Serial MTJ-Based TMR Sensors in Bridge Configuration for Detection of Fractured Steel Bar in Magnetic Flux Leakage Testing

Abstract

Thanks to high sensitivity, excellent scalability, and low power consumption, magnetic tunnel junction (MTJ)-based tunnel magnetoresistance (TMR) sensors have been widely implemented in various industrial fields. In nondestructive magnetic flux leakage testing, the magnetic sensor plays a significant role in the detection results. As highly sensitive sensors, integrated MTJs can suppress frequency-dependent noise and thereby decrease detectivity; therefore, serial MTJ-based sensors allow for the design of high-performance sensors to measure variations in magnetic fields. In the present work, we fabricated serial MTJ-based TMR sensors and connected them to a full Wheatstone bridge circuit. Because noise power can be suppressed by using bridge configuration, the TMR sensor with Wheatstone bridge configuration showed low noise spectral density (0.19 μV/Hz0.5) and excellent detectivity (5.29 × 10−8 Oe/Hz0.5) at a frequency of 1 Hz. Furthermore, in magnetic flux leakage testing, compared with one TMR sensor, the Wheatstone bridge TMR sensors provided a higher signal-to-noise ratio for inspection of a steel bar. The one TMR sensor system could provide a high defect signal due to its high sensitivity at low lift-off (4 cm). However, as a result of its excellent detectivity, the full Wheatstone bridge-based TMR sensor detected the defect even at high lift-off (20 cm). This suggests that the developed TMR sensor provides excellent detectivity, detecting weak field changes in magnetic flux leakage testing.

Current Progress of Magnetoresistance Sensors

“…Breakthroughs are expected to improve the performance and reduce the manufacturing cost of devices with built-in MR units, which overcome existing challenges and benefit the future application of MR sensors. The low magnitude of AMR outputs (∆R/R 0 < 2.5%) Difficult to reduce the size and hard for miniaturization [8,9] Multilayer systems based on GMR and TMR GMR/TMR multilayer systems exhibit high sensitivity for low magnetic fields GMR/TMR multilayer systems can be integrated with the electronic circuit easily Multilayer structures require complicated fabrication processes and specific equipment due to the strict limitations of layer thickness (increasing cost on equipment and extending the fabrication process lead to expensive products) GMR/TMR multilayer systems exhibit limited resistance variation range (working range, especially for TMR) and relatively low MR at room temperature (mostly for GMR) [52][53][54][55][56][57][58][59][60][61][65][66][67] Granular MR systems Granular MR systems bring simplified fabrication procedures and reduced investments in instruments Relatively large MR at room temperature can be achieved by some specifical designed granular MR systems Magnetic field ≥ 50 kOe is the prerequisite to achieve large MR at ambient temperature (relatively small resistance change for low magnetic fields at room temperature) Some granular MR systems require extremely low temperatures for large MR Although granular MR systems can reduce the complexity of the fabrication process, the dependence on specific fabrication techniques (such as magnetron sputtering) remains [23][24][25][68][69][70][71][72][73][74] Layered graphene MR systems Layered graphene MR systems exhibit large MR value and potential to be applied on fabricating next-generation spintronics based on layered graphene Most layered graphene MR systems require extremely low temperatures to achieve large MR Special designed substrates/circuits are required Precise control of layer number and positions is challenging Special fabrication techniques are required for preparing layered graphene, which further increases the production costs and the complexity [30]…”

“…The GMR multilayer systems normally consist of ferromagnetic layers with 4~6 nm thickness and 35 nm non-magnetic conductive spacer layer. The GMR and TMR multilayers exhibit good compatibility for integrating with electronic circuits (Figure 5) [66,67]. GMR and some TMR multilayer systems display inadequate MR under the low magnetic field and room temperature.…”

“…GMR and some TMR multilayer systems display inadequate MR under the low magnetic field and room temperature. In addition, their applications are hindered by the limited working range (especially for TMR multilayer systems) and complicated fabrication processes, which require further investigation to overcome current challenges [67].…”

Bridge Resistance Compensation for Noise Reduction in a Self-Balanced PHMR Sensor

“…Detection of the ultra-low magnetic field has a wide area of interest where sensitive physical sensing holds the key part of applicative technologies such as automotive [ 1 ], magnetic communication [ 2 , 3 , 4 , 5 , 6 ], noninvasive brain mapping [ 7 ], nondestructive materials evaluation [ 8 ], geomagnetism [ 9 , 10 ], and point-of-care diagnostics [ 11 , 12 , 13 , 14 ]. In particular, magnetic field sensors have been applied in the engine-based automotive industry over decades to control the speed, navigation, steering, and parking modules of automatic control systems.…”

Defect Width Assessment Based on the Near-Field Magnetic Flux Leakage Method

“…Magnetic flux leakage (MFL) testing has been used as a high-speed and high-sensitivity non-destructive testing (NDT) method for decades [ 1 , 2 , 3 , 4 ]. The MFL testing has gained good performance in the detection of defects in ferromagnetic objects such as pipelines, wire ropes, tanks, etc.…”