Magnetoresistive Research Articles

Novel Magnetic Field Modulation Concept Using Multiferroic Heterostructure for Magnetoresistive Sensors

The low frequency magnetic field detection ability of magnetoresistive (MR)sensor is seriously affected by 1/f noise. At present, the method to suppress the influence of low frequency noise is mainly to modulate the measured magnetic field by mechanical resonance. In this paper, a novel modulation concept employing a magnetoelectric coupling effect is proposed. A design method of modulation structure based on an equivalent magnetic circuit model (EMCM) and a single domain model of in-plane moment was established. An EMCM was established to examine the relationship between the permeability of flux modulation film (FMF) and modulation efficiency, which was further verified through a finite element simulation model (FESM). Then, the permeability modulated by the voltage of a ferroelectric/ferromagnetic (FE/FM) multiferroic heterostructure was theoretically studied. Combining these studies, the modulation structure and the material were further optimized, and a FeSiBPC/PMN-PT sample was prepared. Experimental results show that the actual magnetic susceptibility modulation ability of FeSiBPC/PMN-PT reached 150 times, and is in good agreement with the theoretical prediction. A theoretical modulation efficiency higher than 73% driven by a voltage of 10 V in FeSiBPC/PMN-PT can be obtained. These studies show a new concept for magnetoelectric coupling application, and establish a new method for magnetic field modulation with a multiferroic heterostructure.

Enhanced magnetic properties at low temperature of Mn substituted Ni-Zn mixed ferrite doped with Gd ions for magnetoresistive applications

Mn substituted Ni-Zn mixed ferrites doped with Gd3+ ions belonging to the series Mn0.2Ni0.6Zn0.2GdxFe2-xO4 (x=0.00, 0.02, 0.04, 0.06) have been synthesized by sol-gel method. The effect of Gd3+ substitution on the structural, cationic distribution, magnetic and dielectric properties has been investigated in detail. X-Ray analysis reveals formation of single phase spinel cubic ferrite nanoparticles with no secondary phases. The average crystallite size estimated using Scherrer’s relation lie in the 11–13 nm range with the size decreasing with increase in Gd ion content. Lattice constants calculated from proposed cation distribution are in close agreement with reported values. Surface morphology studies using SEM and HRTEM show spherical shaped nanoparticles which are slightly agglomerated. EDX analysis confirms the expected stoichiometry. The valence state of materials was characterized by XPS. The dielectric variations with frequency and temperature were analyzed using a precision impedance analyzer. The relative permittivity and dielectric loss show an increasing trend with Gd concentration up to x=0.04 and then decrease with further increase in doping. Increasing the temperature increases the relative permittivity. Measurements of magnetic parameters using VSM were performed at room temperature and at 77K. M−H loops recorded have shown that the synthesized ferrite nanoparticles exhibit soft ferrimagnetism. The saturation magnetization and coercivity decrease with increase in Gd doping. The samples possess high magnetization and coercivity at low temperatures. A remarkable difference in the magnetic moments estimated from measured magnetic parameters and from the cation distribution over A and B sites on the basis of Neel’s two sublattice model of ferrimagnetism was noted in all the samples. Higher Y-K angles obtained imply their potential use in synthesis of magnetoresistive (MR) materials.

Hybrid Rigid-Flexible Magnetoresistive Device Based on a Wafer Level Packaging Technology for Micrometric Proximity Measurements

The sensor technology and the packaging method influence drastically the device performance by conditioning its sensitivity, the operating conditions and the distance between the magnetic source and the sensor. Magnetic encoders based on magnetoresistive (MR) sensors standout as a technology immune to dust, oil and fluids, and a cheaper system than optical solutions. Furthermore, they provide a higher spatial resolution, sensitivity and bandwidth than Hall effect sensors. However, to achieve a competitive prototype for applications with high constraints, it is necessary to combine state-of-the-art MR sensors with a packaging solution that entails a minimum impact on the sensor performance. Therefore, a new packaging method developed at wafer level is proposed for motion and positioning systems in planar architectures. The final prototype consists on a hybrid rigid-flexible MR probe combining a sensing chip with a flexible polyimide (PI) connector. This packaging technique pushes the contacts and establish electrical contact with the outer electronics on the back side or at a lower level parallel to the sensor plane, achieving a minimum distance of $7.5~\mu \text{m}$ between the sensor and the magnetic source. Finally, the device was successfully implemented as a magnetic encoder on a linear magnetic scale with $80~\mu \text{m}$ bit length revealing a sin/cos like signal with an amplitude of 42 mV $_{\mathrm {RMS}}$ at a reading distance of $80~\mu \text{m}$ .

A CMOS Magnetoresistive Sensor Front-End With Mismatch-Tolerance and Sub-ppm Sensitivity for Magnetic Immunoassays.

Magnetic biosensing is an emerging technique for ultra-sensitive point-of-care (PoC) biomolecular detection. However, the large baseline-to-signal ratio and sensor-to-sensor mismatch in magnetoresistive (MR) biosensors severely complicates the design of the analog front-end (AFE) due to the high dynamic range (DR) required. The proposed AFE addresses these issues through new architectural and circuit level techniques including fast settling duty-cycle resistors (DCRs) to reduce readout time and a high frequency interference rejection (HFIR) sampling technique embedded in the ADC to relax the DR requirement. The AFE achieves an input-referred noise of 46.4 nT/√Hz, an input-referred baseline of less than 0.235 mT, and a readout time of 11 ms while consuming just 1.39 mW. Implemented in a 0.18 μm CMOS process, this work has state-of-the-art performance with 22.7× faster readout time, >7.8× lower baseline, and 2.3× lower power than previously reported MR sensor AFEs.

Detection of Magnetomechanical Effect in Structural Steel Using GMR 2nd Order Gradiometer Based Sensors.

The magneto-mechanical behaviour of structural steel specimens stressed up to the plastic deformation stage was investigated using a 2nd order gradiometer based on Giant Magneto Resistive (GMR) sensors. The correlation between the gradient of the magnetization and the dislocation density before the crack initiation inside the test material was reported. The capability of the GMR scanning sensor to detect the residual magnetization due to the tensile stress with a non-invasive technique was demonstrated.

Effect of Na-doping on structural, electrical, and magnetoresistive properties of La0.7(Ag0.3-xNax)0.3MnO3 polycrystalline ceramics

In this work, perovskite manganites, La0.7(Ag0.3-xNax)0.3MnO3 (LANMO, x = 0, 0.05, 0.1, 0.15, and 0.2), were synthesized using facile sol-gel method. Structural, electrical, and magnetoresistive (MR) properties of LANMO samples with different sodium contents were investigated. XRD and Rietveld refinement revealed that all samples crystallized in distorted rhombohedral system with R3‾c space group. The doping of sodium increased average particle sizes and A-site cation vacancies and also reduced formal charges of Mn. Electrical transport properties showed that peaks for temperature coefficient of resistance (TCR) and corresponding temperatures (Tk) improved significantly due to increased Na content. TCR and MR values of 10.4% K−1 and 24.5% for x = 0.1 were obtained at room temperature (297.1 and 299.2 K, respectively). Results suggest that these LANMO ceramics are potentially attractive candidates for applications in room-temperature infrared detector.

Study on nonlinear magnetic droplets in a flow-focusing generator

A nonlinear magnetic droplet in a flow-focusing channel is studied in this letter. The breakup regime of the droplets undergoes a transition from dripping to bifurcation to dripping. A GMR (giant magnetoresistance) sensor is proposed for droplet detection. When this nonlinear droplet passes, the sensor can produce a continuous jitter signal which is significantly different from the one obtained in the dripping mode. The MR (magnetoresistive) ratio is about 3.2%. The droplet magnetic field obtained by molecular dynamics simulation demonstrates that the GMR sensor is effective in the detection.

Reconfigurable Spintronics Wheatstone Bridge Sensors With Offset Voltage Compensation at Wafer Level

Industrial requirements are continuously pushing the sensor limits toward higher signal-to-noise ratio under harsh environments, without compromising production costs by additional electronics. Non-full resistive Wheatstone bridge architectures arise as a compact and cheaper alternative to full-bridge measurement schemes, despite a lower output, linearity, and immunity to thermal drifts promoted by the replacement of the out-of-phase sensing elements by passive elements. However, the combination of state-of-the-art magnetoresistive (MR) sensors with thermally compatible materials used as tunable thin-film resistor at wafer level can lead to competitive prototypes for applications with high constraints. Therefore, the resistivity and the temperature coefficient of resistance (TCR) of Cr, Ti, and TiW thin films were evaluated as potential candidates for a thermal and electrical stable non-full MR bridge. Within a TCR around 1059 ± 12 ppm/°C for a spin-valve sensor lying between the values observed for a Ti thickness range of 300 A up to 500 A (571.9 ± 8.3 and 1347 ± 69 ppm/°C, respectively), a sin/cos magnetic encoder was prototyped in a non-full-bridge architecture with a thermal offset drift of 51.48 ± 0.25 nV/Vcc/°C and an offset voltage of 16 mV/Vcc.

Magnetic-Field-Sensing-Based Approach for Current Reconstruction, Sag Detection, and Inclination Detection for Overhead Transmission System

Noncontact monitoring technique of mechanical and electrical states is increasingly attractive for the overhead transmission system. This paper proposes a new technique to detect the tower inclination and transmission line sag, and measure transmission current simultaneously. The key of this technique is to use the ground magnetoresistive (MR) sensor to measure the magnetic flux density along the center line from the three-phase transmission lines. Moreover, the magnetic field (MF) calculation formulas for a tilted tower are given in this paper. Finally, with MF calculation formulas, an algorithm based on artificial immune system (AIS) is applied to estimate the transmission line sag and tower tilt angles, and reconstruct currents. Model simulations of an overhead transmission system are carried out to verify the idea. The simulation results validate the feasibility of this noncontact approach to estimate mechanical parameters and reconstruct electrical parameters for overhead transmission system.

MgO based specular spin valve with reversible minor loop and higher exchange bias for futuristic linear magnetic field sensor

Specular spin valves (SVs) containing ultrathin MgO, structured as substrate/seed/AF/PL1/MgO/PL2/Cu/FL/MgO/cap, have been fabricated. Both structural and magnetic characterizations of MgO based specular spin valve (SSV) have been performed and compared them with the measured data on naturally oxidized (NO) and conventional spin valves (CSV), grown under optimised condition. Reversible minor loop characteristics, highest exchange bias of 625 G and 10% magnetoresistive (MR) ratio were important observations in MgO based system. Zero hysteresis behavior was confirmed due to the reduction of grain growth of the stacks above the fine-textured MgO layer, through X-ray diffraction measurements. Interestingly, at 10 K, above 100% enhancement in MR ratio was observed in MgO based system with marginal increase in coercivity of the order 1 G. On the other hand, NO based structure has 10% MR, minor loop hysteresis of ∼2 G and exchange bias of ∼560 G at room temperature; however at 10 K, only 75% enhancement in MR ratio with large anomalies in magnetic measurements attributes due to the AFM nature of oxide materials. The above studies reflects the superior performance of MgO based SSV over a wide range of temperature in comparison to other SV structures and may lead to futuristic linear magnetic field sensor applications.

Performance Study on Commercial Magnetic Sensors for Measuring Current of Unmanned Aerial Vehicles

The industrial investment in unmanned aerial vehicles (UAVs) is soaring due to their multiple autonomous applications, such as aerial photography, rescue operations, surveillance, and scientific data collection. Current sensing is critical for determining battery capacity in charging and discharging process and alerting for system fault during the flight. Shunt resistors and Hall-effect sensors are traditionally used in UAVs. Recently, magnetoresistive (MR) sensors are gaining enormous attention from researchers. MR sensors tend to consume less power, and they are smaller in size than the Hall-effect sensors. In this paper, a number of off-the-shelf MR sensors were investigated to evaluate the possibility of applying them for UAVs. Another type of magnetic sensor (fluxgate) and shunt resistor was also studied and compared as a reference. The relative scoring method is adopted to evaluate the sensor performance under different metrics, and the results disclose that the MR sensors are highly suitable for current sensing of UAVs due to their higher accuracy, lower energy consumption, wider temperature endurance, smaller size, and less weight than other magnetic current sensors. They are also very competitive with traditional shunt resistors through the overall comparison. The remanence, thermal stability, and cross-field sensitivity of MR sensors are further discussed. This finding provides insight into the selection strategy of current sensors for UAVs and can enhance the industrial development of UAV potentially.

Magnetoresistive Sensor Development Roadmap (Non-Recording Applications)

Magnetoresistive (MR) sensors have been identified as promising candidates for the development of high-performance magnetometers due to their high sensitivity, low cost, low power consumption, and small size. The rapid advance of MR sensor technology has opened up a variety of MR sensor applications. These applications are in different areas that require MR sensors with different properties. Future MR sensor development in each of these areas requires an overview and a strategic guide. An MR sensor roadmap (non-recording applications) was therefore developed and made public by the Technical Committee of the IEEE Magnetics Society with the aim to provide an research and development (R&D) guide for MR sensors intended to be used by industry, government, and academia. The roadmap was developed over a three-year period and coordinated by an international effort of 22 taskforce members from ten countries and 17 organizations, including universities, research institutes, and sensor companies. In this paper, the current status of MR sensors for non-recording applications was identified by analyzing the patent and publication statistics. As a result, timescales for MR sensor development were established and critical milestones for sensor parameters were extracted in order to gain insight into potential MR sensor applications (non-recording). Five application areas were identified, and five MR sensor roadmaps were established. These include biomedical applications, flexible electronics, position sensing and human–computer interactions, non-destructive evaluation and monitoring, and navigation and transportation. Each roadmap was analyzed using a logistic growth model, and new opportunities were predicted based on the extrapolated curve, forecast milestones, and professional judgment of the taskforce members. This paper provides a framework for MR sensor technology (non-recording applications) to be used for public and private R&D planning, in order to provide guidance into likely MR sensor applications, products, and services expected in the next 15 years and beyond.

Hybrid Magnetic Sensor Combined With a Tunnel Magnetoresistive Sensor and High-Temperature Superconducting Magnetic-Field-Focusing Plates

Magnetoresistive (MR) sensors are widely used, particularly in consumer products. However, in applications requiring extremely sensitive magnetic sensors, superconducting quantum interference devices (SQUIDs) are primarily used. In this study, we develop a hybrid magnetic sensor by combining an MR sensor with two high-temperature superconducting (HTS) plates to achieve sensitivity that lies between those of MR sensors and SQUIDs. In addition, we apply a modulation method for measuring the absolute magnetic field. A nanogranular in-gap tunnel MR sensor is installed inside the slit between the magnetic-field-focusing HTS plates, and the magnetic response is evaluated. Using the magnetic-field-focusing characteristics of the HTS plates (made from YBa2Cu3O7‐δ) and the MR sensor inside the slit between the two plates, the sensitivity and noise characteristics are improved. Adjustment of parameters, such as MR sensor height from the slit, slit width of the HTS plates, and plate size allow sensitivity control depending on the application. Moreover, the absolute magnetic response and low noise in low-frequency regions are obtained through ac modulation.

A multifunctional molecular spintronic platform with magnetoresistive and memristive responses via a self-assembled monolayer

We report the spin-dependent transport and the I-V hysteretic characteristics in molecular-level organic spin valves containing a self-assembled-monolayer (SAM) barrier of 1,4 benzenedimethanethiol (BDMT). X-ray photoelectron spectroscopy confirms the establishment of an ordered self-assembled monolayer of BDMT with the phosphonic groups coordinated onto the ferromagnet surface. The magnetoresistive (MR) and the I-V curves characterize the transport properties of the SAM-based organic spin valves, which exhibit both types of non-volatile memory switching, i.e., the magnetoresistive and the memristive switching. The results reveal the possibility of integrating organic SAM into the future multifunctional molecular-level spintronic device applications.

Eddy Current–Tunneling Magneto-Resistive Sensor for Micromotion Detection of a Tibial Orthopaedic Implant

In this paper, an eddy current loop coupled with the tunneling magneto resistor (EC-TMR) is used as a displacement sensor to detect the micromotion of an orthopaedic implant. The high sensitivity and signal to noise ratio of the TMR sensor are used to achieve high resolution at a large standoff distance. First, a small three-turn rectangular eddy current loop of dimension 2.5 mm × 10 mm is designed and simulated inside the human body using a full-wave EM simulator. Then, it is fabricated and tested using vector network analyzer. The magnetic tunnel junction stack is optimized and a six-element TMR sensor is fabricated and characterized. The eddy current and tunnelling magneto resistive sensor are integrated and heterodyne detection technique is used to obtain the high-resolution micromotion detection at an extended standoff range. This technique can be used for in-vivo detection of the micromotion of the orthopaedic implant which will be useful in reducing the revision surgeries due to the mechanical failures of the implant.

Magnetoresistance and structural characterization of electrospun La1−xSrxMnO3 nanowire network fabrics with x = 0.2

Nonwoven nanowire network fabrics of the material class La1−xSrxMnO3 (LSMO) with x = 0.2 were fabricated employing a sol-gel-process via electrospinning and a subsequent thermal treatment process based on thermal gravity analysis results. Investigations by means of scanning electron microscopy revealed an average diameter of the resulting nanowires of around 230 nm and a length of more than 100 μm. The chemical phases of the samples have been confirmed via X-Ray diffraction. The nanowires are polycrystalline with an average grain size of about 25 nm obtained from transmission electron microscopy. Analyses of the electronic transportation properties and of the magnetoresistive (MR) effects of the nanowire samples were carried out by a four probe measurement inside a bath cryostat in fields up to 10 T. Measurements of magnetization in the temperature range 2 K < T < 350 K reveal the Curie temperature (325 K) and are compared to magnetic thermogravitmetric data. Magnetization loops M(H) at room temperature and 4.2 K were carried out revealing the soft magnetic character of the nanowires.

A decay time approach for linear measurement of electrical conductivity

Nitrogen contamination of weld material affects mechanical properties and results in small change of the electric conductivity. This small difference of resistivity needs to be measured since the material property is affected by the nitrogen contamination level. A major challenge in determining the contamination level is the accurate measurement of resistivity in the presence of lift-off variation, top oxide layer and surface irregularities. In this paper, a novel approach based on Decay Time (DT) using Pulsed Eddy Current (PEC) and a high sensitive magnetoresistive (MR) sensor is proposed to measure small changes in electrical conductivity of materials. The DT parameter defined herewith is linearly correlated with material conductivity allowing it to be used for estimating the conductivity from transient PEC measurements. Moreover, this method is less sensitive to lift-off variations. The operation principle of this method is demonstrated using finite element model (FEM) simulation and validated using a prototype probe comprising an excitation coil and MR sensor. Weld metal samples with varying levels of nitrogen contamination are tested using the prototype probe demonstrating the feasibility of the method.

Magnetocardiography Using a Magnetoresistive Sensor Array.

In previous magnetocardiography studies, magnetocardiograms (MCGs) have been obtained using superconducting quantum interference device (SQUID) systems. SQUID is the most sensitive instrument for measuring low-frequency magnetic fields, but it requires liquid helium for cooling, so operating costs are high. In contrast, magnetoresistive (MR) magnetometers function by detecting the change in resistance, caused by an external magnetic field, and have much lower costs. This study was aimed to evaluate feasibility of the MR sensor array for acquiring MCGs.We used an MR sensor array, which was developed for measuring magnetic fields in the picotesla range, with a reduced noise level (TDK Corporation, Tokyo, Japan). A 30-channel MR sensor array was placed in a magnetically shielded room, and the cardiac magnetic field over the anterior chest walls of five healthy subjects was recorded.For all five subjects, MCGs were successfully recorded using the MR sensor array. The cardiac magnetic field corresponding to P, QRS, and T waves on an electrocardiogram (ECG) was detectable by signals averaging 272 ± 27.5 beats.An MR sensor array can be used to measure cardiac magnetic fields. Our results will contribute to the development of low-cost devices for recording MCGs, which will help develop non-invasive diagnostics in cardiovascular medicine.

Optimizing Dual-Layer Recording Using Antiferromagnetic Exchange Coupling

Optimizing Dual-Layer Recording Using Antiferromagnetic Exchange Coupling

Accurate Position Detection in Wireless Power Transfer Using Magnetoresistive Sensors for Implant Applications

Recently, the utilization of magnetic resonant coupling (MRC) mechanism for wireless power transfer (WPT) has been actively investigated. Among numerous applications of WPT technology, the energization of implanted biomedical devices wirelessly and uninterruptedly from external supply is important because it can eliminate possible device replacement due to battery depletion. Given the implanted receiver is invisible from the external transmitter, the coil misalignment occurs easily which results in low transfer efficiency and high magnetic field leakage, and consequently endangers the human health [1]. Thus, the development of accurate position detection of the implanted receiver from the external transmitter is highly desirable. Presently, most studies of the position detection in WPT are focused on the application of electric vehicles. Although the corresponding technologies such as coil sets [2] or auxiliary multi-coils [3] have achieved fruitful outcome, they are too bulky and complicated to be used in implant applications. Due to the advantages of high stability, small size, low power consumption and high precision, the magnetoresistive (MR) sensor has been widely used in many industrial applications [4]. Generally, they are arranged in an array style to measure the magnetic field vector [5] or a moving magnetic object [6]. To the best of authors' knowledge, the application of MR sensors in WPT is absent in literature. In this paper, a new position detection approach in WPT is proposed and implemented, which is particularly suitable for implant applications. The key is to use a MR sensor array to directly measure the variation of magnetic field so as to precisely detect the relative position of the implanted receiver from the external transmitter. Therefore, the advantages of efficient and compact WPT for implant applications can be achieved.