Magnetoresistive Research Articles

A study on HAMR read/write channel models for double layer magnetic recording

Three-dimensional (3D) magnetic recording with multiple recording layers has garnered considerable attention as a next-generation magnetic recording method that uses dual conventional recording layers and a magnetoresistive (MR) head for reading. In this work, to examine signal processing methods for the 3D magnetic read/write (R/W) channel using heat-assisted magnetic recording (HAMR), we have constructed a HAMR R/W channel using a granular media model. Our 3D HAMR R/W channel model, specialized for signal processing development, can account for the Curie temperatures of the recording layers and the temperature distribution of the heat source.

Characterization of Magnetoresistive Shunts and Its Sensitivity Temperature Compensation.

The main purpose of the paper is to show how a magnetoresistive (MR) element can work as a current sensor instead of using a Wheatstone bridge composed by four MR elements, defining the concept of a magnetoresistive shunt (MR-shunt). This concept is reached by considering that once the MR element is biased at a constant current, the voltage drop between its terminals offers information, by the MR effect, of the current to be measured, as happens in a conventional shunt resistor. However, an MR-shunt has the advantage of being a non-dissipative shunt since the current of interest does not circulate through the material, preventing its self-heating. Moreover, it provides galvanic isolation. First, we propose an electronic circuitry enabling the utilization of the available MR sensors integrated into a Wheatstone bridge as sensing elements (MR-shunt). This circuitry allows independent characterization of each of the four elements of the bridge. An independently implemented MR element is also analyzed. Secondly, we propose an electronic conditioning circuit for the MR-shunt, which allows both the bridge-integrated element and the single element to function as current sensors in a similar way to the sensing bridge. Third, the thermal variation in the sensitivity of the MR-shunt, and its temperature coefficient, are obtained. An electronic interface is proposed and analyzed for thermal drift compensation of the MR-shunt current sensitivity. With this hardware compensation, temperature coefficients are experimentally reduced from 0.348%/°C without compensation to -0.008%/°C with compensation for an element integrated in a sensor bridge and from 0.474%/°C to -0.0007%/°C for the single element.

Measurement of somatosensory evoked magnetic fields using an adjustable magnetoresistive sensor array

An adjustable helmet-style magnetoresistive (MR) sensor array with room-temperature magnetic flux sensors was developed to demonstrate the simultaneous multipoint measurement of the somatosensory evoked magnetic field (SEF). Utilizing the extended sensor length, we designed the array to permit individual radial adjustment of each sensor, thereby achieving a precise fit to the varied head geometries of different subjects. Furthermore, the geometry of the sensor array precisely adjusted for the individual subject was quickly obtained by calibration. The SEF was measured in three healthy subjects using an array of 30-channel MR sensors placed on the left hemisphere of the head with median nerve stimulation in the right wrist and averaged over 8000 measurements. An M20 component considered to originate from the primary somatosensory cortex was observed at an approximate latency of 20 ms of the magnetic field waveform in all cases (maximum amplitude of 725 ± 257 fT, peak latency of 20.5 ± 0.45 ms). The phase inversion observed around C3 in the international 10–20 system corresponded to the palmar area of the primary somatosensory cortex on the contour map of the magnetic field at the M20 peak. The MR sensor, an affordable and easy-to-use magnetic sensor that does not require a zero-field environment nor a cryogenic apparatus, was successfully used for simultaneous multipoint SEF measurements in humans and provides a promising system for realizing magnetoencephalography application devices.

Combining droplet microfluidics and magnetoresistive sensors for the rapid and quantitative detection of Klebsiella pneumoniae in urinary tract infections

Urinary tract infections (UTIs) are one of the most common bacterial infections and are considered a public health problem. This has been exacerbated due to the increasing prevalence of multidrug-resistant bacterial strains which poses a significant concern. Additionally, current diagnostic methods are time-consuming and laborious, hindering appropriate treatment and infection control. Herein a novel microfluidic system for diagnosis of UTIs caused by Klebsiella pneumoniae is presented, relying on the unprecedented combination of magnetoresistive (MR) sensors with droplet microfluidics. Initially, K. pneumoniae suspensions were encapsulated in droplets with magnetic nanoparticles (MNPs) functionalized with a bacteriophage receptor binding protein (RBP) specific for K. pneumoniae. The magnetically labeled cells inside droplets were further detected through an MR readout system. This strategy enabled the refined control over the number of MNPs and bacteria inside each droplet which allowed the clear discrimination between the output signals from samples of K. pneumoniae and non-target Escherichia coli. In addition, a direct correlation of the number of events from the MR sensors with different concentrations of K. pneumoniae was observed, providing a dynamic detection range of this pathogen from 104 to 108 CFU/mL. The developed device was further validated for the detection of K. pneumoniae in urine clinical samples (UCSs) from patients with UTIs. These samples were directly encapsulated with RBP-functionalized MNPs, with no need for sample pre-treatment. This allowed the quantitative detection of K. pneumoniae in UTIs samples with high specificity (100%) and sensitivity (87.5%) in less than 1 h.

Two-dimensional synchronous motion modulation MEMS structure for suppressing 1/f noise in magnetoresistive sensors

Magnetoresistive (MR) sensors have great application prospects in the field of weak magnetic field detection due to their high sensitivity, small size, and low power consumption. However, 1/f noise greatly limits the low-frequency detectivity of MR sensors. In order to suppress 1/f noise, this paper proposes a two-dimensional synchronous motion modulation (TDSMM) structure based on microelectromechanical systems (MEMS). This structure can effectively reduce 1/f noise by modulating the frequency of the measured magnetic field in the high-frequency band. Theoretical analysis and finite element simulation were conducted on three different modulation models: TSDMM, magnetic flux concentrators motion modulation, and MR components longitudinal motion modulation. The results showed that the modulation efficiency of the TDSMM reached as high as 127%, which is currently the highest value in MR-MEMS sensors. The TDSMM MEMS structure has been successfully manufactured, and the resonant frequency of the transverse resonator is twice that of the longitudinal resonator, enabling extremely high modulation efficiency. The noise spectral density of giant-magnetoresistive components on a silicon-on-insulator substrate was tested, and the noise level in the high-frequency band was three orders of magnitude lower than that in the low-frequency band. These results position MR-MEMS sensors with TDSMM structures as highly competitive candidates in the field of ultra-weak magnetic field detection.

Monotone Signal Localization Using Magnetoresistive Sensor Array for Low-Field Magnetic Particle Imaging

Magnetic particle imaging (MPI) demands high sensitivity to be operable under low excitation fields as alternative countermeasure for specific absorption rate and magnetostimulation effects. Owing to capability of magnetoresistive (MR) sensors to unidirectionally detect sub-pT signal, we built a prototype of brain MPI scanner by using MR sensor array. The arrays were 13×13 and 5×13 matrices with 15mm sensor pitch and installed orthogonally relative to a 0.2m drive coil. To preliminarily evaluate 10 kHz-signal localization, we used a current loop embedded with 20-turn coils. We then carefully measured pT field at 234 sensor coordinates for 1mA ac current fed to the loop. The resulting field contour was numerically interpolated to identify the actual loop orientation. We found that the loop three-dimensionally rotated along xyz axes with (1.0°, –1.5°, 2.0°) angles from the expected position. To further estimate spatial resolution of the array, we measured magnetic fields from two adjacent point sources of small coils. The corresponding signal separation appeared dependent on the perpendicular distance to the nearest sensor. This fine signal localization of MR sensor array highlights its potential for tracing magnetic nanoparticles in low-field MPI system.

Detection of Surface and Back-Surface Defects on Metal Plate via Rectangular Wave Eddy Current Testing Using Magnetoresistive Sensor

Eddy current testing (ECT) at different frequencies can identify surface and back-surface defects caused by the skin effect. Traditional ECT uses a pickup coil to measure the magnetic-field signal caused by eddy current. However, according to Faraday’s law of electromagnetic induction, the voltage signal detected at low frequencies is weak; thus, the signal is susceptible to interference from noise. By contrast, magnetoresistive (MR) sensors have high sensitivity at low frequencies. Therefore, we developed a rectangular wave eddy current testing system using an MR sensor. The system uses rectangular wave excitation, which can generate both low- and high-frequency signals, and the MR sensor detects the magnetic-field components. A 12-mm-thick aluminum plate with an artificial circular defect 10 mm in diameter and 4 mm in height was used as a specimen. The measurement results indicated that the developed system can accurately and quickly distinguish between surface and back-surface defects.

Low-Concentration Magnetic Particle Spectroscopy Using Gradiometric Receive Coil-Coupled Magnetoresistive Sensor

In order to realize clinical magnetic particle imaging (MPI), scanner sensitivity must be designed to recognize magnetic nanotracers within safe dosage range. As zero-dimensional MPI scanner, magnetic particle spectroscopy (MPS) is often used to initially characterize complex dynamic of the tracers by applying a sinusoidal magnetic field with high amplitude. However, the resulting magnetization signal can be very small to dominate electromagnetism of cellular matrices with tracer concentration below 2 μg <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Fe</sub> mL <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> . Here, we demonstrated an enhanced MPS sensitivity by coupling typical gradiometric receive coil with a magnetoresistive (MR) sensor under 25 mT/μ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> at 10 kHz. This magnetometric system could distinguish particle signal of Resovist® sample from its 0.1mL liquid medium while diluting iron concentration up to 70 ng <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Fe</sub> . For this case, we confirmed that the field-induced background signal from water-based medium became significant. Depending on ion concentration, such polar solvent exhibited both diamagnetism and Eddy current effect. Even though its harmonic components might disappear technically, magnetization signal of low-concentration sample had phase and magnitude differences relative to those of the medium at fundamental frequency. Owing to its high sensitivity, MPS system using MR sensor can be potentially implemented for liquid-phase biosensing, in addition to MPI technology.

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.

On-chip full bridge bipolar linear spin valve sensors through modified synthetic antiferromagnetic layers

Realizing the rising demand for high-sensitivity magneto-resistive (MR) sensors in the industry, an attempt was made towards the fabrication of a spin valve (SV) based bipolar sensor on chip in the form of a full Wheatstone bridge with push–pull configuration. For a sensor with a push–pull configuration, the opposite arms are configured with opposite sense layers. To achieve this, either odd or even layers of synthetic antiferromagnetic (SAF) structure were introduced in the conventional spin valve structure, through which the magnetization orientation of the sensing layer was regulated by either negative or positive sensitivity to the applied field direction respectively. A detailed theoretical simulation was carried out to achieve the stable antiparallel magnetic state around zero fields by varying individual layer thicknesses in the case of single and double-layer SAF structures. With optimized conditions, experimental data were presented, and a bipolar device with a sensitivity of the order of 0.7 mV/V/Oe in the linear field range of ± 15 Oe was fabricated. The performance of the device was characterized by a wider temperature range and it was observed that the device has high thermal stability in the range of - 40 °C to 125 °C with a thermal coefficient of the voltage around 12 μV/V/∘C. Angular performance of the fabricated sensor with minimized orthogonality error was presented and it was observed that the sensor can measure the angle with an accuracy of +/-2 ° .

Analysis and Elimination of Speed Measurement Ripple in Sinusoidal Coded Gearwheel Encoder for Spindle Motor Drive

The gearwheel encoder with a magnetoresistive (MR) sensor is robust, simple, and widely used in high-speed industrial motor drives. Moreover, a high-precision rotor position detection is possible with the gearwheel encoder due to its large number of output pulses per revolution. The MR sensor outputs two sinusoidal signals that are 90 degrees apart, the period of which matches with one tooth of the gearwheel. However, these output signals are not perfectly sinusoidal because the MR sensor is manually mounted, and the magnitudes of these signals depend on the distance between the MR material and gearwheel. This slight imperfection in the output signals provokes large speed ripples in the speed measurement. This study analyzed the speed measurement ripples, and an effective ripple-elimination method was devised based on the results of the analysis. Ripple components caused by each signal error were calculated from the error signal models, and exact ripple frequencies were extracted. From the calculation results, a frequency adaptive series notch filter algorithm was designed. The simulation and experimental results demonstrated the effectiveness of the analysis and proposed approach. It is obviously shown through the experimental results that the speed measurement ripple is almost eliminated with the proposed method, and, thus, the analysis is valid.

Measuring instantaneous angular speed using a gear wheel as material measure to detect pitting damage during an endurance test

Gearbox condition monitoring is mostly carried out using accelerometers at the housing. The long transfer path interferes with the information on damaged gears. Therefore, new technologies such as measuring the instantaneous angular speed (IAS) are gaining interest. But these require constructive changes in the power-transmitting elements. Therefore, this contribution uses magnetoresistive (MR) sensors and a minimally invasive integration approach to measure IAS. Three sensor classes are integrated: accelerometers at the gearbox housing, MR sensors, which use a gear wheel as a material measure and MR sensors, which use an encoder magnet at the end of the shaft. These sensor concepts are introduced by Koch et al. 2022 [1] and applied to a 3-stage bevel helical gearbox in this contribution. The first results are on artificial tooth flank damage. Therefore, this contribution focuses on real fatigue pitting damage.A back-to-back test stand is set up for the endurance test. The tests are carried out with an overload load at a constant speed to generate fatigue pitting damage. The results are evaluated in the frequency domain by exploring the gear mesh frequency (GMF) and sidebands modulated with the shaft frequency of the damaged pinion. Results show that with all sensor concepts, the damages can be detected. Influences when using a gear wheel as a material measure are described for one measurement concept. A feature to compare the damage-specific information over the endurance test is introduced and the influence of the transfer path is evaluated through analysis of the relative rise of damage-specific amplitudes.

Noise measurement and system calibration on magnetoresistive sensors

PurposeThe noise measurement on magnetoresistive (MR) sensors is generally conducted by techniques including single-channel data sampling and fast Fourier transform (FFT) analysis as well as two-channel cross-correlation. The single-channel method is easy to implement and is widely used in the noise measurement on MR sensors, whereas the two-channel method can only eliminate part of the system noise. This study aims to address two key issues affecting measurement accuracy: calibration of the measurement system and the elimination of system noise.Design/methodology/approachThe system is calibrated by using a low-noise metal film resistor in that the system noise is eliminated through power spectrum subtraction. Noise measurement and analysis are conducted for both thermal noise and detectivity of magnetic tunnel junction (MTJ) sensor.FindingsThe thermal noise measurement error is less than 2%. The detectivity of the MTJ sensor reaches 27 pT/Hz1/2 at 2 kHz.Originality/valueThis study provides a more practical solution for noise measurement and system calibration on MR sensors with a bias voltage and magnetic field.

Enhancement of magnetoresistance with the variation of particle size in La0.68−xYxCa0.32MnO3, x = 0.0, 0.4, 0.5, 0.6

Huge magnetoresistance (MR) has been achieved in La0.68Ca0.32MnO3, by optimal substitution. To explore the origin of this giant enhancement of MR, we have carried out magnetic and magneto transport measurements of La0.68−xYxCa0.32MnO3, x = 0.0, 0.4, 0.5, 0.6 compounds in detail. Our analysis indicates that the disorder introduced by yttrium substitution plays the crucial role in the magneto resistive properties in this series. A large MR has been observed in optimally doped x = 0.5. In this compound, an MR value as high as 1.5 × 107% at a temperature of 50 K has been achieved for an external magnetic field of 90 kOe. The extraordinarily enhanced and large MR value has been described using a phenomenological model of percolation via ferromagnetic clusters. At the specific temperature sizes of these s compounds increases and MR% is likewise increased. For x = 0.6,not enough to create the percolation path.

Effects of Partial Manganese Substitution by Cobalt on the Physical Properties of Pr0.7Sr0.3Mn(1-x)CoxO3 (0 ≤ x ≤ 0.15) Manganites.

We have investigated the structural, magnetic, and electrical transport properties of Pr0.7 Sr0.3 Mn(1-x)Cox O3 nanopowders (x = 0, 0.05, 0.10 and 0.15). The Pechini Sol-gel method was used to synthesize these nanopowders. X-ray diffraction at room temperature shows that all the nano powders have an orthorhombic structure of Pnma space group crystallography. The average crystallite size of samples x = 0, 0.05, 0.10, and 0.15 are 33.78 nm, 29 nm, 33.61 nm, and 24.27 nm, respectively. Semi-quantitative chemical analysis by energy dispersive spectroscopy (EDS) confirms the expected stoichiometry of the sample. Magnetic measurements indicate that all samples show a ferromagnetic (FM) to paramagnetic (PM) transition with increasing temperature. The Curie temperature TC gradually decreases (300 K, 270 K, 250 K, and 235 K for x = 0, 0.05, 0.10, and 0.15, respectively) with increasing Co concentrations. The M-H curves for all compounds reveal the PM behavior at 300 K, while the FM behavior characterizes the magnetic hysteresis at low temperature (5 K). The electrical resistivity measurements show that all compounds exhibit metallic behavior at low temperature (T < Tρ) well fitted by the relation ρ = ρ0 + ρ2T2 + ρ4.5T4.5 and semiconductor behavior above Tρ (T > Tρ), for which the electronic transport can be explained by the variable range hopping model and the adiabatic small polaron hopping model. All samples have significant magnetoresistance (MR) values, even at room temperature. This presented research provides an innovative and practical approach to develop materials in several technological areas, such as ultra-high density magnetic recording and magneto resistive sensors.

Artificial Neural Networks for GMR-Based Magnetic Cytometry

In this work we propose an artificial neural network (ANN) for magnetic micro cytometry pattern recognition and automated counting. The method is tested for detecting analytes in the 2-3 μm range. The cytometer is composed of a disposable cartridge and an acquisition platform. The disposable cartridge contains microfluidic channels with a 10×100 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> cross section on top of a substrate with magnetoresistive (MR) sensors. The custom analog signal chain performs with an integrated noise of 2.99 μV <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RMS</sub> in a 10 kHz bandwidth. To employ the ANN we synthesize a training data set based on the magnetic-dipole equation and several data set expansion methods. The ANN is tested on an experiment with 2.8 μm magnetic particles (MP) and compared with an improved threshold-based method with reduced false positive. The ANN produces a maximum of 90% detection rate, improving on the 30-50% detection rates of other single sensor methods published in literature.

Microgrid Protection Using Magneto-Resistive Sensors and Superimposed Reactive Energy

The concept of microgrids has emerged as an effective way to integrate distributed energy resources (DERs) into distribution networks. The presence of DERs in microgrids leads to challenges in the formulation of protection for microgrids. Protection problems arise in a microgrid due to varying fault current levels in different operating scenarios. In order to overcome the practical challenges arising from varying fault current levels leading to short-circuit faults in microgrids, this paper proposes a MagnetoResistive (MR) sensors-based protection scheme, with fault localization through SuperimposedReactiveEnergy (SRE). The process is initiated by employing highly sensitive non-intrusive magnetic sensors to detect the magnetic field at each end of the distribution line. The magnetic field is then used to calculate the total harmonic distortion and thus detect faults in microgrids. After detection of faults, the proposed scheme uses SRE to identify faulty zones in microgrids. Finally, SI components of the current are extracted for fault classification. Extensive simulations on the International Electro-technical Commission (IEC) microgrid are performed in MATLAB/Simulink to validate the efficacy of the proposed scheme. Simulation results show that the proposed scheme can effectively detect, classify and isolate different faults in microgrids, while operating under various modes with varying fault locations and resistances, with the efficiency of approximately 97–98%.

A Review: Research Progress of Neural Probes for Brain Research and Brain–Computer Interface

Neural probes, as an invasive physiological tool at the mesoscopic scale, can decipher the code of brain connections and communications from the cellular or even molecular level, and realize information fusion between the human body and external machines. In addition to traditional electrodes, two new types of neural probes have been developed in recent years: optoprobes based on optogenetics and magnetrodes that record neural magnetic signals. In this review, we give a comprehensive overview of these three kinds of neural probes. We firstly discuss the development of microelectrodes and strategies for their flexibility, which is mainly represented by the selection of flexible substrates and new electrode materials. Subsequently, the concept of optogenetics is introduced, followed by the review of several novel structures of optoprobes, which are divided into multifunctional optoprobes integrated with microfluidic channels, artifact-free optoprobes, three-dimensional drivable optoprobes, and flexible optoprobes. At last, we introduce the fundamental perspectives of magnetoresistive (MR) sensors and then review the research progress of magnetrodes based on it.

Magnetoresistive Single‐Molecule Junctions: the Role of the Spinterface and the CISS Effect

AbstractThis review is an effort in putting together the latest results about room‐temperature magnetoresistive (MR) effects in nanoscale/single‐molecule electronic devices consisting of one (few) molecule(s) placed in electrical contact between two nanoscale electrodes. Molecules represent powerful building blocks for developing state‐of‐the‐art MR devices, as they bring long spin relaxation timescales, low cost and high tunability of their electrical and magnetic properties via chemical modifications. The capability to control at room temperature and under bespoke electrodes’ magnetization the MR response of a single‐molecule (SM) device has been a longstanding quest. Such SM platforms could serve as fundamental tools to understand what the main mechanistic ingredients of MR effects in a molecular device are, leading to their use as building‐blocks for miniaturization in spintronic applications. The work carried out so far in this field has identified two key components directly involved in the MR response of a single(few)‐molecule(s) device: (i) The molecule|electrode spinterface, defining the interplay between interfacial electrostatics and spin density, has been proven to play a fundamental role in the interpretation of the observed single‐molecule junction's MR effects, which is governed by the electrode material and the electrode‐molecule chemistry. (ii) Two aspects of the molecular structure have been demonstrated to be involved in the spin‐dependent conduction mechanism: (1) the presence of paramagnetic metal centres in the molecular structure and how their orbitals bearing the unpaired electrons couple with the device electrodes, and (2), the degree of chirality within the molecular wire. This contribution will focus on the above points (i‐ii) by making use of specific examples in the literature.

The Effect of Geometrical Overlap between Giant Magnetoresistance Sensor and Magnetic Flux Concentrators: A Novel Comb-Shaped Sensor for Improved Sensitivity

The combination of magnetoresistive (MR) element and magnetic flux concentrators (MFCs) offers highly sensitive magnetic field sensors. To maximize the effect of MFC, the geometrical design between the MR element and MFCs is critical. In this paper, we present simulation and experimental studies on the effect of the geometrical relationship between current-in-plane giant magnetoresistive (GMR) element and MFCs made of a NiFeCuMo film. Finite element method (FEM) simulations showed that although an overlap between the MFCs and GMR element enhances their magneto-static coupling, it can lead to a loss of magnetoresistance ratio due to a magnetic shielding effect by the MFCs. Therefore, we propose a comb-shaped GMR element with alternate notches and fins. The FEM simulations showed that the fins of the comb-shaped GMR element provide a strong magneto-static coupling with the MFCs, whereas the electric current is confined within the main body of the comb-shaped GMR element, resulting in improved sensitivity. We experimentally demonstrated a higher sensitivity of the comb-shaped GMR sensor (36.5 %/mT) than that of a conventional rectangular GMR sensor (28 %/mT).