France



Introduction&lt;br/&gt;Induction hardening treatment is used for a variety of steels because of its short hardening time and ability to hardened steel with complex shapes. In this research, the non-destructive inspection method for the hardened depth in surface of the induction hardened steel plate by an electromagnetic sensor is investigated. The hardened depth in surface of the steel plate is measured by detecting the difference in the permeability and the conductivity between the hardened and the non-hardened layers inside the steel plate by the electromagnetic sensor [1,2]. On the other hand, the detection signal in the electromagnetic sensor is also affected by the changing in the distance (&lt;i&gt;L&lt;/i&gt;&lt;sub&gt;o&lt;/sub&gt;: Lift-off) between the surface of the induction hardened steel plate and the sensor. Therefore, the method for simultaneously measuring the surface hardened depth in the hardened steel plate and the lift-off is proposed.&lt;br/&gt;&lt;br/&gt;The model and the evaluation method&lt;br/&gt;Fig.1 shows the 1/2 region of the proposal inspection model for detecting the surface hardened depth in the surface hardened steel plate of SCM440 steel material. The proposal electromagnetic sensor is consisted of an electromagnetic yoke made of laminated silicon steel plate, an alternating excitation coil, and two types of detection coils. The &lt;i&gt;z&lt;/i&gt;-direction of the flux density (&lt;i&gt;B&lt;/i&gt;&lt;sub&gt;z&lt;/sub&gt;) inside the magnetic yoke is detected by the &lt;i&gt;z&lt;/i&gt;-direction detection coil on one leg of the yoke, and the &lt;i&gt;x&lt;/i&gt;-direction magnetic field distributed (&lt;i&gt;B&lt;/i&gt;&lt;sub&gt;x&lt;/sub&gt;) between the two legs of the yoke is detected by the &lt;i&gt;x&lt;/i&gt;-direction detection coil. Since the permeability and conductivity in the hardened layer are lower than that of the non-hardened layer, the flux density in the &lt;i&gt;x-&lt;/i&gt; and &lt;i&gt;z-&lt;/i&gt; direction of these detection coils is changed when the hardened depth is changed.&lt;br/&gt;However, even if the depth of the hardened layer is constant, their detected magnetic flux in these detection coils is changed when the lift-off is changed. Therefore, both the lift-off and the surface hardened depth are estimated by the &lt;i&gt;B&lt;/i&gt;&lt;sub&gt;x&lt;/sub&gt; and &lt;i&gt;B&lt;/i&gt;&lt;sub&gt;z&lt;/sub&gt;.&lt;br/&gt;&lt;br/&gt;3-D electromagnetic FEM considering the nonlinearity of magnetic properties&lt;br/&gt;The flux density and eddy current inside the surface hardened steel plate are calculated by 3-D nonlinear finite element method (FEM) taking account of the initial magnetization curve and conductivity of the hardened and non-hardened layers in the steel. In an induction hardened steel plate, there is an intermediate layer called a heat-affected zone between the hardened and non-hardened layers. Therefore, in this analysis, the initial magnetization curves and conductivity inside the intermediate layer are calculated by linear interpolation of the initial magnetization curves and conductivity of the hardened and non-hardened layers.&lt;br/&gt;&lt;br/&gt;Measurements results and estimates&lt;br/&gt;Fig.2 shows the obtained values of &lt;i&gt;B&lt;/i&gt;&lt;sub&gt;x&lt;/sub&gt; and &lt;i&gt;B&lt;/i&gt;&lt;sub&gt;z&lt;/sub&gt; in the two detection coils when the hardened depth (&lt;i&gt;D&lt;/i&gt;) and &lt;i&gt;L&lt;/i&gt;&lt;sub&gt;o&lt;/sub&gt; are changed, respectively. This figure denotes that if &lt;i&gt;L&lt;/i&gt;&lt;sub&gt;o&lt;/sub&gt; is constant, &lt;i&gt;B&lt;/i&gt;&lt;sub&gt;x&lt;/sub&gt; is increased and &lt;i&gt;B&lt;/i&gt;&lt;sub&gt;z&lt;/sub&gt; is decreased when &lt;i&gt;D&lt;/i&gt; is increased. This is, because the flux density &lt;i&gt;B&lt;/i&gt;&lt;sub&gt;z&lt;/sub&gt; inside the magnetic yoke is decreased, and the &lt;i&gt;B&lt;/i&gt;&lt;sub&gt;x&lt;/sub&gt; distributed between both feet of the yoke is increased, since the permeability and conductivity in surface domain of the steel plate is decreased when the &lt;i&gt;D&lt;/i&gt; is increased. On the other hand, if &lt;i&gt;D&lt;/i&gt; is constant, &lt;i&gt;B&lt;/i&gt;&lt;sub&gt;x&lt;/sub&gt; is increased and &lt;i&gt;B&lt;/i&gt;&lt;sub&gt;z&lt;/sub&gt; is decreased when &lt;i&gt;L&lt;/i&gt;&lt;sub&gt;o&lt;/sub&gt; is increased. This is, because the flux density &lt;i&gt;B&lt;/i&gt;&lt;sub&gt;z&lt;/sub&gt; in the closed magnetic path between the sensor and the steel plate is reduced when the &lt;i&gt;L&lt;/i&gt;&lt;sub&gt;o&lt;/sub&gt; is increased, and the magnetic flux &lt;i&gt;B&lt;/i&gt;&lt;sub&gt;x&lt;/sub&gt; distributed between both feet of the yoke is increased.&lt;br/&gt;The figure shows that the signals of &lt;i&gt;B&lt;/i&gt;&lt;sub&gt;x&lt;/sub&gt; and &lt;i&gt;B&lt;/i&gt;&lt;sub&gt;z&lt;/sub&gt; due to the changes of &lt;i&gt;D&lt;/i&gt; and &lt;i&gt;L&lt;/i&gt;&lt;sub&gt;o&lt;/sub&gt; are distinguished. Therefore, Both &lt;i&gt;D&lt;/i&gt; and &lt;i&gt;L&lt;/i&gt;&lt;sub&gt;o&lt;/sub&gt; are inspected by the linear interpolation using measured &lt;i&gt;B&lt;/i&gt;&lt;sub&gt;x&lt;/sub&gt; and &lt;i&gt;B&lt;/i&gt;&lt;sub&gt;z&lt;/sub&gt;. Table I shows the interpolated results of &lt;i&gt;D&lt;/i&gt; and &lt;i&gt;L&lt;/i&gt;&lt;sub&gt;o&lt;/sub&gt; using the &lt;i&gt;B&lt;/i&gt;&lt;sub&gt;x&lt;/sub&gt; -&lt;i&gt; B&lt;/i&gt;&lt;sub&gt;z&lt;/sub&gt; plane of Fig.2. The measured values are almost in agreement with the obtained ones. The table illustrates that both &lt;i&gt;D&lt;/i&gt; and &lt;i&gt;L&lt;/i&gt;&lt;sub&gt;o&lt;/sub&gt; can be detected from the &lt;i&gt;B&lt;/i&gt;&lt;sub&gt;x&lt;/sub&gt; and &lt;i&gt;B&lt;/i&gt;&lt;sub&gt;z&lt;/sub&gt; in two detection coils.&lt;br/&gt;&lt;br/&gt;Conclusions&lt;br/&gt;The results obtained are summarized as follows:&lt;br/&gt;(1)It is possible to estimate the depth (&lt;i&gt;D&lt;/i&gt;) of surface hardened steel plate by detecting the change of flux density in two detection coils due to the difference of the permeability and the conductivity in the steel with and without hardened layers. However, the flux density detected by the search coil is influenced by the change of both lift-off and depth of surface hardened steel.&lt;br/&gt;(2)The flux density (&lt;i&gt;B&lt;/i&gt;&lt;sub&gt;x&lt;/sub&gt;) in the detection coil of &lt;i&gt;x&lt;/i&gt;-direction is increased and the flux density (&lt;i&gt;B&lt;/i&gt;&lt;sub&gt;z&lt;/sub&gt;) in the detection coil of &lt;i&gt;z&lt;/i&gt;-direction is decreased when the surface hardened depth (&lt;i&gt;D&lt;/i&gt;) of the induction hardened steel or the lift-off (&lt;i&gt;L&lt;/i&gt;&lt;sub&gt;o&lt;/sub&gt;) are increased. However, the signals of &lt;i&gt;B&lt;/i&gt;&lt;sub&gt;x&lt;/sub&gt; and &lt;i&gt;B&lt;/i&gt;&lt;sub&gt;z&lt;/sub&gt; in two detection coils due to the changes of &lt;i&gt;D&lt;/i&gt; and &lt;i&gt;L&lt;/i&gt;&lt;sub&gt;o&lt;/sub&gt; are distinguished. Therefore, it is possible to inspect the &lt;i&gt;D&lt;/i&gt; of the surface hardened steel without the influence of &lt;i&gt;L&lt;/i&gt;&lt;sub&gt;o&lt;/sub&gt; by detecting both the &lt;i&gt;B&lt;/i&gt;&lt;sub&gt;x&lt;/sub&gt; and &lt;i&gt;B&lt;/i&gt;&lt;sub&gt;z&lt;/sub&gt; of two detection coils in the proposed inspection sensor.

**References:**

[1] Y.Gotoh, S.Onita, T.Horino, and Y. Misaka,” Detecting Method of Hardened Depth in Surface Hardened Steel by Magnetic Field on Steel”,&lt;i&gt; IEEE Transactions on Magnetics&lt;/i&gt;, vol.54, no.11, Article Sequence Number:6202404, 2018.&lt;br/&gt;&lt;br/&gt;[2] Y.Gotoh, K.Sakurai, N.Takaoka, Y.Misaka, K.Kawasaki, and N.Takahashi,” Proposal of Electromagnetic Inspection Method of Hardened Depth of Steel using 3-D Nonlinear FEM Taking Account of Hysteresis”, &lt;i&gt;IEEE Transactions on Magnetics&lt;/i&gt;, vol.47, no.10, pp.3546-3549, 2011.

**Images:**

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/e862db6def1a56d0093f74acb2e22a85.jpg)
Fig.1. Proposal inspection model (&lt;i&gt;x&lt;/i&gt;-&lt;i&gt;z&lt;/i&gt; plane 1/2 region).

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/c3974039805ce679ad07cfffd241dc00.jpg)
Fig.2. Experimental results for magnetic flux density (&lt;i&gt;B&lt;/i&gt;&lt;sub&gt;x&lt;/sub&gt;,&lt;i&gt;B&lt;/i&gt;&lt;sub&gt;z&lt;/sub&gt;) at each hardened depth (&lt;i&gt;D&lt;/i&gt;) and lift-off (&lt;i&gt;L&lt;/i&gt;&lt;sub&gt;o&lt;/sub&gt;).&lt;br /&gt;
&lt;br /&gt;
Table 1. The interpolated results of &lt;i&gt;D&lt;/i&gt; and&lt;i&gt; L&lt;/i&gt;&lt;sub&gt;o&lt;/sub&gt;.

INTERMAG 2021

 Proposal of Inspection Method for Lift-Off and Surface Hardened Depth of Induction Hardened Steel Plate Using by Alternating Magnetic Field

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Introduction Induction hardening treatment is used for a variety of steels because of its short hardening time and ability to hardened steel with complex shapes. In this research, the non-destructive inspection method for the hardened depth in surface of the induction hardened steel plate by an electromagnetic sensor is investigated. The hardened depth in surface of the steel plate is measured by detecting the difference in the permeability and the conductivity between the hardened and the non-hardened layers inside the steel plate by the electromagnetic sensor [1,2]. On the other hand, the detection signal in the electromagnetic sensor is also affected by the changing in the distance (Lo: Lift-off) between the surface of the induction hardened steel plate and the sensor. Therefore, the method for simultaneously measuring the surface hardened depth in the hardened steel plate and the lift-off is proposed. The model and the evaluation method Fig.1 shows the 1/2 region of the proposal inspection model for detecting the surface hardened depth in the surface hardened steel plate of SCM440 steel material. The proposal electromagnetic sensor is consisted of an electromagnetic yoke made of laminated silicon steel plate, an alternating excitation coil, and two types of detection coils. The z-direction of the flux density (Bz) inside the magnetic yoke is detected by the z-direction detection coil on one leg of the yoke, and the x-direction magnetic field distributed (Bx) between the two legs of the yoke is detected by the x-direction detection coil. Since the permeability and conductivity in the hardened layer are lower than that of the non-hardened layer, the flux density in the x- and z- direction of these detection coils is changed when the hardened depth is changed. However, even if the depth of the hardened layer is constant, their detected magnetic flux in these detection coils is changed when the lift-off is changed. Therefore, both the lift-off and the surface hardened depth are estimated by the Bx and Bz. 3-D electromagnetic FEM considering the nonlinearity of magnetic properties The flux density and eddy current inside the surface hardened steel plate are calculated by 3-D nonlinear finite element method (FEM) taking account of the initial magnetization curve and conductivity of the hardened and non-hardened layers in the steel. In an induction hardened steel plate, there is an intermediate layer called a heat-affected zone between the hardened and non-hardened layers. Therefore, in this analysis, the initial magnetization curves and conductivity inside the intermediate layer are calculated by linear interpolation of the initial magnetization curves and conductivity of the hardened and non-hardened layers. Measurements results and estimates Fig.2 shows the obtained values of Bx and Bz in the two detection coils when the hardened depth (D) and Lo are changed, respectively. This figure denotes that if Lo is constant, Bx is increased and Bz is decreased when D is increased. This is, because the flux density Bz inside the magnetic yoke is decreased, and the Bx distributed between both feet of the yoke is increased, since the permeability and conductivity in surface domain of the steel plate is decreased when the D is increased. On the other hand, if D is constant, Bx is increased and Bz is decreased when Lo is increased. This is, because the flux density Bz in the closed magnetic path between the sensor and the steel plate is reduced when the Lo is increased, and the magnetic flux Bx distributed between both feet of the yoke is increased. The figure shows that the signals of Bx and Bz due to the changes of D and Lo are distinguished. Therefore, Both D and Lo are inspected by the linear interpolation using measured Bx and Bz. Table I shows the interpolated results of D and Lo using the Bx - Bz plane of Fig.2. The measured values are almost in agreement with the obtained ones. The table illustrates that both D and Lo can be detected from the Bx and Bz in two detection coils. Conclusions The results obtained are summarized as follows: (1)It is possible to estimate the depth (D) of surface hardened steel plate by detecting the change of flux density in two detection coils due to the difference of the permeability and the conductivity in the steel with and without hardened layers. However, the flux density detected by the search coil is influenced by the change of both lift-off and depth of surface hardened steel. (2)The flux density (Bx) in the detection coil of x-direction is increased and the flux density (Bz) in the detection coil of z-direction is decreased when the surface hardened depth (D) of the induction hardened steel or the lift-off (Lo) are increased. However, the signals of Bx and Bz in two detection coils due to the changes of D and Lo are distinguished. Therefore, it is possible to inspect the D of the surface hardened steel without the influence of Lo by detecting both the Bx and Bz of two detection coils in the proposed inspection sensor.

**References:**

[1] Y.Gotoh, S.Onita, T.Horino, and Y. Misaka,” Detecting Method of Hardened Depth in Surface Hardened Steel by Magnetic Field on Steel”, IEEE Transactions on Magnetics, vol.54, no.11, Article Sequence Number:6202404, 2018. [2] Y.Gotoh, K.Sakurai, N.Takaoka, Y.Misaka, K.Kawasaki, and N.Takahashi,” Proposal of Electromagnetic Inspection Method of Hardened Depth of Steel using 3-D Nonlinear FEM Taking Account of Hysteresis”, IEEE Transactions on Magnetics, vol.47, no.10, pp.3546-3549, 2011.

**Images:**

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/e862db6def1a56d0093f74acb2e22a85.jpg)
Fig.1. Proposal inspection model (x-z plane 1/2 region).

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/c3974039805ce679ad07cfffd241dc00.jpg)
Fig.2. Experimental results for magnetic flux density (Bx,Bz) at each hardened depth (D) and lift-off (Lo). 
 
Table 1. The interpolated results of D and Lo.

poster

In recent years, magnetoelectric (ME) composites consisting of magnetostrictive and piezoelectric phases mechanically coupled to each other have received widely attention due to the tremendous potential applications for functional devices such as magnetic sensors, phase shifters, transformers or gyrators, current sensors and energy harvesters. In the field of magnetic sensors, the ME magnetic sensors are composed of ME composites that show many excellent advantages, such as low cost, high sensor performances (sensitivity, linearity, resolution), small volume perfect and simplified electronic circuits [1]. These advantages offer the potential of ME magnetic sensor to be a new generation of magnetic sensors. There are enormous investigations activities in recent years on ME magnetic sensor. Previous research shows that ME magnetic sensor exhibit high AC/DC magnetic sensor performances at room temperature under resonance conditions [2]. Additionally, the ME magnetic field sensors have been drawn significant interests for numerous applications, such as bio-magnetism detecting apparatuses, earth magnetic field detection, non-destructive material testing, and aerospace instruments[3]. However, these ME magnetic sensor need to be strictly parallel to the magnetic field when measuring the magnetic field due to the ME laminates of configurations of square, or rectangular geometries have high anisotropy, otherwise will cause enormous deviation and affect the stability of ME magnetic sensors. It is thus desirable to develop ME magnetic sensor with ultra-lower anisotropy, along with high sensor performances. In this study, a toroidal ME magnetic sensor in composite of strictly concentric ferrites/piezoelectric rings with toroidal solenoid wound around was fabricated and developed. Compared with the previously reported ME magnetic sensor of configurations of square, or rectangular geometries, the toroidal ME magnetic sensor has ultra-lower shape-induced anisotropy, high stability and high sensor performances, which voltage output is almost unaffected by the angle between the ME magnetic sensor and the magnetic field. These excellent stability and sensor performances characteristics offer unique possibilities for possible use of ME magnetic sensor.

**References:**

[1] J. Zhai, Z. Xing, S. Dong, Appl. Phys. Lett. Vol. 88, p.062510 (2006). [2] H. Xi, M. Lu, Q. Yang, Sensor Actuat A-Phys. Vol. 311, p.112064 (2020). [3] J. Gao, L. Shen, Y. Wang, J. Appl. Phys. Vol. 109, p.074507 (2011).

**Images:**

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/ec5e0db2329771e70db9c72da54c82bb.jpg)
Fig.1. Schematic diagram of the presented magnetic field sensor.

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/c6f133ff964c062f2b382cef753f9eca.jpg)
Fig.2. The effect of DC magnetic field angle on the ME output voltage measured for toroidal ME sensor. The inset indicates the direction of the DC magnetic fields in a top view

 Ultra-lower anisotropy magnetic field sensor in ferrite/piezoelectric toroidal magnetoelectric composites

1. Background and problem considered Metal tube and wire are used in variety parts of infrastructures. Defects like holes and cracks in the products can cause serious accidents. They are inspected in manufacturing processes by using Non-Destructive Techniques (NDTs). The inspection speed and sensitivity are important because the tube and wire are long. The Eddy Current Testing (ECT) is one of NDTs and widely used in the processes. The ECT is better than the ultrasonic testing, the X-ray testing and other NDTs on the speed and sensitivity. However tube and wire manufacturers need faster inspection speed and better inspection sensitivity, because the speed is closely related to the production speed. The speed is determined by the vibration of the inspection sample caused by the transport of the sample. The vibration has an impact on detection signals of the ECT [1]. In previous studies, signals caused by vibration are removed from the detection signal by suppressing the vibration of the transport device, and/or signal processing [2-4]. We propose a new ECT probe to suppress the vibration component in the detection signal. The novelty of the proposed probe is that it has two outer coils for electromagnetically suppressing the vibration components outside of the two inner coils for flaw detection. The principle that the proposed method reduces the detection signal due to the vibration is explained. 2. Vibration Signal Suppression Probe The proposed probe has four coils and is used with a variable amplifier and a phase shifter which is known as an all-pass filter. It is possible that effect of the vibration in the detection signal can be equal to zero by using the variable amplifier and phase shifter which adjust the amplitude and the phase of the excitation current flowing through the outer coil. Fig. 1 shows the structure of the probe with four coils. L2 and L3 are defined as the inner coils. L1 and L4 are defined as the outer coils. The inside coils form a self-induced differential sensor. The outer and inner coils form a mutually induced differential sensor with the outer coil as excitation and the inner coil as detection. The detection signals of the sensors due to the displacement of the rod is defined as the displacement signal. The displacement signal of the probe is equal to the total output of the self-induced and mutual-induced sensors. We simplify discussion by limiting static displacement. The displacement is defined the offset between axes of the rod and coils and rotation of the rod. As shown in Fig. 1, axis of the rod has the offset from axis of coils to the radius direction. The rod rotates around a point on the plane containing the axes of the rod and coil. To demonstrate the effectiveness of the method, the displacement signals of the ECT were simulated by the 3D finite element method. In the simulation, excitation current was 10 mA at 16 kHz, a brass rod with a conductivity of 1.26×107 S/m was used, and dCoils was 6 mm. Simulated detection signals are shown in Fig. 2. The detection signal of the ECT is obtained as a point on the complex plane. The horizontal and vertical axes of the plane represent in-phase and quadrature components of the signals for the excitation current. The solid and dotted lines in Fig. 2 show simulated results of displacement signals when the inner or outer coils are used as the excitation coils. The lines show the displacement signals generated when the balance of the electromotive force was lost due to the displacement. The phases of the displacement signals were constant. The amplitudes of the displacement signals were proportional to the rod of offset and rotation. The dash dot line shows that displacement signal with the amplitude and phase of the outer excitation signal adjusted using a variable amplifier and phase shifter. The amplitude and phase were adjusted so that it is equal to the inverted phase of the inner excitation signal and the amplitude of the inner excitation signal. The displacement signals cancel each other when inner and outer coils are used as excitation coil at the same time. In this case, the gain of the amplifier was 4.59 and the phase of the phase shifter was -44.8 degrees. The detail of the experimental method and results will be presented in our presentation.

**References:**

[1] C. Reboud, D. Prémel, G. Pichenot, and D. Lesselier, International Journal of Applied Electromagnetics and Mechanics, Vol. 25, No. 1-4, pp. 313-317(2007) [2] Cheng-Chi Tai, James H. Rose, and John C. Moulder, In Review of Progress in Quantitative Nondestructive Evaluation, pp. 1593-1600(1997). [3] L. Janousek, Z. Chen, N. Yusa, and K. Miya, NDT & E International, Vol. 38, No. 6, pp. 508-515(2005) [4] W. Yin, and A. J. Peyton, NDT & E International, Vol. 40, No. 1, pp. 43-48(2007)

**Images:**

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/374558b7ee841ebcdba2ff4816bbbf11.jpg)
Fig. 1. Structure of the vibration suppression probe. The inside coils, L2 and L3, form a self-induced differential sensor. The outer, L1 and L4, and inner coils form a mutually induced differential sensor with the outer coil as excitation and the inner coil as detection.

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/256978b252c8d9537c973c2b60778bbb.jpg)
Fig. 2. Simulated displacement signals. The solid line is the signal with the inner excitation. The dot line is the signal with the outer excitation. The dash dot line is the signal adjusted from the outer excitation signal. The total signal of inner and adjusted outer signals is equal to zero.

 New Eddy Current Probe for Vibration Signal Suppression

Many ferromagnetic substances are naturally magnetized under the influence of the geomagnetic field, and information about the corresponding objects can be obtained based on the analysis of monitoring the magnetic fields generated around magnetic targets. The detection and tracking of magnetic targets is the current research hotspot, and the target monitoring based on magnetic signals as well as widely used in various fields. Currently, the magnetic target localization method is commonly used to obtain the relative position of the target by inversion of the magnetic field gradient tensor matrix. However, this method has a detection blind spot, and even if the final results are corrected by mean filtering, there is still a certain amount of error. Therefore, the correction of the localization results needs to be based on the information of the magnetic field gradient matrix collected at the detection point. This paper uses the magnetic field gradient tensor target localization algorithm as the basis of single-point localization, analyzes its error in detection, and proposes a weighted route fitting method based on magnetic field gradient matrix information for the characteristics of route calculation which is vulnerable to great errors. After simulation experiments, the method can reduce the fluctuation of detection results to a certain extent, and can effectively propose the influence of the extreme error generated by the detection blind area on the positioning results. To investigate the sources of error in target localization, we designed a series of experiments for the magnetic field gradient matrix-based localization method. The location of the detection blind zone is verified by changing the relative position of the magnetic target to the detection point. When the position vector is perpendicular to the magnetic moment vector, the error rate increases significantly. The experimental results are consistent with the theoretical formula, and the higher the accuracy of the magnetic sensor, the smaller its detection blind zone. When the detection point out is in the detection blind area, its magnetic field gradient matrix is a non-full-rank matrix, so the accuracy of target detection is often inversely proportional to the dispersion of its matrix eigenvalues. Therefore, by introducing a weighted fitting method based on the gradient matrix, the detection error can be reduced to a certain extent and the interference caused by abnormal data can be eliminated. Experiments are designed to verify the feasibility of the method. The schematic diagram of the positioning detection experiment is shown in Figure 1 below, where ABCED represents the location of different detection points, and the oval dashed line is its detection range. The magnetic target is set to come out from the coordinates (-0.5m,0m) and advance along a straight line to (0.5m,0m), and the comparison of its route monitoring results is shown in Figure 2 below. Based on the results, it can be concluded that the error-corrected route is closer to the set simulation route.

**References:**

Zhao G, Han Q, Tong X, et al. Adaptive filtering method for magnetic anomaly detection[J]. Journal of Applied Remote Sensing, 2018, 12(2):1-10. Kim K Y, Jeong E, Kim S, et al. Magnetic Anomaly Detection Using Continuous Angle Alignment of Three-Axis Magnetic Signal[J]. IEEE sensors journal, 2019, 19(2):743-750. Zheng J, Fan H, Zhang Q, et al. Magnetic Anomaly Target Recognition Based on SVD and SVMs[J]. IEEE Transactions on Magnetics, 2019, 55(9):1-8. Shen Y, Wang J, Shi J, et al. Interpretation of Signature Waveform Characteristics for Magnetic Anomaly Detection Using Tunneling Magnetoresistive Sensor[J]. Journal of Magnetism and Magnetic Materials, 2019, 484(AUG.):164-171. Huang Y, Wu L H, Sun F. Underwater Continuous Localization Based on Magnetic Dipole Target Using Magnetic Gradient Tensor and Draft Depth[J]. IEEE Geoscience and Remote Sensing Letters, 2014, 11(1):178-180. Nara T, Ito W. Moore–Penrose generalized inverse of the gradient tensor in Euler's equation for locating a magnetic dipole[J]. Journal of Applied Physics, 2014, 115(17PT.3):3646-3648.

**Images:**

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/c3e0fd30c505b2a24e40c70d1a568e97.jpg)
Fig.1 Schematic diagram of magnetic target movement route monitoring

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/18c7f44678b5fffecd2245e1eb1c5fc9.jpg)
Fig.2 Comparison chart of route fitting correction results

 Magnetic Target Motion Monitoring Based on Weighted Route Fitting

Low frequency and weak magnetic field sensors based on magnetoelectric effect have received extensive attention due to their valuable characteristics, such as low-cost materials and fabrication, room-temperature response, low size, self-powered and non-contact sensing, high sensing resolution and large dynamic range1. In order to improve the sensitivity of magnetoelectric sensors, it is necessary to use soft magnetic materials (such as magnetic amorphous alloys, nickel, permendur, galfenol, Terfenol, ferrites) with low coercivity, low residual magnetization and high permeability as magnetostrictive phase2. The characteristics of soft magnetic field determine that the magnetoelectric sensor can quickly respond to the change of the external magnetic field, obtain the induced electromotive force, and timely recover to the state of complete demagnetization when the external magnetic field disappears. However, the output voltage generated by the magnetoelectric composite is an ambiguous function of the applied DC magnetic field due to non-negligible hysteresis loss in soft magnetic materials. The intrinsic hysteresis loop of magnetic materials is a multivalued function, which causes the magnetoelectric composites to output an uncertain voltage under the same external DC magnetic field. The non-negligible hysteresis behavior of magnetic media leads to the absolute measurement error of DC magnetic field sensor and reduces the signal-to-noise ratio (SNR). This will limit the measurement stability and accuracy of magnetoelectric sensors. Therefore, the use of magnetostrictive materials with smaller remanence may be a way to further improve the working stability of magnetoelectric sensors. In this study, by comparing three typical magnetostrictive materials with different properties, the effect of intrinsic hysteretic properties of the materials on the stability of the sensor will be studied. Furthermore, the advantages of permalloy such as low coercivity and low remanence have been used to fabricate a ME sensor with high stability. Each vibration mode of permalloy shows consistency for at least 110 hours without any demagnetization. The stable vibration mode indicates that the stability of magnetoelectric sensor based on permalloy will reach a higher order of magnitude than NZFO and Terfenol-D. Compared to previously reported counterparts, this ME sensor can achieve higher precision measurement and produce higher stability signal.

**References:**

1 Hu J M, Chen L Q and Nan C W, Adv. Mater., Vol. 28, p.15-39 (2016) 2 Burdin D A, Chashin D V and Ekonomov N A, J. Magn. Magn. Mater., Vol. 449, p.152-156 (2018)

**Images:**

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/325c0e3bc1c866f9e9b23231c59ea03d.jpg)
Three different magnetostrictive material magnetization curves. The insert shows initial part of the curve in expanded scale.

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/c6859ec992e58cfac796facb7c4412d0.jpg)
The dynamic d33 spectrum which sampled under the condition of resonance and the same applied magnetic field for 120h.

 High-stability magnetic sensors in permalloy/piezoelectric magnetoelectric heterostructure using inappreciable hysteresis

With the development of the Internet of things, the demand for wearable flexible sensors is increasing. In order to research the new type of flexible magnetic sensors, in this paper, aiming at the vacancy of the application surface acoustic wave(SAW) devices in the field of wearable flexible magnetic sensors, the SAW magnetic sensors with ZnO/FeGa/PI structure are modeled and simulated[1,2], and the resonant characteristics and wave modes are studied. The frequency domain analysis of four kinds of devices with different thickness of 1.2 μm, 2 μm, 3.4 μm and 4 μm is carried out respectively. It is found that the device has multiple wave modes in each case, and the resonance peak of the device shifts to high frequency with the increase of the thickness of the piezoelectric layer. By comparing the modal deformation figure, it is found that the main modes of piezoelectric film with different thickness are various. For example, for 1.2 μm, the main mode is mode 2, while for 2 μm, the main mode is mode 4. The reason is that the device is composed of multi-layer thin films, in which the acoustic wave is not only affected by the piezoelectric medium, but also by the magnetostrictive layer and flexible substrate. Therefore, the optimal working mode is different for different thickness devices. In this paper, the flexible magnetic sensor is designed according to different thickness structure, and its optimal working mode is studied. When the maximum resonant mode is used as the working mode, the linear region of the magnetic sensor is basically unchanged, which is between 2 and 10 Oe. When the thickness of ZnO is 2 μm, the sensitivity in the linear region is the largest, about 253.3 kHz/Oe, and the working frequency is about 70MHz. When the thickness of ZnO is 3.4 μm, the sensitivity is only 30.5 kHz/Oe, but the working frequency is the highest, about 106MHz. This provides a theoretical basis for the development and application of flexible magnetic sensors. It is also found that when the thickness of ZnO is 1.2 μm, the maximum absolute value of admittance is only 0.0564, while for the structure with 4 μm, the maximum absolute value of admittance is 10.3413, which is of great significance for the subsequent design of impedance matching of wireless SAW magnetic sensor.

**References:**

[1] Datta S, Atulasimha J, Mudivarthi C, et al. Stress and magnetic field-dependent Young's modulus in single crystal iron-gallium alloys[J]. Journal of Magnetism and Magnetic Materials, 2010, 322(15): 2135-2144. [2] Chung G S, Phan D T. Finite Element Modeling of Surface Acoustic Waves in Piezoelectric Thin Films[J]. Journal of the Korean Physical Society, 2010, 57(3): 446-450.

**Images:**

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/7b7faa45b7f12d0b6a8f402c537530e5.jpg)
FIG. 1 Absolute value of admittance and deformed shape plot of piezoelectric layers with different thickness (a) 1.2 μm (b) 2 μm (c) 3.4μm (d) 4 μm.

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/d20a6bf2d577a547df1976e022bde57b.jpg)
FIG. 2 (a) Relationship between resonant frequency of main mode and magnetic field intensity when the thickness of ZnO is 1.2 μm. (b) Sensitivity of main modes corresponding to different ZnO thickness.

 Simulation of wave mode of flexible SAW magnetic sensor based on ZnO/FeGa/PI structure

Magnetic field sensors using the magnetoelectric (ME) effect in the composite multiferroic structures have attracted intensive attention due to their convenient operating at room temperature and high sensitivity. Large works have focused on the improvement of sensitivity and linearity by designing novel device structures but put rare attention on the optimization of equivalent noise that is also an important parameter for actual device application. In this work, we present a DC magnetic field sensor consisting of PZT/Metglas/PZT composites. With applying a voltage bias on the bottom PZT piezoelectric layer, the ME coefficient of the top PZT/Metglas bilayers can be increased obviously, as shown in Fig.1(b). This effective increase of ME coefficient will benefit the improvement of the magnetic field-to-voltage transfer coefficient and the reduction of the noise in the magnetoelectric sensor that have been shown in Fig.1(b). By increasing the electric field from 0 V/mm to 360 V/mm, the sensitivity of the sensor can increase from 13.04 mV/Oe to 32.85 mV/Oe at the resonant frequency of 131.6 kHz, and the equivalent magnetic noise of the sensor will decrease from 20 nT/√Hz to 9.8 nT/√Hz, as shown Fig.1(c). This three times increase of sensitivity and two times reduction of equivalent noise in the DC magnetic field detection by using ME effect may offer a new route for the development of novel magnetic field sensors or arrays.

**References:**

1. J. L. Hockel, T. Wu, and G. P. Carman, Journal of Applied Physics, Vol. 109, p. 064106 (2011). 2. V. N. Serov, Y. K. Fetisov, and A. A. Berzin, IEEE Sensors Journal, Vol. 18, p. 8256 (2018). 3. Y. Shen, D. Ma, and J. Gao, IEEE Electron Device Letters, Vol. 39, p. 1417 (2018).

**Images:**

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/4f90ba99bf370ba612a719e50bcc089f.jpg)
(a) Dependence of the output voltage with the DC magnetic field at different bias voltages; (b) Dependence of the sensitivity with the bias electric field for the proposed magnetic field sensor; (c) Frequency dependent equivalent noise density spectra at different bias electric fields of the proposed sensor.

 Optimization of Equivalent Noise in the DC Magnetic Field Sensor Based on Magnetoelectric Effect

Nowadays，the detection of the ultra-weak magnetic fields (&lt; 1 mOe) in micro- or nano-scope (&lt; 100 nm) is a hot research field. The magnetic sensor based on the principle of anisotropic magnetoresistance effect is attractive in the application of detecting the ultra-weak magnetic field in a contactless manner [1]. In this paper, a sensitive, 2-axis magnetic sensor based on anisotropic thin-film structure is proposed and fabricated. As the sensor’s induction layer, the anisotropic films of permalloy Ni (80%) Fe (20%) with different thicknesses (20 nm and 50 nm) are deposited by means of magnetron sputtering [2]. Seed layer under NiFe layer is used to control the grain size in the NiFe layer, which is an important parameter for magnetoresistance. In order to obtain the NiFe layer with good performance, tantalum is deposited as the seed layer, making the NiFe layer with &lt;111&gt; crystallographic texture with suitable grain size [3]. Figure 1 shows the schematic structure of the designed sensor for the ultra-weak magnetic field detection. The characterizations are conducted on the prepared layer, including surface roughness by atomic force microscope (AFM) and the crystalline structure by X-ray diffraction (XRD). According to the test results of AFM, it is found that the film with a thickness of 20 nm has a smoother surface with an average roughness of 1.985 nm, which is less than that of the film with a thickness of 50 nm with an average roughness of 3.005 nm. With the difference of surface roughness, the coercivity of the two thickness films is also different, and it ultimately has an effect on the film properties. Figure 2 shows the characterization results of the surface roughness of the deposited NiFe thin film. Based on the characterized results, the magnetoresistance related to the anisotropic effect is demonstrated and analyzed. The influence of the thickness of the anisotropic film on the characteristics of the magnetization and the dynamic range of the sensors, as well as the range of the magnetic field sensitivity is studied and analyzed in the paper [4]. The deposited film with 20 nm thickness shows obvious anisotropy with low coercivity, resulting in high sensitivity and outstanding detecting ability in the weak magnetic field.

**References:**

[1] N. A. Djuzhev, N. S. Mazurkin, and V. S. Pozdnyakov, Semiconductors, Vol. 49, no. 13, pp. 1739-1742 (2015) [2] M. Y. Chinenkov et al., NANOCON 2010 - 2nd International Conference, Conference Proceedings, pp. 270-273 (2010) [3] S. V. Luong, A. T. Nguyen, T. L. Nguyen, and A. T. Nguyen, IEEE Transactions on Magnetics, Vol. 54, no. 99, pp. 1-4 (2018) [4] D. Murzin, V. Belyaev, and F. Groß, Japanese Journal of Applied Physics, Vol. 59 (2019)

**Images:**

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/7d4e34ba9f63b52244bdc965e672fa73.jpg)
The model of the magnetic sensor. Internal figure (a) Local top view of the model. (b) Enlarged view of permalloy stripes and parallelograms which are biased

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/ebdc03eef8f2668ca164a2570b87437a.png)
Results of Ni80Fe20 films' surface roughness with different thicknesses tested by atomic force microscope (AFM). (a) The thickness is 20nm. (b) The thickness is 50nm.

 A Sensitive, 2-Axis Magnetic Sensor Based on Anisotropic Magnetoresistance Effect

There are many different ways and methods for measuring displacement. The influence of the magnetic material on the inductance of the inductor in its vicinity has been well investigated and used for the realization of various sensors, as well as displacement sensors [1, 2]. However, it has been shown that an improvement in the inductance of the inductor can be achieved if the grooves are realized on the surface of the magnetic film [3], which gives the great potential for the realization of magnetic sensors with better characteristics. With advances in development of materials in recent years, 3D printing technology allows a very fast fabrication of prototypes. It can be used for development of electronic components, sensors and actuators. It is a very simple, high-quality and precise way to create complex geometric structures using different materials which enable the realization of sensors of different characteristics. This paper presents a wireless magnetic displacement sensor realized in heterogeneous integration technology. The sensor is realized using additive 3D printing technology and magnetic disc with improved design. The wireless measurement method enables simpler and more precise measurement and data collection compared to the classic wired method. The exploded 3D model of the sensor is shown in the Fig. 1. The proposed sensor consists of three parts: the inductor, the sensor’s casing with spacer and the ferrite disk with grooves. The inductor windings are turned downwards towards the ferrite. The inductor is realized in Printed Circuit Board Technology, in the form of a spiral with an outer diameter of 25 mm and 22 turns (the width of the conducting lines is 500 μm and the conductor spacing is 730 μm). Using the Mn0.5Zn0.5Fe2O4 powder, the ferrite disc with grooves is pressed and sintered at a temperature of 1275 °C for 2 hours in a chamber furnace. The ferrite disc has a diameter of 25 mm. It has three concentric rings and a circle in the center on its surface. The part of the sensor that allows the sensor to be sensitive to displacement is the spacer. The spacer is printed using a 3D technology. It was made of Ninjaflex, one of the leading flexible 3D filaments. It is in the shape of the ring with the thickness of 500 µm. Using a Polylactic acid (PLA) filament, the sensor’s housing is 3D printed in which the sensor parts are connected and fixed. The applicator of displacement that acts on the surface of the sensor is also 3D printed. Displacement measurement is performed wirelessly using an impedance analyzer HP4194A. A coil (antenna) is placed around the sensor and the impedance and the phase of the antenna-sensor system are measured, from which the resonant frequency is determined. The displacement operation on the sensor is performed using a manual translation stage which enables the precise movement of the sensor surface in the steps of 50 μm. The characteristic of the antenna-sensor is shown in the Fig. 2. During the action of the displacement, the spacer is compressed and the ferrite approaches the inductor, which causes an increase in inductance, i.e. a decrease in the resonant frequency of the antenna-sensor system.

**References:**

1. M. Kisic, N. Blaz, Lj. Zivanov and M. Damnjanovic, AIP Advances, Vol. 10, paper no. 015017, p. 1-4 (2020) 2. M. G. Kisic, N. V. Blaz, Lj. D. Zivanov and M. S. Damnjanovic, IEEE Transactions on Magnetics, Vol. 53, no. 11, Art no. 4004104, p. 1-4 (2017) 3. X. Zhou, Y. Wen, Y. Mu and P. Li, 64th Annual Conference on Magnetism and Magnetic Materials, MMM2019, November 4-8. 2019, Las Vegas, Nevada, Book of abstracts, paper no. DV-05, p. 437-438 4. Available online at: https://ninjatek.com/ninjaflex/, accessed 05.01.2021.

**Images:**

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/eb2563e4c8b8f330d15ac98f997429ff.jpg)
Fig.1. The exploded 3D view of the sensor.

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/4dd0398f6f53d10c2624bcdc7dd3ddf0.jpg)
Fig. 2. The dependence of the resonant frequency on the displacement.

 Heterogonous Integrated Displacement Sensor with Grooved Ferrite

1. INTRODUCTION The non-destructive testing technology is appropriate to inspect high-priced product because it can inspect while maintaining the shape of the structure. And as the structure of the inspection body becomes complex and diversified with the development of the industrial society, the demand for non-destructive inspection is increasing mainly in developed countries. These techniques investigate structures of inspection by detecting energy deformation caused by the defects or constitution of products using specific physical energy to the target. Depending on the type of energy used for investigation, non-destructive testing is divided into technologies such as magnetic-particle testing, eddy-current testing, ultrasonic testing, radiation transmission testing, liquid-penetration testing and is applied to a wide range of application such as automobiles, nuclear power, steel structures, and pipelines [1]. Oil and gas pipes are mostly buried underground, so it is necessary to regularly manage corrosion or internal defects in pipes. Non-destructive testing is mainly used for safe long-distance transportation of oil and gas, among which Eddy-Current Testing (ECT), which does not have direct contact with the test object, is widely used. In ECT, operating frequencies have a significant impact on detecting faults. Traditional ECT mostly operates within the frequency range of a few kHz to MHz [2]. When the following high frequencies applied at the structure of the inspection, there is some difficulty to detect defects under pipe because eddy current does not flow to the bottom of the pipe due to the skin effect. In order to compensate for these limitations, the industry recently started to use the Magnetic Eddy Current technique that combines the principle of eddy current testing and magnetic flux leakage testing. The principle of this technology is based on the change in the pipe permeability due to defects by applying a certain amount of magnetic flux to the pipe using a permanent magnet or an electromagnet. In this technique, the magnetic flux used here does not need to go to the saturation point like MFL. In general, the magnetic field size required for MFL is at least 10 KA/m, but in MEC, a magnetic field of 3 KA/m is sufficient [3]. In the MEC technique, the value of magnetization level applied to the structure of the inspection has a significant influence on the defect determination performance. If the magnetic field is too small, it is impossible to determine the defects on the outer part of the pipe, and if the magnetic field is too large, defects of various sizes cannot be distinguished. Therefore, it is important to properly apply magnetic flux to the pipe so that defects can be found with high resolution in a given driving condition and the defect position can be distinguished. However, it has not been long since MEC technology has been introduced and researched, so there are insufficient papers on what driving conditions the sensor should operate under for optimal defect detection performance. For this reason, in this paper, a study was conducted to compare the defect detection performance according to the applied magnetic field point and to find the optimal applied magnetic field under a given driving condition. In order to analyze the distribution of the magnetic field around the defect according to the applied magnetic field value, a 3D model is set as shown in Fig.1-(a) and analyzed using 3D FEM. The defect underneath the pipe increases the density of magnetic flux in the pipe, thereby the permeability of pipe decreases and the impedance of the coil changes accordingly to generate a defect signal. The thickness of the defect used in the simulation is fixed at 1.5mm, which is 30% of the pipe thickness, and the diameter of the defect is fixed at 20mm. The value of magnetic field applied within the pipe was changed by changing the current value using an electromagnet. Fig.1-(b) show the respective magnetic field point of the pipe when 1A, 2A, 3A, 4A current is applied to the electromagnet coil. When a current of 1A is passed, it is difficult to determine the defects outside the pipe due to the large permeability value, and at 3A and 4A, the magnetic field applied to the pipe reaches almost saturation and the magnetic permeability is close to 1, making it inappropriate for detection of various size of defects. However, at 2A, it has a permeability, which is the middle value of the previous values, so it can detect various size of defects. Thus, using MEC techniques allows optimal defect detection performance at lower magnetic flux than MFL. In order to compare the defect detection performance in a real situation according to the applied magnetic field value, a prototype was made as shown in Fig.2-(a) and the experiments were conducted to verify compare to the simulated values. As shown in Fig.2-(b_Above), a specimen with artificial defects was produced and an actual MEC driving experiment was conducted. The specimens consist of defects of 10%, 30%, and 50% depth of the pipe thickness, where the solid line represents the external defect of the pipe and the dotted line represents the internal defect of the pipe. And defects werer detected by using coils as shown in Fig.2-(b_Below). The setting and procedures of experiments and the discussion of the results will be covered in the manuscript.

**References:**

[1] Seung-Seok Lee, Ki-Bok Kim, Journal of the korean society for nondestructive testing, vol. 24, pp. 64-85 (2015) [2] K.Reber, A.Boenisch, Stefanie Asher, PPSA Seminar in Scotland, pp.1-12 (2015) [3] Mohd Aufa Hadi Putera Zaini, Mohd Mawradi Saari, Nurul Ain Nadzri, 2020 11th IEEE Control and System Graduate Research Colloquium(ICSGRC 2020), Malaysia, pp.1-4 (2020)

**Images:**

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/77bba74437caf3ebf9387a3d20c9fd21.jpg)
Fig. 1. (a) MEC simulation models. (b) Magnetic field points according to the applied magnetic flux.

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/f9a621e0e02b8adcb115167ec3f92c7a.jpg)
Fig.2. (a) MEC prototype. (b) specimen with artificial defects(Above) and MEC coil(Below).

 Selection of Applied Magnetic Field for Optimal Detectability in Eddy Current Testing with DC bias

Tunneling magnetoresistive (TMR) sensors can easily be used for magnetic sensing due to high sensitivity, small size, and low power consumption. [1] They also have extremely short response times and are suitable for precise sensing applications. The magnetic hysteresis in the sensing layer of TMR sensors usually has a negative effect on the linearity and reversibility of the sensors. Therefore, reduced magnetic hysteresis, as well as low power and low noise, is favored in low magnetic field measurement. Previous research has shown that the magnetic hysteresis in TMR sensors can be reduced using AC modulation magnetic fields. [2] In this study, a much smaller AC modulation amplitude is used for lower overall power consumption of TMR sensors and the linear response range of the sensors under a small modulation field is investigated. The magnetic tunnel junction (MTJ) stack in the studied TMR sensor was designed as Si/150 SiO2/ 1.5 Ta/25 Ru/7 Ir20Mn80 / 2.2 Co30Fe70 /0.85 Ru/ 2.8 Co40Fe40B20/ 2 MgO /1.3 Co40Fe40B20 / 0.6 MgO/ 50 Co40Fe40B20 /2 Ta/15 Ru (in nm). The TMR ratio is about 180 % and the coercivity of the free layer is about 0.5 mT after magnetic annealing at 340 oC for 2 hour. The MTJs were connected into a Wheatstone bridge to form a sensor. An alternating current driven modulation coil was added surrounding the TMR sensor during magnetic field measurement with the AC modulation field applied along the sensing direction. The DC voltage output of the TMR sensor with different measurement conditions is shown in Fig. 1. The sensor was powered by a 1.5 Volt battery and a sinusoidal modulation field was applied with a frequency of 1 kHz and an amplitude of 0.3 mT. The output voltage of the sensor was measured in these DC magnetic field ranges: (a) -3 mT to 3 mT, marked in black in the figure, (b) -1.8 mT to 1.8 mT, marked in green, (c) -0.6 mT to 0.6 mT, marked in blue, and (d) -0.1 mT to 0.1 mT, marked in red. The figure shows that magnetic hysteresis in TMR sensor was effectively reduced if measured in a small range of DC magnetic field. With the amplitude of AC modulation field being as small as 0.3 mT, the sensor exhibits a linear response in the range of -0.6 mT to 0.6 mT, where the sensitivity is 20.6%/mT. It’s demonstrated that based on a simple construction, TMR sensors with small AC modulation fields can be used for low field sensing with good linearity. ACKNOWLEDGMENT This work was supported by Ministry of Science and Technology of Taiwan under grant number: 108-2314-B-003 -001 and 109-2314-B-003 -004. The MTJ samples were prepared in the Nebraska Nanoscale Facility: National Nanotechnology Coordinated Infrastructure and the Nebraska Center for Materials and Nanoscience, which are supported by the National Science Foundation under Award ECCS: 2025298, and the Nebraska Research Initiative.

**References:**

[1] Zheng, C. et al. Magnetoresistive Sensor Development Roadmap (Non-Recording Applications). IEEE Trans. Magn. 2019, 55, 1–30. [2] S. Liao et al., "Hysteresis Reduction in Tunneling Magnetoresistive Sensor with AC Modulation Magnetic Field," IEEE Transactions on Magnetics,(2020) doi: 10.1109/TMAG.2020.3009900.

**Images:**

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/064c1f75dc42a064ec6e3200b38de2c4.jpg)
Fig.1 The output voltage of the sensor as a function of the applied DC magnetic field in the range of four conditions: -0.1 mT to 0.1 mT, -0.6 mT to 0.6 mT, -1.8 mT to 1.8 mT, and -3 mT to 3 mT. The sensor was powered by a 1.5 Volt battery and a AC modulation field was applied with a frequency of 1 kHz and an amplitude of 0.3 mT for all conditions of DC field range.

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