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.

Hall devices using anomalous Hall effect of ferromagnetic Fe–Sn nanocrystalline thin films exhibit high sensitivity for detection of magnetic field comparable to that of semiconductor devices. For applying this material to magnetometry, superior detectivity owing to low electrical noise is a critical requirement. We investigate the magnetic-field sensitivity and the low-frequency noise spectrum as a detectivity for Fe–Sn Hall-cross devices. We detected typical noise behavior of 1/f and thermal noise. The noise-equivalent magnetic field reaches 0.3 μT/Hz1/2, which is comparable to that of semiconductor-based Hall sensors. A sufficiently low-noise spectrum evidences the potential applicability of Fe–Sn to Hall magnetometry.

Magnetic sensors to detect magnetic nanoparticles (MNPs) towards biomedical applications require very high sensitivity at low magnetic fields. Here we report a magnetic sensor consisting of a magnetic tunnel junction (MTJ) with a synthetic antiferromagnetic free layer. This sensor exhibits a low magnetic anisotropy and sensitivities of over 18%/Oe at low fields in the range of 0 to 3 Oe. We employ superparamagnetic MNPs with a large diameter of 200 nm. The sensor’s transfer curves show the magnetoresistance (MR) variations as a function of MNP concentration. We demonstrate the detection capability of MNP amounts of below 500 ng and low MNP concentrations of 0.01, 0.1, and 1 mg/ml in solvents. This result suggests that the combination of high-sensitivity TMR sensors and large MNPs has a substantial potential for biomarker detection applications.

Magnetic tunnel junctions (MTJs) that consist of two ferromagnets separated by a thin insulator are among the core devices used in spintronic applications such as magnetic sensors. Since magnetic sensors require high sensitivity for nondestructive eddy current testing, we developed and demonstrated magnetic sensors based on various configurations of serial MTJs. We fabricated sensors with 4, 16, 28, 40, and 52 serial MTJs in various numbers of rows (1, 4, 7, 10, and 13) to detect surface cracks via eddy current testing. All of the sensors could detect and discriminate between surface cracks 0.1 mm in width and 0.1 to 1.0 mm in depth on an aluminum specimen. Systematic studies on the effect of the number of MTJs showed a signal to noise ratio as high as 115 dB when detecting 0.1 mm deep cracks with 28 serial MTJs in 7 rows. This suggests that suitably configured serial MTJ sensors can offer an excellent performance in the detection of tiny surface defects via eddy current testing.

Magnetic flux leakage (MFL) testing is a method of non-destructive testing (NDT), whereby the material is magnetized, and when a defect is present, the magnetic flux lines break out of the material. The magnitude of the leaked magnetic flux decreases as the lift-off (distance from the material) increases. Therefore, for detection at high lift-off, a sensitive magnetic sensor is required. To increase the output sensitivity, this paper proposes the application of magnetic tunnel junction (MTJ) sensors in a bridge circuit for the NDT of reinforced concrete at high lift-off. MTJ sensors were connected to a full-bridge circuit, where one side of the arm has two MTJ sensors connected in series, and the other contains a resistor and a variable resistor. Their responses towards a bias magnetic field were measured, and, based on the results, the sensor circuit sensitivity was 0.135 mV/mT. Finally, a reinforced concrete specimen with a 1 cm gap in the center was detected. The sensor module (with an amplifier and low pass filter circuits) could determine the gap even at 50 cm, suggesting that MTJ sensors have the potential to detect defects at high lift-off values and have a promising future in the field of NDT.