Tunnel Junction Sensors: A tunnel junction is a barrier between two electrically conducting materials in electronics and spintronics, such as a thin insulating layer or electric potential. Tunneling allows electrons (or quasiparticles) to travel past the barrier. Traditionally, there is no chance at all that the electron will get through the barrier. The electron does, however, have some probability of passing through the barrier since, by quantum mechanics, its wave amplitude is non-zero in the barrier. There are numerous uses for tunnel intersections. In magnetic tunnel junctions, electrons pass from one magnetic material to another through a tiny insulating barrier. This might be the foundation of a magnetic detector [1]. Fig: Schematic representation of an electron tunneling through a barrier [1]. A sort of instrument called a tunnel junction sensor is used to monitor several physical parameters like temperature, pressure, magnetic fields, and more. They rely on the quantum tunneling concept, which relies on quantum mechanical phenomena to allow electrons to flow through a tiny insulating barrier known as the tunneling barrier. A tunnel junction sensor's fundamental components are two conductive electrodes separated by a thin insulating layer. Electrons may tunnel through the insulating barrier when a voltage is applied across the electrodes, producing a detectable current. The characteristics of the barrier and the physical quantity being measured affect how much current flows through them. In scanning tunneling microscopy (STM), tunnel junction sensors are frequently used. STM is a method for observing surfaces at the atomic scale by moving a pointed probe over them and detecting the current that tunnels between them. Scientists can map the surface topography with extraordinary resolution by keeping an eye on the current. Magnetic tunnel junctions (MTJs) utilize tunnel junction sensors as well to detect magnetic fields. A tunnel junction is created by two ferromagnetic layers sandwiched between thin insulating layers in MTJs. The relative alignment of the magnetization in the two ferromagnetic layers affects the electrical resistance of the MTJ, enabling the detection of magnetic fields. Tunnel junction sensors have also been investigated for use in temperature, gas, and strain sensing, among other applications. By altering the insulating barrier's characteristics and the materials used to make the device, tunnel junction sensors' sensitivity, and operating range can be customized. With potential applications in areas including materials science, nanotechnology, and biomedical research, tunnel junction sensors provide a flexible and sensitive way to measure numerous physical properties at the atomic and nanoscale [2] [3] [4]. Fig: Schematic of the magnetic tunnel junction. In a process known as magnetic flux leakage (MFL) testing, the material is magnetized, and when a defect is present, the magnetic flux lines emerge from the material. As lift-off (distance from the material) increases, the amplitude of the leaked magnetic flux decreases. Therefore, a sensitive magnetic sensor is needed for detection at high lift-off. This research suggests using magnetic tunnel junction (MTJ) sensors in a bridge circuit for the NDT of reinforced concrete at high lift-off to improve output sensitivity. Two MTJ sensors were wired into a full-bridge circuit, with a resistor and a variable resistor on one side of the arm and two MTJ sensors linked in series on the other. Although it strengthens a structure's structural integrity, it is nevertheless corrosive and brittle when subjected to extreme conditions or too much force. Therefore, NDT must manage and maintain these structures to avoid their collapse, which could result in human casualties and environmental harm. Due to the ferromagnetic nature of the steel bars used in reinforced concrete, flaws like corrosion and cracks can alter the structure of magnetic domains and their macro-properties. Magnetic flux leakage (MFL) testing is taken into consideration for its use by utilizing these characteristics. In theory, the steel rebar is magnetized, however, due to a defect's alteration of the magnetic characteristics, magnetic flux lines emerge from the steel rebar. The size and shape of the flaw affect the amount of magnetic flux that leaks. A sensitive magnetic sensor is required to find these leaks. The impact of the lift-off (the separation between the sensor and the steel rebar) on fault identification is another aspect that must be taken into account. Since the biggest magnetic flux leak occurs at low lift-off levels, where most research concentrates, the detection is more precise. However, in some instances, the steel rebar is placed further from the concrete's surface, resulting in a very modest magnetic flux leak whose size makes identification challenging. Hall effect sensors are frequently employed in conventional MFL to find magnetic flux leaks. The extremely limited sensitivity range of these sensors, however, may make it challenging to perform measurements at high lift-off. Due to their high sensitivity and low detective range, magnetic tunnel junction (MTJ) based sensors have gained a lot of attention. MTJ sensors' sizes can be reduced in comparison to other magnetic sensors on the market, such as Hall sensors, magneto-impedance sensors, and inductive coils. Additionally, MTJ sensors' sensitivity is unaffected by field frequency. MTJ sensors have become popular in the field of non-destructive testing sensors in addition to attracting attention in the field of bio-magnetic sensors as a result of their great characteristics. To maximize the sensitivity of the sensor and lower noise, studies on MTJ sensors concentrate on determining the ideal free layer structure of the MTJ film stack or optimizing the integration of the MTJ sensor. However, some investigations have noted that the sensitivity of the entire system was increased by incorporating the MTJ sensors in bridge circuits. A better response is also produced by other elements, such as the sensor circuit (amplifier circuit and filter circuit) [5] [6]. References: [1] "https://en.wikipedia.org/wiki/Tunnel_junction". [2] G. &. R. H. Binnig, " Scanning tunneling microscopy," IBM Journal of Research and Development, vol. 30, no. 4, pp. 355-369, 1986. [3] S. S. P. H. M. &. T. L. Parkin, "Magnetic domain-wall racetrack memory," Science, vol. 320, no. 5873, pp. 190-194, 2008. [4] M. &. A. Y. Tsunoda, "Magnetic Tunnel Junction Sensors for High-Density Magnetic Recording.," Sensors, vol. 14, no. 10, pp. 18366-18381, 2014. [5] M. N. S. Z. J. M. O. a. Y. A. Muhamad Arif Ihsan, "Investigation of a Magnetic Tunnel Junction Based Sensor for the Detection of Defects in Reinforced Concrete at High Lift-Off," Sensors (Basel), vol. 19, no. 21, 2019. [6] .. B. P.C.R., "Magnetic Flux Leakage Testing: Basics," J. Non-Destr. Test. Eval. , vol. 13, p. 7–19, 2012.
