Giant Magnetoresistance Spin Valves Research Articles

A GMR enzymatic assay for quantifying nuclease and peptidase activity.

Hydrolytic enzymes play crucial roles in cellular processes, and dysregulation of their activities is implicated in various physiological and pathological conditions. These enzymes cleave substrates such as peptide bonds, phosphodiester bonds, glycosidic bonds, and other esters. Detecting aberrant hydrolase activity is vital for understanding disease mechanisms and developing targeted therapeutic interventions. This study introduces a novel approach to measuring hydrolase activity using giant magnetoresistive (GMR) spin valve sensors. These sensors change resistance in response to magnetic fields, and here, they are functionalized with specific substrates for hydrolases conjugated to magnetic nanoparticles (MNPs). When a hydrolase cleaves its substrate, the tethered magnetic nanoparticle detaches, causing a measurable shift in the sensor's resistance. This design translates hydrolase activity into a real-time, activity-dependent signal. The assay is simple, rapid, and requires no washing steps, making it ideal for point-of-care settings. Unlike fluorescent methods, it avoids issues like autofluorescence and photobleaching, broadening its applicability to diverse biofluids. Furthermore, the sensor array contains 80 individually addressable sensors, allowing for the simultaneous measurement of multiple hydrolases in a single reaction. The versatility of this method is demonstrated with substrates for nucleases, Bcu I and DNase I, and the peptidase, human neutrophil elastase. To demonstrate a clinical application, we show that neutrophil elastase in sputum from cystic fibrosis patients hydrolyze the peptide-GMR substrate, and the cleavage rate strongly correlates with a traditional fluorogenic substrate. This innovative assay addresses challenges associated with traditional enzyme measurement techniques, providing a promising tool for real-time quantification of hydrolase activities in diverse biological contexts.

Two spin-valve GMR thin films on half wheatstone bridge circuit for detecting green-synthesized Fe3O4@Ag nanoparticles-labeled biomolecule

The availability of low-cost, rapid, and easy-to-use instruments for detecting biomolecules is critical in giant magnetoresistance (GMR) based biosensors. This research reports the performance of two spin-valve (SV) GMR thin films on half Wheatstone bridge circuits for detecting biomolecules. The SV thin films with the structure of Ta(2 nm)/Ir20Mn80(10 nm)/Co90Fe10(3 nm)/Cu(2.2 nm)/Co84Fe10B4(10 nm)/Ta(5 nm) used as sensing element was fabricated using RF magnetron sputtering on Si/SiO2 substrate. The Fe3O4 magnetic nanoparticles as a label were synthesized by coprecipitation method based on the green synthesis route using Moringa oleifera (MO) extract. Furthermore, the utilization of Ag nanoparticles to form Fe3O4@Ag was also green-synthesized by the aqua-solution method using MO extract. The nanoparticles exhibited soft ferromagnetic behavior with the saturation magnetization of 55.0 emu/g for Fe3O4 and 43.4 emu/g for Fe3O4@Ag. A small amount of magnetic nanoparticles-ethanol solution was dropped onto each sensing element surface on half Wheatstone bridge. For comparison, one thin film was also investigated as a quarter Wheatstone bridge circuit. Based on this research, two sensors have faster magnetic switching and saturation than one sensor, confirmed by the left-shifting of Hs(sv) from 12.4 Oe down to 10.5 Oe for two sensors of the Fe3O4 sample. BSA is used as a biomarker to observe the performance of biosensors. The sensor on the half-bridge circuit in detecting Fe3O4@Ag/BSA achieved a better sensitivity of 0.51 mV/mg/mL than that of the quarter bridge of 0.46 mV/mg/mL. A small limit-of-detection of 0.5 mg/mL was achieved for Fe3O4@Ag/BSA. Moreover, the excellent reproducibility and stability of the sensor were confirmed by relative signal deviation ranges of 2–13 % for 60 s. Therefore, the half-bridge configuration is reliable for achieving competitive performance with green-synthesized magnetic labels in GMR as biosensors in encouraging more responsive detection.

Co/Pd-based spin-valves with perpendicular magnetic anisotropy on flexible substrates. Direct deposition vs transfer-and-bonding approaches

Flexible spintronics is an emerging field of research that has received increasing attention due to the additional functionalities that are allowed (lightweight, flexibility, shape-ability, wearability) with respect to conventional rigid systems. In this work, different strategies for the fabrication of flexible spintronic devices with perpendicular magnetic anisotropy are compared, i.e., transfer-and-bonding approaches exploiting wet and dry lift-off methods, and direct deposition on flexible substrates. To evaluate the potential of the proposed strategies, Co/Pd-based giant magneto-resistive spin-valves including a synthetic antiferromagnet reference electrode were investigated. Such stacks represent a demanding model system, owing to the large number of interfaces whose quality strongly affects the overall magnetic and electric performances. The advantages and drawbacks of the different strategies are discussed to provide crucial indications for the development of flexible spintronic devices of any complexity. Based on the results, the most suitable option for achieving high-quality heterostructures on large area surfaces via direct deposition is using polyethylene naphthalate (Teonex®) tapes, provided that the processing and operating temperatures are relatively low (<525 K). On the other hand, if the process requires higher temperatures, the dry lift-off method exploiting the low adhesion between an Au underlayer and the SiOx/Si(100) substrate is the preferred alternative.

Giant magnetoresistance (GMR) spin-valve based magnetic sensor with linear and bipolar characteristics for low current detection

High sensitivity linear, bipolar, and hysteresis-free magnetic field sensor is necessary for low current measurement of the order of 1000 mA with a resolution of 1 mA. Magnetoresistive technology has evolved into popular magnetic sensing technology where magnetic multilayered structures were tailored to obtain linear output characteristics. In this report, a GMR spin-valve structure was used to develop magnetic field sensing elements. Mechanical assembly of the sensing elements in the form of a Wheatstone bridge configuration was made to have the linear and bipolar characteristics of the sensor. Application of the constant DC biasing field perpendicular to the sensitivity axis was made to have fully hysteresis-free output characteristics and the maximum sensitivity was achieved of the order 2 to 2.5 mV/G with a supply of 1 mA current to the bridge circuit. The in-house developed spin valve sensor was then used to measure the current in the range of ± 1 A, having a sensitivity of the order of 2 mV/mA with a resolution down to 1 mA by using non-intrusive current sensing technology. A core-based sensor has a large aperture size that accommodates a current-carrying conductor with an ampacity of 1 kA. The calibrated output shows the sensor can measure the field in the range of ± 0.4 mT with a resolution of ∼ 200 nT and has the potential for floating current measurement application in the battery management system.

Thin-Film Heterostructures Based on Co/Ni Synthetic Antiferromagnets on Polymer Tapes: Toward Sustainable Flexible Spintronics.

Synthetic antiferromagnets with perpendicular magnetic anisotropy (PMA-SAFs) have gained growing attention for both conventional and next-generation spin-based technologies. While the progress of PMA-SAF spintronic devices on rigid substrates has been remarkable, only few examples of flexible thin-film heterostructures are reported in the literature, all containing platinum group metals (PGMs). Systems based on Co/Ni may offer additional advantages with respect to devices containing PGMs, i.e., low damping and high spin polarization. Moreover, limiting the use of PGMs may relieve the demand for critical raw materials and reduce the environmental impact of related technologies, thus contributing to the transition toward a more sustainable future. Here, we discuss for the first time the realization of Co/Ni-based PMA-SAFs on polymer tapes and exploit it to obtain flexible giant magneto-resistive spin valves (GMR-SVs) with perpendicular magnetic anisotropy. Several combinations of buffer and capping layers (i.e., Pt, Pd, and Cu/Ta) are also investigated. High-quality flexible SAFs with a fully compensated antiferromagnetic region and SVs with a sizable GMR ratio (up to 4.4%), in line with the values reported in the literature for similar systems on rigid substrates, were obtained in all cases. However, we demonstrate that PGMs allows achieving the best results when used as a buffer layer, while Cu is the best choice as a capping layer to optimize the properties of the stacks. We justify the role of buffer and capping layers in terms of different interdiffusion mechanisms occurring at the interface between the metallic layers. These results, along with the high robustness of the samples' properties against bending (up to 180°), indicate that complex and bendable Co/Ni-based heterostructures with reduced content of PGMs can be obtained on flexible tapes, allowing for the development of novel flexible and sustainable spintronic devices for applications in many fields including wearable electronics, soft robotics, and biomedicine.

Two-Dimensional Position Tracking Using Gradient Magnetic Fields

In this work, a two-dimensional (2D) position-detection device using a single axis magnetic sensor combined with orthogonal gradient coils was designed and fabricated. The sensors used were an induction coil and a GMR spin-valve sensor GF807 from Sensitec Inc. The field profiles generated by the two orthogonal gradient coils were analyzed numerically to achieve the maximum linear range, which corresponded to the detection area of the tracking system. The two coils were driven by 1-kHz sine wave currents with a 90° phase difference to generate the fields with uniform gradients along the x- and y-axis in the plane of the tracking stage. The gradient fields were detected by a single-axis sensor incorporated with a digital dual-phase lock-in detector to retrieve the position information. A linearity correction algorithm was used to improve the location accuracy and to extend the linear range for position sensing. The mean positioning error was found to be 0.417 mm, corresponding to the relative error of 0.21% in the working range of 200 mm × 200 mm, indicating that the proposed tracking system is promising for applications requiring accurate control of the two-dimensional position.

Detection of green-synthesized magnetite nanoparticles using spin-valve GMR-based sensor and their potential as magnetic labels

Exploration of magnetic nanoparticle labels for giant magnetoresistance (GMR) biosensing technology is crucial because their characteristics determine GMR-based sensor performance, related to their stray field and compatibility to biomolecule. We investigated the potency of the green-synthesized magnetite (Fe3O4) nanoparticle as a green-magnetic label on a spin-valve (SV) GMR sensor based on a [Ta (2 nm)/Ir20Mn80 (10 nm)/Co90Fe10 (3 nm)/Cu (2.2 nm)/(Co84Fe10B4 (10 nm)/Ta (5 nm)] structure that generated a 7.3 Oe exchange-bias field and 5.73% magnetoresistance. Fe3O4 nanoparticles were synthesized by a chemical coprecipitation method utilizing Moringa oleifera (MO) leaf extract as a natural reducing agent. The green-synthesized Fe3O4 nanoparticles exhibited a cubic inverse spinel structure as a pristine Fe3O4 characteristic. The crystallite size of green-synthesized Fe3O4 was larger than that of Fe3O4, reaching ∼ 14 nm, and formed a particle size of 29.1 nm. Its magnetic properties included soft ferromagnetic behavior with 55.5 emu/g saturation magnetization and 54 Oe coercivity; its saturation was lower than that of Fe3O4 due to the presence of the phytochemical compound of MO extract. In addition, to observe its potency as a magnetic label for biosensing, conventionally synthesized nanoparticles, surface modification using polyethylene glycol (PEG), and attachment of an α-amylase enzyme as a biomarker sample on green nanoparticle labels were also investigated. When measuring various concentration labels using the SV-GMR thin film, the green-synthesized Fe3O4 achieved a linear relationship and a better sensitivity of 0.098 mV/mg/mL than that of the conventional compound. The sensor could detect Fe3O4-labelled α-amylase enzyme as indicated by the decreasing output voltage, a consequence of altering the surface conditions of the nanoparticles. Therefore, green-synthesized nanoparticles are reliable and achieve competitive performance as magnetic labels in GMR sensors.

Giant magnetoresistance in spin valves realized by substituting Y-site atoms in Heusler lattice

‘All-Heusler’ spin-valve constructed by two half-metallic Heusler electrodes and a non-magnetic Heusler spacer contains two interfaces that have a crucial influence on the magnetoresistance. In order to reduce the disorder at the interface and protect the half metallicity of the electrode at the same region, we propose a scheme to construct a spin valve by replacing the Y-site atoms in the half-metallic Heusler electrode to obtain the corresponding non-magnetic spacer based on the Slater–Pauling rule. In this way, the lattice and band match of the two materials can be ensured naturally. By using Co2FeAl as electrode and Co2ScAl as the spacer materials, we construct the Co2FeAl/Co2ScAl/Co2FeAl(001)-spin valve. Based on the first-principles calculation, the most stable FeAl/CoCo-interface is determined both from the phonon spectra and the formation energy when the spacer Co2ScAl grows on the FeAl-terminated (001) surface of electrode material Co2FeAl. By comparing the projected density of states of the interfacial atoms with the corresponding density of states of the bulk electrode material, only the value of spin-up state of Al changes from 0.17 states/atom/eV to 0.06 states/atom/eV before and after substitution, the half metallicity at the interface is maintained. As a result, the spin-dependent transport properties show significant theoretical magnetoresistance MRop which can reach up to 1010% and much larger than 106% reported before.

Giant Magnetoresistance Biosensors in Biomedical Applications.

The giant magnetoresistance (GMR) effect has seen flourishing development from theory to application in the last three decades since its discovery in 1988. Nowadays, commercial devices based on the GMR effect, such as hard-disk drives, biosensors, magnetic field sensors, microelectromechanical systems (MEMS), etc., are available in the market, by virtue of the advances in state-of-the-art thin-film deposition and micro- and nanofabrication techniques. Different types of GMR biosensor arrays with superior sensitivity and robustness are available at a lower cost for a wide variety of biomedical applications. In this paper, we review the recent advances in GMR-based biomedical applications including disease diagnosis, genotyping, food and drug regulation, brain and cardiac mapping, etc. The GMR magnetic multilayer structure, spin valve, and magnetic granular structure, as well as fundamental theories of the GMR effect, are introduced at first. The emerging topic of flexible GMR for wearable biosensing is also included. Different GMR pattern designs, sensor surface functionalization, bioassay strategies, and on-chip accessories for improved GMR performances are reviewed. It is foreseen that combined with the state-of-the-art complementary metal-oxide-semiconductor (CMOS) electronics, GMR biosensors hold great promise in biomedicine, particularly for point-of-care (POC) disease diagnosis and wearable devices for real-time health monitoring.

Prospect of core-shell Fe3O4@Ag label integrated with spin-valve giant magnetoresistance for future point-of-care biosensor

Magnetic-based biosensors are the analytical instruments that convert the biological recognition into the electrical signal through the generating of the stray-field of the magnetic nanoparticles (MNPs) attached to the biomolecule target. The magnetic biosensor feature relies on the transducer and the MNPs label selection. Recently, the biosensor with a point-of-care feature is the most expected device in the nowadays medical diagnostic field. So that, a review of the recent research related to the novel integration of magnetoresistance-based transducers with MNPs for biosensor application is vital for the point-of-care diagnostic development. Hence, the basic principle of biosensors and the giant magnetoresistance (GMR) with exchange bias phenomena are introduced. Furthermore, we provide a review of the cutting edge method in GMR biosensor with spin-valve structure (SV-GMR) which is integrated to MNPs for biomolecule labelling. As review results, among the nano-sized magnetoresistance transducer, the SV-GMR has some predominance, i.e. electrical robustness and moderate magnetoresistance ratio. Meanwhile, as compared to the other proposed MNPs such as pure Fe3O4, Fe2O3, and hybrid Fe3O4-graphene, the core-shell Fe3O4@Ag is potent to be used, which offers not only moderate saturation magnetisation but also good protein affinity, antimicrobial activity, and minimal cytotoxicity. According to the sensor performance comparison, the usage of Fe3O4@Ag for biomolecule labelling in synergy with SV-GMR transducer is prospective to be developed. The Ag shell espouses the protein immobilisation to the surface of the MNPs label that improves the sensor sensitivity. Furthermore, the SV-GMR possessed two modes of the Fe3O4@Ag rapid detection, which are through the moderate voltage change and the switching field shifting. Meanwhile, the concentration increase of Fe3O4@Ag can be well quantified. Moreover, the Fe3O4@Ag/SV-GMR system had a low operating magnetic field with rapid data collection. In conclusion, the Fe3O4@Ag/SV-GMR biosensor system is believed to be applied as a real-time, portable, and cost-effective biosensor.

A Spin-Valve GMR Based Sensor with Magnetite@silver Core-Shell Nanoparticles as a Tag for Bovine Serum Albumin Detection

The availability of rapid-portable instruments to monitor the bovine serum albumin (BSA) concentration is vital in the early detection of disease. In this research, an original biosensing method based on a spin-valve giant magnetoresistance as a transducer and magnetite@silver core–shell magnetic nanoparticles (MNPs) as a tag has been proposed for detecting BSA. The synthesized magnetite@silver MNPs tag size is ≈15 nm with ≈52 emu g−1 of saturation magnetization. This proposed biosensor performs rapid detection of BSA through the shifting of the switching magnetic field and the increasing of the output voltage. The effectivity of the sensor in monitoring the BSA concentration was shown by a strong linear correlation with ≈0.06 mV/(mg/ml) of the sensitivity and ≈0.44 mg ml−1 of the detection limit. Moreover, the measurement result can be acquired rapidly up to 1 min with low external magnetic field assistance. Therefore, this biosensing technique can be promoted as a real-time portable biosensor.

Detection of Fe3O4/PEG nanoparticles using one and two spin-valve GMR sensing elements in wheatstone bridge circuit

Detection of Fe3O4/PEG nanoparticles using one and two spin-valve GMR sensing elements in wheatstone bridge circuit

Clusters of Spin Valve Sensors in 3D Magnetic Field of a Label.

Magnetic field sensors based on the giant magnetoresistance (GMR) effect have a number of practical current and future applications. We report on a modeling of the magnetoresistive response of moving spin-valve (SV) GMR sensors combined in certain cluster networks to an inhomogeneous magnetic field of a label. We predicted a large variety of sensor responses dependent on the number of sensors in the cluster, their types of interconnections, the orientation of the cluster, and the trajectory of sensor motion relative to the label. The model included a specific shape of the label, producing an inhomogeneous magnetic field. The results can be used for the optimal design of positioning devices.

Surface and Subsurface Eddy-Current Imaging With GMR Sensor

The high sensitivity for low-frequency fields makes the tiny giant magnetoresistance (GMR) sensors suitable for high-resolution eddy-current (EC) imaging. In this work, we proposed a novel design of the miniature differential EC probe with a spin-valve GMR sensor for surface and subsurface EC imaging. The differential EC probe consists of a full-bridge GMR chip mounted vertically on the edge of a tiny printed circuit board to reduce the liftoff distance. The excitation signal is perpendicular to the sensing axis of the GMR chip, which makes it sensitive to the unbalance EC signal on the surface of a conductive sample. The miniature EC probe operated at frequencies from 5 to 65 kHz is sensitive to the hidden defects on a flat conductor. The spatial resolution and stability of the device are verified by taking the 2-D scan images over the copper film samples with hidden defects, showing that the spatial resolution is better than 2 mm for the flaw 0.5 mm below the top surface of a multilayer structure. The proposed device is useful in the detection of small surface and subsurface defects, such as cracks, metal loss, and other mechanical damages.

Core-shell Fe3O4@Ag magnetic nanoparticles detection using spin-valve GMR sensing element in the wheatstone bridge circuit

Core–shell Fe3O4@Ag magnetic nanoparticles (MNPs) integrated with a Wheatstone bridge-giant magnetoresistance (GMR) sensor provide access to GMR-based biosensors. The Fe3O4 nanoparticles synthesized using the coprecipitation method demonstrated 77 emu g−1 of magnetization saturation (M S), 51 Oe of coercivity (H C), and particle size of 11 nm. Furthermore, core–shell Fe3O4@Ag MNPs prepared by the aqua-solution method possessed 53 emu g−1 of M S, 145 Oe of H C, and 17 nm of particle size. This high M S of nanoparticles not only offer a large induced magnetic field but is sufficient for particle penetration within the biofilms. It was discovered that the sensor can distinguish between the bare Fe3O4 with the Fe3O4@Ag nanoparticles through an output voltage increase corresponding to a decrease in M S. The output signal of the sensor responds linearly to an increase in the core–shell Fe3O4@Ag nanoparticle concentration, owing to an increase in the induced-field. The sensor exhibits better sensitivity when applied in detecting less than 2 g L−1 of nanoparticle concentration, that is, 0.76 mV per unit of concentration (g/L).

An aptamer-based magnetic flow cytometer using matched filtering

Facing unprecedented population-ageing, the management of noncommunicable diseases (NCDs) urgently needs a point-of-care (PoC) testing infrastructure. Magnetic flow cytometers are one such solution for rapid cancer cellular detection in a PoC setting. In this work, we report a giant magnetoresistive spin-valve (GMR SV) biosensor array with a multi-stripe sensor geometry and matched filtering to improve detection accuracy without compromising throughput. The carefully designed sensor geometry generates a characteristic signature when cells labeled with magnetic nanoparticles (MNPs) pass by thus enabling multi-parametric measurement like optical flow cytometers (FCMs). Enumeration and multi-parametric information were successfully measured across two decades of throughput (37 — 2730 cells/min). 10-μm polymer microspheres were used as a biomimetic model where MNPs and MNP-decorated polymer conjugates were flown over the GMR SV sensor array and detected with a signal-to-noise ratio (SNR) as low as 2.5 dB due to the processing gain afforded by the matched filtering. The performance was compared against optical observation, exhibiting a 92% detection efficiency. The system achieved a 95% counting accuracy for biomimetic models and 98% for aptamer-based pancreatic cancer cell detection. This system demonstrates the ability to perform reliable flow cytometry toward PoC diagnostics to benefit NCD control plans.

Giant magnetoresistive biosensors for real-time quantitative detection of protease activity

Proteases are enzymes that cleave proteins and are crucial to physiological processes such as digestion, blood clotting, and wound healing. Unregulated protease activity is a biomarker of several human diseases. Synthetic peptides that are selectively hydrolyzed by a protease of interest can be used as reporter substrates of unregulated protease activity. We developed an activity-based protease sensor by immobilizing magnetic nanoparticles (MNPs) to the surface of a giant magnetoresistive spin-valve (GMR SV) sensor using peptides. Cleavage of these peptides by a protease releases the magnetic nanoparticles resulting in a time-dependent change in the local magnetic field. Using this approach, we detected a significant release of MNPs after 3.5 minutes incubation using just 4 nM of the cysteine protease, papain. In addition, we show that proteases in healthy human urine do not release the MNPs, however addition of 20 nM of papain to the urine samples resulted in a time-dependent change in magnetoresistance. This study lays the foundation for using GMR SV sensors as a platform for real-time, quantitative detection of protease activity in biological fluids.

Manipulation of free-layer bias field in giant-magnetoresistance spin valve by controlling pinned-layer thickness

The manipulation of the bias field of the free-layer in giant magnetoresistance spin-valves is of great importance in sensor applications because this feature dominantly affects the low-field sensitivity of magnetoresistance. In this study, it is demonstrated that the bias field of the free-layer can be manipulated by controlling the thickness of the pinned-layer deposited afterward. The key to success is the utilization of the magnetostatic interactions between the free-poles formed on the Néel walls in both free- and pinned-layers. Magnetostatic interactions play a role in stabilizing the antiparallel magnetization state and hence in suppressing the magnetization switching of the free-layer from an antiparallel to a parallel state. A nearly zero bias field is achieved for a Ta-buffered sample with a pinned-layer thickness of 1.75 nm, where a very high low-field sensitivity of 7.7 mV/mA·Oe is obtained.

Self-assembly of highly sensitive 3D magnetic field vector angular encoders.

Novel robotic, bioelectronic, and diagnostic systems require a variety of compact and high-performance sensors. Among them, compact three-dimensional (3D) vector angular encoders are required to determine spatial position and orientation in a 3D environment. However, fabrication of 3D vector sensors is a challenging task associated with time-consuming and expensive, sequential processing needed for the orientation of individual sensor elements in 3D space. In this work, we demonstrate the potential of 3D self-assembly to simultaneously reorient numerous giant magnetoresistive (GMR) spin valve sensors for smart fabrication of 3D magnetic angular encoders. During the self-assembly process, the GMR sensors are brought into their desired orthogonal positions within the three Cartesian planes in a simultaneous process that yields monolithic high-performance devices. We fabricated vector angular encoders with equivalent angular accuracy in all directions of 0.14°, as well as low noise and low power consumption during high-speed operation at frequencies up to 1 kHz.

Property of Giant Magnetoresistance-Spin Valve NiO/NiFe/Cu/NiFe Multilayer According to Width Variation and the Development of a Self-Driving Spin-Device

Property of Giant Magnetoresistance-Spin Valve NiO/NiFe/Cu/NiFe Multilayer According to Width Variation and the Development of a Self-Driving Spin-Device