Spin Valve Sensors Research Articles

Spin valves with Conetic based synthetic ferrimagnet free layer

The detection of low magnetic field requires hysteresis-free magnetoresistive sensor with high sensitivity. Giant magnetoresistance spin valve (SV) sensors with CoFeB based synthetic antiferromagnetic (SAF) pinned layer and Conetic based synthetic ferrimagnet (SF) free layer are developed in this paper. Low free layer coercivity (HC) of 0.27 Oe is achieved through adapting synthetic antiferromagnetic pinned layer in SV with Conetic single free layer. In SV with SAF pinned layer and SF free layer, the MR ratio, HC, switching field (HSW) and offset field (HO) increase with the thickness of the Conetic alloy layer right above the Cu spacer. The HC, HO, and HSW show oscillatory behavior with the Ru spacer thickness in the SF free layer. Through introducing CoFeB/SF free layer, an increased sensitivity of 0.27%/Oe and a small coercivity of 0.3 Oe are attained, which holds promise for low field sensing applications.

Spintronic Memristor as Interface Between DNA and Solid State Devices

Recently biomolecular computing platforms have been widely investigated with great potentials in both biomedical research and practices, such as using molecular structures of DNA to present the data bits and to operate the logic. Emerging CMOS/molecular hybrid (CMOL) circuitry demonstrates many overwhelming advantages compared with pure biomolecular circuitry, including the design flexibility and compatibility with the traditional CMOS process. In this work, spintronic devices are utilized to detect the spatial information of DNA by translating the magnetic signals associated with the DNA pieces to electrical signals. The physical mechanisms and sensing performances of various magnetic sensor structures are discussed and evaluated, including giant magneto-resistance (GMR) spin valve sensors, tunnel magneto-resistance (TMR) sensors and our newly proposed spintronic GMR/TMR memristor sensors. A on-chip readout scheme is also proposed with a telecommunication method-frequency division multiplexing (FDM) technique to efficiently transmit useful information and filtrate the noise which could achieve high SNR up to 70 dB. The new approach can be adopted as not only the interface between the DNA structure and CMOS circuitry, but also a promising sensing mechanism in the DNA hybridization detection technique.

Simple Mathematical Operation-Based Calibration Method for Giant Magnetoresistive Current Sensor Applying B-Spline Modeling

Nowadays, giant magnetoresistance (GMR)-based sensor technology is widely used in numerous industrial applications. However, an inexpensive and efficient method for compensating the nonlinear output of GMR spin valve sensors is still under the industrial prospect, which limits the application of the GMR sensors. By applying the B-Spline modeling, excellent results can be achieved; nevertheless, the sensor has to be calibrated in the whole temperature range. In this paper, a simple mathematical operation-based calibration method for GMR spin valve-type current sensors applying B-Spline modeling is introduced. Instead to measure dozens of temperatures and per temperature several hundreds of points, this method allows a calibration for each further sensor applying B-Spline modeling with only two temperatures and per temperature only 15 points. In comparison with the original calibration measurement over temperature for B-Spline modeling, the new calibration with minimized efforts shows outstanding measurement accuracy of ±0.6% at a nominal current of 50 A in a temperature range from −20 °C to 80 °C.

Stretchable Spin Valve with Stable Magnetic Field Sensitivity by Ribbon-Patterned Periodic Wrinkles.

A strain-relief structure by combining the strain-engineered periodic wrinkles and the parallel ribbons was employed to fabricate flexible dual spin valves onto PDMS substrates in a direct sputtering method. The strain-relief structure can accommodate the biaxial strain accompanying with stretching operation (the uniaxial applied tensile strain and the induced transverse compressive strain due to the Poisson effect), thus significantly reducing the influence of the residual strain on the giant magnetoresistance (GMR) performance. The fabricated GMR dual spin-valve sensor exhibits the nearly unchanged MR ratio of 9.9%, magnetic field sensitivity up to 0.69%/Oe, and zero-field resistance in a wide range of stretching strain, making it promising for applications on a conformal shape or a movement part.

Ultra-Compact 100 × 100 μm² Footprint Hybrid Device with Spin-Valve Nanosensors.

Magnetic field mapping with micrometric spatial resolution and high sensitivity is a challenging application, and the technological solutions are usually based on large area devices integrating discrete magnetic flux guide elements. In this work we demonstrate a high performance hybrid device with improved field sensitivity levels and small footprint, consisting of a ultra-compact 2D design where nanometric spin valve sensors are inserted within the gap of thin-film magnetic flux concentrators. Pole-sensor distances down to 400 nm are demonstrated using nanofabrication techniques combined with an optimized liftoff process. These 100 × 100 μm pixel sensors can be integrated in modular devices for surface mapping without moving parts.

Integration of Magnetoresistive Sensors With Atomic Force Microscopy Cantilevers for Scanning Magnetoresistance Microscopy Applications

This paper reports on the development and the fabrication of a novel atomic force microscopy cantilever with integrated magnetoresistive sensors at its tip for scanning magnetoresistance applications. A 2-D scan over an area of $100\times 100~\mu \text{m}^{2}$ simultaneously generates topographic and magnetic maps with a spatial resolution better than 1 $\mu \text{m}$ . The magnetic field is detected using two spin valve sensors with a field sensitivity of 61.33 $\mu \text{V}$ /Oe. The device operation is demonstrated with a ferromagnetic test feature of $1\times 20~\mu \text{m}^{2}$ . The results demonstrate that this scanning technique is capable of accurately quantifying the absolute magnitude of magnetic stray fields.

Detecting Antibody-Labeled BCG MNPs Using a Magnetoresistive Biosensor and Magnetic Labeling Technique

Tuberculosis is still a major global health concern, causing the estimated death of 1.5 million people per year and being associated with high morbidity. The development of point-of-care diagnostic tools for tuberculosis is mandatory, especially because the fast and accurate detection of the slow-growing Mycobacterium tuberculosis by the conventional diagnostic tests is difficult.The objective of this work was to develop the first steps to achieve a portable method for the diagnosis of tuberculosis, by a sandwich-immunoassay combined with magnetoresistive biochip technology.With the purpose of conjugating 250 nm streptavidin-coated magnetic nanoparticles with anti- M.tuberculosis biotinylated antibodies, Mycobacteriumbovis Bacillus Calmette-Guérin was used as a surrogate for M. tuberculosis bacteria. After magnetic capture, target bacteria were brought in contact with the surface of the magnetoresistive biochip previously functionalized with a secondary anti-M. tuberculosis antibody. Magnetically labeled cells were detected by an array of spin-valve sensors, which change their electrical resistance in the presence of the fringe field of the magnetic particles. Optimization studies on the efficiency of the magnetic capture and further recognition of the bacteria by the secondary antibody on the biochip surface were conducted. The results on the magnetoresistive biochip showed a clear difference in the signal between specific and control (non-specific) sensors, suggesting the usefulness of this technique as a potential biorecognition tool for the development of a point-of-care diagnostic method for tuberculosis.

Detecting Antibody-Labeled BCG MNPs Using a Magnetoresistive Biosensor and Magnetic Labeling Technique

Tuberculosis is still a major global health concern, causing the estimated death of 1.5 million people per year and being associated with high morbidity. The development of point-of-care diagnostic tools for tuberculosis is mandatory, especially because the fast and accurate detection of the slow-growing Mycobacterium tuberculosis by the conventional diagnostic tests is difficult.The objective of this work was to develop the first steps to achieve a portable method for the diagnosis of tuberculosis, by a sandwich-immunoassay combined with magnetoresistive biochip technology.With the purpose of conjugating 250 nm streptavidin-coated magnetic nanoparticles with anti- M.tuberculosis biotinylated antibodies, Mycobacteriumbovis Bacillus Calmette-Guérin was used as a surrogate for M. tuberculosis bacteria. After magnetic capture, target bacteria were brought in contact with the surface of the magnetoresistive biochip previously functionalized with a secondary anti-M.tuberculosis antibody. Magnetically labeled cells were detected by an array of spin-valve sensors, which change their electrical resistance in the presence of the fringe field of the magnetic particles. Optimization studies on the efficiency of the magnetic capture and further recognition of the bacteria by the secondary antibody on the biochip surface were conducted. The results on the magnetoresistive biochip showed a clear difference in the signal between specific and control (non-specific) sensors, suggesting the usefulness of this technique as a potential biorecognition tool for the development of a point-of-care diagnostic method for tuberculosis.

Strong ferromagnetically-coupled spin valve sensor devices for droplet magnetofluidics.

We report a magnetofluidic device with integrated strong ferromagnetically-coupled and hysteresis-free spin valve sensors for dynamic monitoring of ferrofluid droplets in microfluidics. The strong ferromagnetic coupling between the free layer and the pinned layer of spin valve sensors is achieved by reducing the spacer thickness, while the hysteresis of the free layer is eliminated by the interplay between shape anisotropy and the strength of coupling. The increased ferromagnetic coupling field up to the remarkable 70 Oe, which is five-times larger than conventional solutions, brings key advantages for dynamic sensing, e.g., a larger biasing field giving rise to larger detection signals, facilitating the operation of devices without saturation of the sensors. Studies on the fundamental effects of an external magnetic field on the evolution of the shape of droplets, as enabled by the non-visual monitoring capability of the device, provides crucial information for future development of a magnetofluidic device for multiplexed assays.

Real-Time Monitoring of Magnetic Nanoparticles Diffusion in Lateral Flow Microporous Membrane Using Spin Valve Sensors

This paper investigates real-time monitoring and detection of 10.5 nm magnetic nanoparticles (MNPs) (Fe3O4) flowing in a nitrocellulose (NC) membrane over two spin valve sensors. The fabricated device gives information on the concentration of MNP flowing through the NC membrane of a typical lateral flow test, and gives relevant dynamic characteristics, such as flow rate and MNP distribution on capillary-driven systems. Sensor output varies linearly with MNP concentration. Experiments show average flow rates varying from 0.2 to 0.3 mm/s depending on MNP concentration. Signal shape [ ${V}$ ( ${t}$ )] gives information on MNP spatial distribution at the particle front flowing over the sensor.

Optimized Monolithic 2-D Spin-Valve Sensor for High-Sensitivity Compass Applications

We have designed and fabricated 2-D giant magnetoresistance spin-valve sensors on the basis of exchange-biased NiFe-CoFe/Cu/CoFe/IrMn nanolayers in monolithic integration for high-sensitivity compass applications. For a maximum signal-to-noise ratio, we have realized a focused double full-bridge layout with an antiparallel alignment of the pinned layer magnetization for neighboring meanders. This precise alignment is achieved with microscopic resolution by laser heating and subsequent in-field cooling. Striving for high-signal sensitivity and low hysteresis, we study in detail how the geometry of the constituent single meanders influences their magnetic structure and the resulting electronic transport properties. The investigated geometrical parameters include stripe width, stripe length, U-turn material, and total meander length. Moreover, the influence of the relative alignment between reference magnetization and shape anisotropy is studied. We compare our experimental results to the predictions of tailored micromagnetic simulations. Applying the best-suited meander geometry, we demonstrate how the developed 2-D sensor may be readily employed to determine the direction of small magnetic fields, such as that of the Earth, as a 2-D vector with high spatial (~1mm) and temporal (~1ms) resolution.

Dynamical Detection of Magnetic Nanoparticles in Paper Microfluidics With Spin Valve Sensors for Point-of-Care Applications

In the pursuit of point-of-care systems that present high sensitivity and fast turnaround time, lateral flow is considered a remarkable promising class of in vitro diagnostics, and popularized by the pregnancy tests. A variety of lateral flow tests have emerged able to provide cardiac panels in emergency conditions, infectious disease screening, drug abuse, among others. However, most commercial tests only provide qualitative results, limiting their applicability for quantification thresholds of most biomarkers. In this paper, we propose a novel detection architecture for quantification of biomolecules labeled with magnetic nanoparticles employing two spin valve sensors. The main goal of this paper is to demonstrate that this architecture can detect and monitor MNPs flowing in a LFT strip for detection and quantification of different solutions varying in MNP concentration. In addition, we present preliminary analytical results that support our strategy for achieving biomolecular quantification using the proposed solution.

Nanoscale Magnetic Tunnel Junction Sensing Devices With Soft Pinned Sensing Layer and Low Aspect Ratio

Highly sensitive nanosensors with high spatial resolution provide the necessary features for high-accuracy imaging of isolated magnetic nanoparticles or mapping of magnetic fields. Here, we fabricated nanosensor devices based on MgO-magnetic tunnel junctions with soft pinned sensing layer. The exchange interaction at the free-layer is tuned to yield distinct linear operation ranges for the nanosensors. Circular (diameter D = 120-500 nm) and elliptical pillars with low aspect ratio (120 nm × 130 nm- 120 nm × 200 nm) displaying a linear non-hysteretic transfer curves with tunnel magnetoresistance values up to 143% were obtained. A noticeable improvement in the sensitivity for circular structures from an average value of ~1%/mT up to ~2%/mT is observed with the use of a CoFe/CoFeB/Ta/NiFe/MnIr free-layer. The sensitivity values are almost independent on the size for circular devices, consistent with a linear operation range dominated by the exchange field strength. For elliptical devices, a high sensitivity is also observed, although displaying a dependence on the size, due to a competition with the demagnetizing field. The low-frequency noise features were also addressed revealing a detectivity in the tens of μT/√Hz with Hooge parameters within 1-3 × 10 -9 μm 2 in the linear range. Nevertheless, such high sensitivity values are a major improvement in comparison with those reported previously for nanometric sensors, and extremely competitive with values reported for micrometric spin-valve sensors, with the advantage of providing a reduced device footprint suitable for highly resolved measurements.

Towards sub-200 nm nano-structuring of linear giant magneto-resistive spin valves by a direct focused ion beam milling process

In this work, we present a detailed investigation of a focused ion beam (FIB) assisted nano-structuring process for giant magneto-resistive (GMR) spin valve sensors. We have performed a quantitative study of the dependence of the GMR ratio as well as the sensor resistance on the ion dose, which is implanted in the active region of our sensors. These findings are correlated with the decrease of magneto-resistive properties after micro- and nano-structuring by the FIB and reveal the importance of ion damage which limits the applicability of FIB milling to GMR devices in the low μm range. Deposition of a protective layer (50 nm SiO2) on top of the sensor structure before milling leads to a preservation of the magneto-resistive properties after the milling procedure down to sensor dimensions of ∼300 nm. The reduction of the sensor dimensions to the nanometer regime is accompanied by a shift of the GMR curves, and a modification of the saturation behavior. Both effects can be explained by a micromagnetic model including the magnetic interaction of free and pinned layer as well as the effect of the demagnetizing field of the free layer on the sensor behavior. The results demonstrate that the FIB technology can be successfully used to prepare spintronic nanostructures.

A 256 pixel magnetoresistive biosensor microarray in 0.18μm CMOS.

Magnetic nanotechnologies have shown significant potential in several areas of nanomedicine such as imaging, therapeutics, and early disease detection. Giant magnetoresistive spin-valve (GMR SV) sensors coupled with magnetic nanotags (MNTs) possess great promise as ultra-sensitive biosensors for diagnostics. We report an integrated sensor interface for an array of 256 GMR SV biosensors designed in 0.18 μm CMOS. Arranged like an imager, each of the 16 column level readout channels contains an analog front- end and a compact ΣΔ modulator (0.054 mm2) with 84 dB of dynamic range and an input referred noise of 49 nT/√Hz. Performance is demonstrated through detection of an ovarian cancer biomarker, secretory leukocyte peptidase inhibitor (SLPI), spiked at concentrations as low as 10 fM. This system is designed as a replacement for optical protein microarrays while also providing real-time kinetics monitoring.

Field Detection in Spin Valve Sensors Using CoFeB/Ru Synthetic-Antiferromagnetic Multilayers as Magnetic Flux Concentrators

This work compares the performance of spin valve sensors comprising magnetic flux concentrators (MFCs) composed of Co93Zr3Nb4 (CZN) or (Co70Fe30)80B20-based synthetic-antiferromagnet (SAF) multilayer stacks. In addition, the influence of a tapered MFC tip is also studied. When compared to CZN films, SAFs have the disadvantage of lower magnetic susceptibility (χ = 196 for SAF vs. χ = 753 for CZN), which affects negatively the field gain in gap (10 for SAF vs. 43 for CZN). However, from the overall noise spectrum, one can conclude that the magnetic field detections for sensors incorporating CoFeB/Ru multilayers as MFCs are close to the ones obtained with CZN, being mainly determined by the sensor intrinsic properties instead. For low frequencies, field detection levels at 10 Hz improved from ~ 61 nT/Hz0.5 for single spin valve sensors down to ~ 1.8 nT/Hz0.5 when CZN concentrators with a steep-profile are used.

A Magneto-Nanosensor Immunoassay for Sensitive Detection of Aspergillus Fumigatus Allergen Asp f 1

We report a magneto-nanosensor biochip for fungal detection. The chip is made of arrays of giant magnetoresistive (GMR) spin-valve sensors, and is able to detect protein biomarkers at low concentrations in solutions. As a demonstration, a standard curve for fungal pathogen Asp f 1 was obtained by measuring signals from various concentrations of Asp f 1 spiked in PBS solutions, indicating a detection limit of ~100 pg/ml. Five positive and negative Asp f 1 solution samples were discriminated correctly in blind experiments. Our data suggest that the magneto-nanosensor biochips are very promising as sensitive diagnostic devices for fungal pathogens. Given the generality of the detection scheme used in the magneto-nanosensor, we anticipate that the platform will be very useful for the detection of many types of biomarkers.

Stretchable Spin Valves on Elastomer Membranes by Predetermined Periodic Fracture and Random Wrinkling

The first highly stretchable and sensitive spin valve sensor on elastomeric membranes are demonstrated. The sensor elements exhibit stable GMR behavior up to tensile strains of 29% in in situ stretching experiments and show no fatigue over 500 loading cycles. This remarkable stretchability is achieved by a predetermined periodic fracture mechanism that creates a meander-like pattern upon stretching.

Electrical ammeter based on spin-valve sensor

The present work shows an electrical ammeter for laboratory purpose based on a magnetoresistive (MR) spin-valve (SV) sensor. The proposed ammeter measures a 10 A maximum current and offers a maximum frequency response between 150 and 800 kHz depending on the electronics whole gain. These features are due to the use of a new generation MR-SV current sensor and a conditioning electronics that compensates in frequency and temperature the sensor response. With little adjustments in the electronics and changing the position of the sensor with respect to current carrying conductor, the designed instrument is able to measure higher current levels. The work shows the proposed ammeter with its different subsystems and describes the procedure used to test the instrument. Also a discussion of the obtained experimental results is included.

Influence of Temperature on Structure and Magnetic Properties of Exchange Coupled TbCo/FeNi Bilayers

Among amorphous films of rare earth-transition metal (RE-TM) alloys as exchange-biasing layers in magnetoresistive heads and spin-valve sensors, the amorphous Tb-Co films have most high practical potential. In the present work the influence of the temperature and the heat treatment parameters on the structure and magnetic properties was studied for exchange bias FeNi/Tb35Co65 bilayers annealed in vacuum or a nitrogen flow. A simple explanation of the dependence of the magnetic properties on the temperature and the heat treatment parameters connected with structural changes in each one of the layers was proposed.