Spin-valve Giant Magnetoresistance Sensor Research Articles

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.

Significant manipulation of output performance of a bridge-structured spin valve magnetoresistance sensor via an electric field

Through principles of spin-valve giant magnetoresistance (SV-GMR) effect and its application in magnetic sensors, we have investigated electric-field control of the output performance of a bridge-structured Co/Cu/NiFe/IrMn SV-GMR sensor on a PZN-PT piezoelectric substrate using the micro-magnetic simulation. We centered on the influence of the variation of uniaxial magnetic anisotropy constant (K) of Co on the output of the bridge, and K was manipulated via the stress of Co, which is generated from the strain of a piezoelectric substrate under an electric field. The results indicate that when K varies between 2 × 104 J/m3 and 10 × 104 J/m3, the output performance can be significantly manipulated: The linear range alters from between −330 Oe and 330 Oe to between −650 Oe and 650 Oe, and the sensitivity is tuned by almost 7 times, making it possible to measure magnetic fields with very different ranges. According to the converse piezoelectric effect, we have found that this variation of K can be realized by applying an electric field with the magnitude of about 2–20 kV/cm on a PZN-PT piezoelectric substrate, which is realistic in application. This result means that electric-control of SV-GMR effect has potential application in developing SV-GMR sensors with improved performance.

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.

Bare PCB Inspection System With SV-GMR Sensor Eddy-Current Testing Probe

This paper describes bare printed circuit board (PCB) inspection based on eddy-current testing (ECT) technique with high scanning speed. A high-frequency ECT probe composed of a meander coil as an exciting coil and the spin-valve giant magnetoresistance (SV-GMR) sensor was fabricated and is proposed. The ECT probe was designed based on crack inspection over flat surface, especially suitable for microdefect detection on high-density bare PCB. The ECT signal detected by the SV-GMR sensor was acquired by high-speed A/D converter for applying the signal processing based on digital technique. Harmonic analysis based on Fourier transform was used to analyze the ECT signal at fundamental frequency in order to increase inspection speed and this technique allowed the ECT probe to scan bare PCB, with high sampling frequency and with high-spatial resolution inspection. Experimental results verified the possibility and the performance of the proposed PCB inspection system based on ECT technique

Applications of SV-GMR sensor for detecting micro non-magnetic and ferromagnetic material

This paper describes the application of recognition of micro non-magnetic (conductive) and ferromagnetic materials (Fe) using a Spin-Valve Giant Magneto-Resistance (SV-GMR) as a sensor. High frequency and static magnetic fields were applied to the specimens. The proposed Eddy Current Testing (ECT) probe with a SV-GMR sensor was used to detect the magnetic field variation from the specimens. Experiments were performed to detect both non-magnetic single and array (PbSn, 125 μm radius) using the SVGMR sensor. The analytical results are agreed with the experimental results. Furthermore, the experimental results of magnetic particles (Fe, size of 20 60 μm) were also successfully detected.

Conductive Microbead Array Detection Based on Eddy-Current Testing Using SV-GMR Sensor and Helmholtz Coil Exciter

This paper describes the detection of both single and array conductive microbead (PbSn) by using a spin-valve giant magnetoresistance (SV-GMR) as a sensor and the Helmholtz coil as an exciter based on eddy-current testing (ECT). Experiments were performed to detect a single and array conductive microbead, with three models. Each model consists of 4times4 microbeads but the microbead diameter and pitch were slightly different for each array. The microbead radius was 125 mum and the pitch was 500 mum. The ECT method was used to estimate the position of the centers of the microbeads and the error in the measurement was plotted on a plane. A good level of position resolution has been achieved and the signals were quite clear

Conductive microbead array detection by high-frequency eddy-current testing technique with SV-GMR sensor

Applying of eddy-current testing (ECT) probe with spin-valve giant magnetoresistive (SV-GMR) sensor to high frequency excitation provides the feasibility of micro-defect detection for example high-density printed circuit board inspection. In this paper, the detection of conductive micro-bead with 250 to 760 /spl mu/m diameter, and its feasibility are discussed. The analytical model is discussed and compared with experimental results to verify that the proposed technique is able to detect conductive micro-bead.

Eddy-Current Testing Probe With Spin-Valve Type GMR Sensor for Printed Circuit Board Inspection

This paper proposes an eddy-current testing (ECT) probe composed of a spin-valve giant magnetoresistance (SV-GMR) sensor and a meander coil for the inspection of bare printed circuit board. The SV-GMR sensor serves as a magnetic sensor for the ECT probe to sense the variation of the magnetic field distribution occurred on the printed circuit board. The SV-GMR sensor is used specifically to detect the changing magnetic field distribution occurred at the defect point. The characteristics of the proposed probe are discussed in this paper. The comparisons of signal-to-noise ratios obtained from ECT probe with SV-GMR sensor and with solenoid coil verify that the applying of SV-GMR sensor to the ECT probe can improve the PCB inspection results.

ＥＣＴ応用を目指したＳＶ−ＧＭＲセンサの高周波数微小磁界振幅特性

The eddy-current testing technique is an interesting method for detecting defects on bare printed circuit boards. This technique detects various defects without mechanical stress to PCB patterns. Usually various pickup coils are used in an ECT probe as a magnetic sensor. Pickup coil sensors, however, do not show enough S/N ratio to detect defects in high-density PCBs, whose conductors are less than 100μm. We propose a new ECT probe with a spin-valve giant magnetoresistive (SV-GMR) sensor for inspection of PCBs. SV-GMR sensors show high sensitivity to a small magnetic field in one direction. This sensor senses a magnetic field as varying its resistance unlike inductive sensors such as a pickup coil.In this paper, we investigated the sensing properties of a SV-GMR sensor with changing axes, frequencies, and magnetic field amplitudes, and discussed problems that could occur in replacing a pickup coil with SV-GMR.