnetic scale has been prepared, and its output voltage is about 4.7–4.9 mV V−1. On the basis of the Stoner–Wohlfarth model and with considering the non-uniformity of the demagnetizing field along the width direction of the strips, both the static and dynamic responses of the AMR sensors have been calculated. The results have shown that the calculated results are in agreement with the experimental data. The magnetization rotation in the magnetic sensor strongly depends on the nonuniform demagnetizing field along the width direction. The magnetization at the center is easily rotated into the field direction, and the magnetization at the edge is difficult to be rotated. The smaller the width of the magnetoresistive strip is, the larger both the demagnetizing field at the edge and the saturation field of the magnetic sensor are. The results are helpful for understanding the magnetization rotation of magnetic sensors and developing the magnetic sensors with high performance.

Comprehensive Introspection of Magnetoresistive sensors applied in biomedical diagnostics.

Over the recent years, magnetoresistive (MR) sensors in biosensing technologies have played a pivotal role in detecting and quantifying biomarkers. The article highly focuses on the potential implications of tunneling magnetoresistance (TMR), giant magnetoresistance (GMR), anisotropic magnetoresistance (AMR), and hybrid MR sensors over conventional prototypes. The study mainly elaborates on the sensor characteristics and their implementation in the biomedical domain. The encompassing evaluation reveals the findings that the TMR sensors are remarkably stable and sensitive, whereas the GMR sensors are highly robust and inexpensive, as determined by the detection level, accuracy, sensing distance, and sensitivity. In addition, it is stated that hybrid MR sensors have lower error rates than AMR sensors utilized in the limited research area.

Magnetic anisotropy instability due to magnetic field annealing in Ta/MgO/NiFe/MgO/Ta anisotropic magnetoresistive sensors

Ta/MgO/NiFe/MgO/Ta anisotropic magnetoresistive thin film sensors, which can be used for magnetic scales, were prepared. To improve the magnetoresistance ratio of the sensors, these sensors were vacuum-annealed with the magnetic field applied, and then, both the static and dynamic magnetoresistive responses of the sensors were obtained. The experimental results have shown that the instability of magnetic anisotropy occurs in the sensors after vacuum-annealing, leading to the significant hysteresis and abnormal output voltage peaks. To investigate the underlying physics, the distribution of the non-uniform demagnetizing field along the width of an anisotropic magnetoresistive (AMR) strip is considered, and both the static and dynamic responses of the AMR elements have been calculated on the basis of the Stoner–Wohlfarth model. The results have shown that the calculated results are in good agreement with the experimental data. The calculated results have revealed that the angles between the anisotropic field and the external magnetic field are different during the field cyclings of Hmax (maximum magnetic field) to −Hmax and −Hmax to Hmax. The angle difference is up to 6°, leading to different magnetoresistive responses. This paper is helpful for the understanding about the magnetization rotations in magnetic sensors and the manufacturing of sensors.

Overview of Magnetic Field Sensor

This article summarizes the commonly used in magnetic sensors Hall sensors, Anisotropic magnetoresistive sensor (AMR), Giant magnetoresistance effect sensor (GMR) and Tunneling magnetoresistance sensor (TMR). The structure and working principle of each sensor are introduced. In addition, some error sources of magnetic sensors and the calibration techniques used are introduced, and some typical application examples of each sensor are introduced.

Effect of TaN buffer layer on the sensitivity of ASIC-integrated AMR sensors

This study investigates the impact of a TaN buffer layer on the performance of ASIC-integrated Anisotropic Magnetoresistive (AMR) position sensors. Our findings demonstrate that the inclusion of a TaN buffer layer significantly enhances the sensitivity and stability of the sensors, with a measured sensitivity of 0.678 ± 0.03 Gauss/mV. In contrast, devices without the buffer layer exhibit a notable decline in sensitivity, decreasing to 0.165 ± 0.02 Gauss/mV after 450℃ annealing. In addition, AMR sensors fabricated on non-ASIC substrates showed no such deterioration. Magneto-optical Kerr effect (MOKE) measurements reveal that the change in sensitivity is primarily attributed to variations in the anisotropy field. We propose that the increased anisotropy field is a result of magnetostatic energy induced by alterations in the PECVD-deposited SiO2 surface granular structure, supported by surface morphology characterization and X-ray diffraction experiments. These findings highlight the critical role of the TaN buffer layer in optimizing the performance and stability of ASIC-integrated AMR sensors.

Pulsed Eddy Current Sensor for Cascade Electrical Conductivity and Thickness Estimation in Nonferrous Metal Plates

This paper presents a pulsed eddy current (PEC) sensor design for cascade electrical conductivity and thickness estimation in non-ferrous metal plates. With a single excitation coil design, the combined change in magnetic flux density caused by the excitation coil and the induced eddy current (EC) in the test plate is detected by a compact anisotropic magneto-resistive (AMR) sensor. The effects of the varying input pulse width are discussed with both numerical simulations in commercial finite element analysis (FEA) software and experimental results, and the time-domain characteristics of the responses are analyzed in relation to the properties of the test plates. It is revealed that the electrical conductivity can be gauged based on the peak value of the short pulse response, and with a known electrical conductivity value, plate thickness can be subsequently derived with the rise time of the long pulse response. A cascade estimation process is therefore proposed to calculate the parameters sequentially by adopting hybrid input signals with both short and long input pulses. Verified by the FEA results, this cascade estimation process was then experimentally validated with a prototype of the PEC sensor.

Highly flexible anisotropic magnetoresistance sensor for wearable electronics

The emerging market of wearable devices for tracking and positioning requires the development of highly flexible magnetic sensors. Due to the stable magnetoresistance ratio and simple fabrication process, sensors based on the anisotropic magnetoresistance (AMR) effect have been proposed as promising candidates. However, the stability of AMR sensors under different bending situations has rarely been investigated. In this work, we proposed a flexible AMR magnetic sensor deposited on an ultra-thin Kapton substrate, which exhibits excellent anti-fatigue behavior at different bending curvatures ranging from 1/3 to 1/10 mm−1. Experimental results show that the sensitivity of our proposed flexible AMR sensor remains as high as 0.25 Oe−1 after being bent 500 times, showing promising potential for application in wearable electronic devices.

Large 3D anisotropy of magnetoresistance in MoAlB

Materials with large three-dimensional (3D) anisotropic magnetoresistance (MR) have very important applications in anisotropic magnetoresistance sensors and magnetometers. Herein, we have observed a large 3D anisotropy of MR in nonmagnetic MoAlB single crystal. Specially, the MR ratio (MRa: MRb: MRc) reaches up to 140.5 : 1: 22, when electric current is along a-/b-/c-axis with an applied magnetic field of 8.5 T at 5 K. To clarify the origins of 3D anisotropic MR, angular MR and direction-dependent Hall coefficient measurements as well as first-principles calculations were performed. The nearly electron-hole compensation and the mixed d-p orbital texture are the important origins of the MR effect along a-/c-axis, and the anisotropy of MR originates from the anisotropic Fermi surface and carrier scattering along a-/c-axis in MoAlB. Whereas, the open orbit is a dominant origin along b-axis of MoAlB. Furthermore, the factors affecting the magnitude of MR are given by using two-band model, and possible control strategies are proposed. This work not only proposes a new idea for exploring single-phase nonmagnetic 3D anisotropic MR material, but also provides a key material for the design and development of new 3D anisotropic magnetoresistance sensors and magnetometers in the future.

Development of a Magnetic/Eddy-Current Sensing System For Simultaneous Estimation of Electrical Conductivity and Thickness in Non-Ferrous Metal Plates

This article presents the development of a nonferrous–metal magnetic/eddy-current (NFM-M/EC) sensing system for simultaneous estimation of electrical conductivity and thickness in nonferrous metal plates. The distributed current source (DCS) method models the axisymmetric magnetic/eddy-current fields for the sensor design. An anisotropic magnetoresistive sensor is used to detect the change in magnetic flux density caused by the induced eddy currents in the test plates. The frequency-mapping method for sweep frequency measurements is numerically validated. Calibration between the model and experimental data by the mesh refinement method is introduced to improve the accuracy of estimations efficiently. The DCS method is verified numerically by comparing it with finite-element analysis. The proposed sensing system design is tested on four different materials with varying thicknesses. Accurate estimations in experiments substantiate the NFM-M/EC sensing system as a new alternative for nondestructive detection.

High-Performance Implantable Sensors based on Anisotropic Magnetoresistive La0.67Sr0.33MnO3 for Biomedical Applications.

We present the design, fabrication, and characterization of an implantable neural interface based on anisotropic magnetoresistive (AMR) magnetic-field sensors that combine reduced size and high performance at body temperature. The sensors are based on La0.67Sr0.33MnO3 (LSMO) as a ferromagnetic material, whose epitaxial growth has been suitably engineered to get uniaxial anisotropy and large AMR output together with low noise even at low frequencies. The performance of LSMO sensors of different film thickness and at different temperatures close to 37 °C has to be explored to find an optimum sensitivity of ∼400%/T (with typical detectivity values of 2 nT·Hz-1/2 at a frequency of 1 Hz and 0.3 nT·Hz-1/2 at 1 kHz), fitted for the detection of low magnetic signals coming from neural activity. Biocompatibility tests of devices consisting of submillimeter-size LSMO sensors coated by a thin poly(dimethyl siloxane) polymeric layer, both in vitro and in vivo, support their high suitability as implantable detectors of low-frequency biological magnetic signals emerging from heterogeneous electrically active tissues.

Key Parameters for Detectivity Improvement of Low Noise Anisotropic Magnetoresistive Sensors Made of La2/3Sr1/3MnO3 Single Layers on Vicinal Substrates

The current trend in magnetoresistive sensors development is to increase the sensitivity of single sensing elements by using multilayer structures and to design them into arrays. Such arrays are designed to compensate the excess low frequency noise of individual elements, which limits their magnetic resolution. Here, we report the modeling, design, and fabrication of single layer anisotropic magnetoresistive (AMR) sensors using low noise epitaxial La2/3Sr1/3MnO3 (LSMO) oxide thin films deposited on vicinal SrTiO3 substrates. The fabrication process is simple, and the operation of the sensor is based on a step-induced uniaxial magnetic anisotropy, described using the Stoner–Wohlfarth model. A coherent magnetization reversal process is observed by magneto-optical Kerr effect imaging. A good agreement between experimental data and the expected sensor response confirms the correct operation of the device. Three main fabrication parameters, namely the vicinal angle of the substrate, the deposition temperature, the thin film thickness, and their effects on film anisotropy field and device detectivity have been studied. Detectivity levels as low as 1.4 nT Hz–1/2 at 1 Hz and 240 pT Hz–1/2 in the white noise region are achieved with a single Wheatstone bridge element operating at 310 K. Compared to GMR and AMR sensors, these results are promising for further development and for their use as single layer LSMO low field AMR sensors, including applications as implantable biomedical devices.

Passing Vehicle Road Occupancy Detection Using the Magnetic Sensor Array

The increasing presence of vehicles on roads necessitates intelligent traffic management solutions in areas where video cameras cannot be utilized. Currently, there are limited choices for depersonalized vehicle reidentification systems. This paper introduces a system that later will be used for vehicle reidentification. The system uses anisotropic magnetoresistive sensors and is based on the hypothesis that each vehicle leaves unique magnetic signatures which can be used for comparison and matching. Vehicle location on the road perpendicular to sensor array detection methodology is presented in this work. An array of magnetic sensors is installed in asphalt across the vehicle’s driving direction. The system continuously measures Earth’s natural magnetic field and detects distortions when vehicles pass an array of a sensors and then logs magnetic signatures. Useful parameters from raw sensor axes are calculated– modules and derivatives. Applying signal-to-noise ratio calculation for module derivatives between ambient noise and signal gives important features for neural network input. Different types of neural network architectures and output result interpretation techniques are investigated. Further, after evaluating network output it is possible to label sensor nodes that are directly beneath the vehicle. Experiment results show that implemented algorithm is highly sufficient for valid sensors under the vehicle selection. Correct sensor selection is important for further re-identification algorithms.

Performance of an Anisotropic Magneto-Resistive Sensor Electrical Energy Meter

This paper presents a digital electrical energy meter based on an Anisotropic Magneto-Resistive (AMR) current sensor and an Arduino micro microcontroller. This study aimed at designing and fabricating a digital electrical energy meter using the AMR sensor which overcomes some of the shortcomings of traditional current sensors used in most energy meters, display electrical energy, consumer‟s terminal voltage, supply current, power factor and „real-time‟ power consumption. The study was carried out in the Department of Electrical and Control Engineering, Egerton University between September 2018 and December 2020. The meter was designed using Proteus 8 Professional software and fabricated on a printed circuit board. Algorithms were developed in C-language and stored in the microcontroller to continuously sample voltage and current signals derived from a successive supply voltage divider and the AMR current sensor, respectively. The sampling frequency was 2 kHz and every 10,000 samples were carried out to compute the Root Mean Square (RMS) values of voltage and current. The fabricated meter digitally computed the cumulative electrical energy in kilo-Watt-hours (kWhrs) and the other variables and displayed them on a 16×2 Liquid Crystal Display (LCD) after a consumption of every 0.01 kWhrs. Statistical results on the displayed variables indicated that the meters tested were not significantly different at 0.95 confidence level implying that those meters had similar performance. The extra displays are important and useful to power consumers and service providers in energy utilization and improvement of the quality of the power supplied. Power factor is an exceptional feature in the prototype meter beneficial to both consumers and power agencies in the decision-making process of reducing bills and power distribution costs leading to improved efficiency and service delivery.

Easy‐Cone Magnetic State Induced Ultrahigh Sensitivity and Low Driving Current in Spin‐Orbit Coupling 3D Magnetic Sensors

AbstractMeasurement of 3D vector magnetic field is of vital importance for the development of magnetic navigation, biomedical diagnosis, and microimaging. Traditional 3D magnetic sensors require cooperation of multiple sensors on three orthogonal planes, resulting in disadvantages of bulky size and low spatial resolution. Recently proposed spin orbit torque sensor based on ferromagnetic/heavy‐metal heterostructures can detect three magnetic field components individually due to the different symmetries of current‐polarity‐dependent magnetization dynamic. However, the large driving current density and complex driving procedure hinder their practical application, especially in AC magnetic field detection. Herein, 3D magnetic sensors with dramatically reduced driving current density are reported, one fifth of the original value, by exquisite engineering of the magnetic anisotropy in Pt/Co/Ta heterostructures. With further reduced perpendicular magnetic anisotropy, the sensor in the easy‐cone state demonstrates a record‐high sensitivity of 31196 V A−1 T−1. More importantly, the easy‐cone state sensor can work with an ultralow driving current density of 3.8 kA cm−2, which is three orders lower than previous results. Although easy‐cone state sensor can only measure the z‐axis field, highly compact 3D magnetic sensor can be realized by adoption of two anisotropic magnetoresistance sensors, promising great potential application in space‐ and energy‐restricted scenarios.