Giant Magnetoresistive Sensors Research Articles

The influence on hysteresis from the ending pinning design of GMR free layer

With the discovery of Giant Magneto Resistance (GMR), the spin valve GMR sensor based on the effect gets developed rapidly in all areas. However, there is always error in measuring result of spin valve GMR sensor because of hysteresis, which will limit widespread use of the sensor. Started with sensor structure, the paper puts coercivity as an important index to evaluate hysteresis characteristics. And a new method is proposed to reduce the hysteresis, namely with anti-ferromagnetic materials to pin the end of free layer. On this basis, the influence of pinning shape and pinning angle on hysteresis is focused research by theory and simulation at the same time. The results show that with the constant pinned area, the hysteresis is irrelevant with pinning shape, and different pinning angle will induce the change of hysteresis characteristic. The investigation will establish a theoretical basis for designing a low coercivity spin valve GMR sensor.

Mobile ions generated by external direct current electric field influence direct current measurement of giant magnetoresistance current sensors

The application of giant magnetoresistance (GMR) current sensors in power grid and other industrial fields has a great prospect benefitting from their wide bands, high sensitivity, and good linearity. This paper studies the influence of mobile ions on current measurement of GMR sensor under high external electric field. The R-H curves of GMR multilayer sensor were depicted when the external electric and magnetic fields were both changed under three orthogonal electric field orientations. The experiment results indicate slightly varying resistances of GMR sensor when the external electric field was changed, and simulation analysis reveals that the resistance variation is attributed to the movement of surface ions under high external electric field. Therefore, a Faraday box is essential for GMR sensors to avoid interferences under high-strength field conditions, which is especially significant for their application as current sensors of the power grid.

Imperceptible magnetoelectronics.

Future electronic skin aims to mimic nature’s original both in functionality and appearance. Although some of the multifaceted properties of human skin may remain exclusive to the biological system, electronics opens a unique path that leads beyond imitation and could equip us with unfamiliar senses. Here we demonstrate giant magnetoresistive sensor foils with high sensitivity, unmatched flexibility and mechanical endurance. They are <2 μm thick, extremely flexible (bending radii <3 μm), lightweight (≈3 g m−2) and wearable as imperceptible magneto-sensitive skin that enables proximity detection, navigation and touchless control. On elastomeric supports, they can be stretched uniaxially or biaxially, reaching strains of >270% and endure over 1,000 cycles without fatigue. These ultrathin magnetic field sensors readily conform to ubiquitous objects including human skin and offer a new sense for soft robotics, safety and healthcare monitoring, consumer electronics and electronic skin devices.

Direct transfer of magnetic sensor devices to elastomeric supports for stretchable electronics.

A novel fabrication method for stretchable magnetoresistive sensors is introduced, which allows the transfer of a complex microsensor systems prepared on common rigid donor substrates to prestretched elastomeric membranes in a single step. This direct transfer printing method boosts the fabrication potential of stretchable magnetoelectronics in terms of miniaturization and level of complexity, and provides strain-invariant sensors up to 30% tensile deformation.

Magnetic Current Imaging of a TSV short in a 3D IC

In this paper we show Magnetic Field Imaging (MFI) is the best method for Electric Fault Isolation (EFI) of short failures in 2.5/3D Through Silicon Via (TSV) devices in a true non-destructive way by imaging the current path. To confirm the failing locations and to do Physical Failure Analysis (PFA), a Dual Beam-Plasma FIB (DB-PFIB) system was used for cross sectioning and volume analysis of the TSV structures and high resolution imaging of the identified defects. Magnetic Current Imaging (MCI) is a sub technique of MFI which has been used by the semiconductor industry for more than a decade to find electrical shorts and leakage paths and which has the capability to “look through” all materials typically used in the semiconductor industry, allowing global imaging without the need for physical de-processing [1, 2, 3]. MCI utilizes two types of sensors: a Superconducting Quantum Interference Device (SQUID) sensor for low current and large working distances and a Giant Magneto Resistance (GMR) sensor for sub micron resolution current imaging at wafer/die level [3]. The sample investigated in this work is a triple-layer stack, in which 2 layers of 50 μm thick test chip (Chip 1 and Chip 2 in Figure 1) were assembled on a 200 μm thick bottom chip (Chip 0 in Figure 1). The test chips were manufactured by imec's standard 65 nm CMOS Back End of Line (BEOL) process, 5×50 μm via-middle TSV technology [4], and fine pitch micro bumping process [6]. Further details of the test vehicle and assembly process can be found elsewhere [5]. The sample had a short between daisy chain a1 and a2, which were supposed to be electrically separated. The probe tests that was used for this experiment is shown in Table 1. The signal was injected into the respective daisy chains by probing V+ to V− on the bottom chip. To send a signal between daisy chain a1 and a2 one could probe V− to V− and V+ to V+. The MCI scans were done using the GMR sensor only. The sample was attached to a vacuum chuck and raster scanned. From Fig. 2 one can see that the current enters the top layer (Chip 2) at TSV 18 and goes back down again to Chip 1 at TSV 28. Since the sample clearly has multiple shorts, the short located at TSV pair 23 was chosen for PFA using the PFIB. A short is found between the 2 BEOL layers of Chip 1, causing the current to leak into Chip 2 (Fig. 3).

High Temperature Anisotropic Magnetoresistive (AMR) Sensors

Magnetic field sensors are employed in down-hole oil, gas, and geothermal well-drilling applications for azimuth sensing, orientation/rotation sensing, and magnetic anomaly detection. Key requirements of these applications include high measurement accuracy in the near-DC frequency regime, high-operating temperatures, high mechanical shock and vibration, and severe size constraints. Silicon manufacturing processes enable the development of rugged components with small size compatible with assembly processes used for adjacent electronics in hermetically sealed hybrid and/or ceramic packages. Silicon-based magnetic sensors include Anistotropic Magnetoresistive (AMR), Giant Magnetoresistive (GMR) and Tunnelling Magnetoresistive (TMR) sensors. Commercially available GMR and TMR sensors generally cannot be operated much above 150°C. While GMR and TMR have enabled great areal density growth for magnetic recording industry over the past two decades, AMR sensors provide high accuracy measurements in the near-DC regime above 150°C. This is in part due to simplicity of their construction, but also due to their low noise characteristics at low frequencies compared to GMR and TMR. This paper will describe the extension of Honeywell's low noise AMR sensors into high temperature regime up to 225°C. Sensors being reported have room temperature bridge resistance of ~700 Ω, open loop sensitivity of ~2.5 mV/V-Gauss, with a temperature coefficient of sensitivity of −2500 ppm/°C. The low-frequency minimum detectable field monotonically increases with increasing temperature. At room temperature it is ~2.2 μG/√Hz@1 Hz and reaches a value of ~26μG/√Hz@1 Hz at 225°C. Signal and noise density both increase with increasing sensor bias voltage such that low-frequency signal-to-noise ratio does not vary in the bias voltage range of 2.5 V to 10V. These sensors have also been configured in a closed loop format using low noise electronics. Measurements of closed loop transfer function in the range of ±0.8 Gauss were made. The sensor was placed in a thermal chamber while the feedback electronics were placed outside at room temperature. The linearity of the transfer function is quite excellent; deviation from linearity increases monotonically with increasing temperature reaching &amp;lt; 0.002% of full scale or 29 μGauss at 225°C. Closed loop operation of a typical sensor shows 1-σ measurement variability of 21 μGauss at 220°C. By a combination of averaging and closed loop operation an input step from 0 to 75 μGauss is replicated at the output to within 0.1 μG at 225°C.

Hysteretic Modeling of Output Characteristics of Giant Magnetoresistive Current Sensors

Current sensing based on the giant magnetoresistance (GMR) effect has been gaining attention due to its outstanding merits. In this paper, hysteretic models of the output characteristics of GMR sensors are presented, and corresponding algorithms are successfully applied to practical GMR sensors to compensate for measurement error due to hysteresis, which is particularly important for high-frequency applications. A 91% decrease in nonlinear error is achieved by the proposed advanced hysteretic model, and the applicable frequency range of the GMR sensor can be extended to around the cutoff frequency of sensor hardware, which is nearly 50 times larger than that based on the conventional linear models. This paper provides optimized solutions for GMR current sensors in different frequency ranges.

Enhanced Signal-to-Noise Ratio in Current-Perpendicular-to-Plane Giant-Magnetoresistance Sensors by Suppression of Spin-Torque Effects

We report a ~4 dB enhancement in signal-to-noise ratio of current-perpendicular-to-the-plane giant magnetoresistance sensors with Heusler alloy magnetic layers due to insertion of an In-Zn-O electrically conductive oxide into an Ag-based metallic spacer layer. Signal and noise device characterization at various magnetic bias conditions shows that this enhancement arises from an increase in magnetoresistance signal at higher bias currents without spin-torque-induced magnetic instability. The best obtained signal-to-noise, over 30 dB for 1 GHz bandwidth, promises application of these sensors in magnetic recording heads for hard-disk drives at recording densities above 1 Tbit/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> .

Rotating Field EC-GMR Sensor for Crack Detection at Fastener Site in Layered Structures

Eddy current-based techniques have been investigated for the inspection of embedded cracks under fastener heads in riveted structures. However, these techniques are limited in their ability to detect cracks that are not perpendicular to induced current flows. Further, the presence of a steel fastener of high permeability produces a strong signal that masks relatively smaller indication from a crack. In this paper, a rotating electromagnetic field is designed to rotate the applied magnetic fields and related eddy currents electrically so that the sensor shows uniform sensitivity in detecting cracks in all radial directions around fastener sites. Giant magnetoresistive sensors are employed to image the normal component of this rotating field, to detect different crack orientations at aluminum and ferromagnetic fastener sites. Numerical model-based studies and experimental validation are presented.

Microfluidics for the Rapid Detection of Pathogens Using Giant Magnetoresistance Sensors

This paper presents an integrated solution toward an on-chip microfluidic diagnostic system using the magnetically induced motion of functionalized magnetic microparticles (MPs) in combination with giant magnetoresistance (GMR) sensors. The innovative aspect of the proposed method is that the induced velocity on MPs in suspension, while imposed to a magnetic field gradient, is inversely proportional to their volume. Specifically, a velocity variation of suspended functionalized MPs inside a detection microchannel with respect to a reference velocity, specified in a parallel reference microchannel, indicates an increase in their nonmagnetic volume. This volumetric increase of the MPs is caused by the binding of pathogens (e.g., bacteria) to their functionalized surface. The new formed compounds, which have an increased nonmagnetic volume, are called loaded MPs (LMPs). Experiments with functionalized MPs and LMPs with Escherichia coli attached to their surface were conducted as a proof of concept. Their movement was demonstrated optically by means of a microscope with a mounted CCD camera as well as by measuring the resistance change of the integrated GMR sensors.

Magnetic Leakage Testing Using Linearly Integrated Hall and GMR Sensor Arrays to Inspect Inclusions in Cold-Rolled Strips

Inclusions in steel are entrapped foreign materials, which may be metallic or non-metallic; they may be distributed throughout the volume of steel strips during the rolling process and may propagate as narrow cracks in steel strips. In this paper, for inspecting inclusions in steel strips, we propose the use of magnetic flux leakage testing involving a magnetizer and magnetic field sensor arrays, such as linearly integrated Hall sensors (LIHaS) in differential mode and linearly integrated giant magnetoresistance sensors (LIGiS). In the proposed method, images of the distribution of Hall voltage (V H ) and giant magnetoresistance voltage (V G ) due to the magnetic leakage field around an inclusion are obtained by means of LIHaS and LIGiS, respectively. The magnetic leakage field images are obtained at different magnetization directions relative to the rolling direction by rotating the magnetizer. Different data processing techniques, such as differentiation, noise reduction, and fusion of image data of the inclusion obtained by LIHaS and LIGiS, are applied to enhance the detectability of the inclusion. The effectiveness of the proposed technique was experimentally verified by detecting actual inclusions situated in the rolling direction in a cold-rolled steel strip.

Multiscale modeling in micromagnetics: Existence of solutions and numerical integration

Various applications ranging from spintronic devices, giant magnetoresistance sensors, and magnetic storage devices, include magnetic parts on very different length scales. Since the consideration of the Landau–Lifshitz–Gilbert equation (LLG) constrains the maximum element size to the exchange length within the media, it is numerically not attractive to simulate macroscopic parts with this approach. On the other hand, the magnetostatic Maxwell equations do not constrain the element size, but cannot describe the short-range exchange interaction accurately. A combination of both methods allows one to describe magnetic domains within the micromagnetic regime by use of LLG and also considers the macroscopic parts by a nonlinear material law using the Maxwell equations. In our work, we prove that under certain assumptions on the nonlinear material law, this multiscale version of LLG admits weak solutions. Our proof is constructive in the sense that we provide a linear-implicit numerical integrator for the multiscale model such that the numerically computable finite element solutions admit weak H1-convergence (at least for a subsequence) towards a weak solution.

Construction and validation of simple magnetic nanoparticle detector based on giant magnetoresistive effect

The finding of giant magnetoresistive(GMR) effect develops a new field for the sensing application with magnetic nanoparticles(MNPs) labeling. A convenient GMR sensor was built with a permanent magnet to excite the MNPs in this work. The sensing element contained a Wheatstone bridge with the GMR material as one of its branches. The magnetic field from MNPs unbalanced the Wheatstone bridge. After being amplified, the output signals were recorded. The construction and optimization of the magnetoresistive sensing platform were discussed in detail. The detection of three kinds of MNPs validated the performance of the proposed GMR sensor. The sensor showed a fast response to the addition or removal of MNPs. Because of its simplicity, this kind of GMR sensor can be developed in a routine laboratory. The finding of this new GMR sensor will promote the development of the method of probing biomolecules and the study on the biomolecular interaction after being labeled magnetically.

Comparative analysis of several GMR strip sensor configurations for biological applications

Giant magnetoresistance (GMR) sensor strips are fabricated and modeled with the purpose of detecting a uniform distribution of Fe3O4 magnetic nanoparticles (MNPs) for biological application. We find that the free and fixed layers of GMR sensors play a dominant role in exciting the MNPs, and consequent MNP stray fields lead to increased free layer susceptibility. This result persists even when MNPs are confined to the interior of the sensing region but is enhanced when MNPs are allowed near exterior edges. Our analysis includes three different fabrication configurations on two different spin valve film stacks and agrees well with experimental results.

Study on Localization With Magnetic Liquids Based on GMR

Giant magnetoresistance (GMR) effect is a phenomenon that the resistivity of the magnetic material will be a huge variation with external magnetic field than without external magnetic field. The GMR sensor is more sensitive to magnetic field than other sensors to detect the magnetic liquids. This paper describes an approach to locate the magnetic liquids by measuring the magnetic field around the magnetic liquids based on the GMR effect. The simulations and experimental models of localization system are built to analyze the results and errors. The sensitivity of the GMR sensor in our study is nearly 120 A/m per centimeter. The experimental results have shown that this approach can locate the three-dimensional coordinates of the magnetic liquids within a 10 cm wide and 20 cm high range. Considering the difficulties of human body nontrauma lesion localization, it can be used to locate the lesion with magnetic liquids.

Defect classification using phase lag information of EC-GMR output

Eddy current testing (ECT) is an important method in non-destructive testing. Classification of defects is a challenging problem in ECT. In this paper, we propose a method in which the phase lag is adopted, and in order to detect the deep sub-surface defects, giant magneto resistance sensor is chosen to detect the magnetic field influenced by the defects. The phase of the magnetic field intensity is used to help classify the defects. Results from the experiments carried out show that the method is effective.

Non-destructive testing of inclusions in cold-rolled strip steels using hall and giant magnetoresistance sensor arrays

Inclusions in steel are entrapped foreign materials, which may be metallic or non-metallic. The inclusions disrupt the structural homogeneity of the steel and affect its mechanical properties. Moreover, nonmetallic inclusions such as calcium and magnesium can be propagated as narrow cracks in steel strips after the rolling process. In this paper, we propose the use of magnetic flux leakage testing involving the fusion of images obtained by arrays of linearly integrated Hall sensors (LIHaS) and linearly integrated giant magnetoresistance sensors (LIGiS) for inspecting inclusions in steel strips. In the proposed method, the normal (Bz) and tangential (Bx) components of the magnetic leakage field around an inclusion are detected by means of LIHaS and LIGiS,respectively. The images of the magnetic leakage fieldwere obtained at different scanning (magnetization) directions relative to the rolling direction. The Bz and Bx images of the inclusion are then combined by using image fusion technique to enhance the detection of the inclusion. The effectiveness of the proposed image fusion technique was experimentally verified by using LIHaS and LIGiS images of an inclusion in a cold-rolled steel strip.

Two-step thinning fabrication of giant magnetoresistance sensors for flexible applications

Spin valve giant magnetoresistance (GMR) sensors were prepared by a two-step thinning method combining grind thinning and inductively coupled plasma (ICP) etching together. The fabrication processes of front GMR sensors and backside ICP etching were described in detail. Magnetoresistance ratio of about 4.24% and coercive field of approximately 11 Oe were obtained in a tested bendable GMR sensor. The variations of the magnetic property in GMR sensors were explained mainly from the temperature, ion beam damage and mechanical damage generated by the fabrication process.

Controlled Trapping and Detection of Magnetic Particles by a Magnetic Microactuator and a Giant Magnetoresistance (GMR) Sensor

This paper presents the design and testing of an integrated micro-chip for the controlled trapping and detection of magnetic particles (MPs). A unique magnetic micro-actuator consisting of square-shaped conductors is used to manipulate the MPs towards a giant magnetoresistance (GMR) sensing element which rapidly detects the majority of MPs trapped around the square-shaped conductors. The ability to precisely transport a small number of MPs in a controlled manner over long distances by magnetic forces enables the rapid concentration of a majority of MPs to the sensing zone for detection. This is especially important in low concentration samples. The conductors are designed in such a manner so as to increase the capture efficiency as well as the precision and speed of transportation. By switching current to different conductors, MPs can be manipulated and immobilized on the innermost conductor where the GMR sensor is located. This technique rapidly guides the MPs towards the sensing zone. Secondly, for optimum measurement capability with high spatial resolution the GMR sensor is fabricated directly underneath and all along the innermost conductor to detect the stray fields originating from the MPs. Finally, a microfluidic channel is fabricated on top of this micro-chip. Experiments inside the microchannel were carried out and the MPs were successfully trapped at the sensing area.

External-field-free magnetic biosensor

In this paper, we report a magnetic nanoparticle (MNP) detection scheme without the presence of any external magnetic field. The proposed magnetic sensor uses a patterned groove structure within the sensor so that no external magnetic field is needed to magnetize the MNPs. An example is given based on a giant magnetoresistance (GMR) sensing device with a spin valve structure. For this structure, the detection of MNPs located inside the groove and near the free layer is demonstrated under no external magnetic field. Micromagnetic simulations are performed to calculate the signal to noise level of this detection scheme. A maximum signal to noise ratio (SNR) of 18.6 dB from one iron oxide magnetic nanoparticle with 8 nm radius is achieved. As proof of concept, this external-field-free GMR sensor with groove structure of 200 nm × 200 nm is fabricated using a photo and an electron beam integrated lithography process. Using this sensor, the feasibility demonstration of the detection SNR of 9.3 dB is achieved for 30 μl magnetic nanoparticles suspension (30 nm iron oxide particles, 1 mg/ml). This proposed external-field-free sensor structure is not limited to GMR devices and could be applicable to other magnetic biosensing devices.