We demonstrated extremely high sensitivity in a tunnel magneto-resistive (TMR) sensor utilizing a free layer with a magnetic vortex structure having a small aspect ratio, and a pinned layer with a small diameter below 100 nm. A high TMR ratio above 200% was achieved in the magnetic tunnel junctions using a CoFeSiB/Ta/CoFeB free layer. Moreover, by decreasing the thickness of the free layer disk, thereby its aspect ratio, the saturation magnetic field was lowered to approximately 100 Oe. In addition, by miniaturizing the size of the pinned layer, the effective saturation magnetic field H-s, at which the magnetic vortex core moves to the outside of the pinned layer, was significantly reduced to 0.3 Oe. Thanks to the high TMR ratio and low Hs, the maximum sensitivity of 268%/Oe within a magnetic field range of -0.3 Oe < H-x < 0.3 Oe was successfully obtained in a vortex-type TMR sensor with a free layer with thickness of 70 nm and diameter of 10 mu m and a pinned layer of 80 nm. In addition, the small coercivity was observed in the minor magneto-conductance curves due to the reversible motion of the vortex core, reflecting the intrinsically hysteresis-free nature of the vortex structure. We realized both a substantial enhancement in sensitivity and a significant reduction in magnetic hysteresis for the vortex-type TMR sensors. The dramatic improvement in the performance of the TMR sensors will greatly contribute to magnetic sensing applications in biomedical and industrial fields.

We developed an integrated dataset by combining experimentally measured magnetocrystalline anisotropy constants (K1) of FeCoNi alloy thin films with density of states (DOS) calculated from first-principles calculations without spin–orbit coupling (SOC). The DOS was transformed into statistical descriptors and directly used as features for machine learning (ML) models to predict K1. The highest predictive performance was achieved when using DOS features within approximately ± 0.1 eV of the Fermi level, yielding a sign prediction accuracy of 83%. This result demonstrates that low K1 materials can be efficiently identified from the DOS even without the SOC. Furthermore, feature importance analysis revealed that the minority-spin DOS near the Fermi level plays the dominant role in the model predictions, reflecting the fact that most alloys in the dataset are Ni–rich with fcc structures, where contributions from majority-spin DOS are negligible.

Amorphous CoFeSiB, which is currently used as the soft magnetic layer in TMR sensors, suffers from crystallization when annealed above 330 °C, leading to degradation of device performance. To overcome this limitation, we developed TMR sensors using amorphous $\text{Co}-\text{Zr}-\text{Ta}$ as the free layer which exhibits higher thermal stability. After magnetic-field annealing at 400 °C, an anisotropy field of $H_{\mathrm{k}}=2.7 \text{Oe}$ was obtained for the single $\text{Co}-\text{Zr}-\text{Ta}$ layer, and a high TMR ratio of 223% was achieved in the sensor. These results indicate that the use of $\text{Co}- \text{Zr}-\text{Ta}$ enables further enhancement of the sensitivity of TMR sensors.

Superparamagnetic tunnel junctions (SMTJs) have recently attracted significant attention as a core technology for ultra-low-power and highly efficient non-conventional computing systems, and intensive studies have been conducted on their design and operation. In addition, SMTJ-based magnetic sensors exhibit hysteresis-free and highly sensitive magnetic field responses. To further enhance the magnetic field sensitivity of SMTJs, It is essential to realize a magnetocrystalline anisotropy constant of $\boldsymbol{K}_{1} \approx \mathbf{0}$ in the free layer. In this study, We focus on FeAlSi thin films with a reported $K_{1} \approx 160 \text{erg} / \text{cm}^{3}$. which have demonstrated tunnel magnetoresistance ratios exceeding 100% in magnetic tunnel junctions, and suggest FeAlSi as a promising candidate for the free layer in SMTJs. To fabricate SMTJs employing FeAlSi as the free layer, optimal metallic buffer layers were investigated.

This paper reports magnetic microscopy using high-sensitivity room-temperature tunnel magnetoresistance (TMR) devices for thin geological sections. The sensitivity region of the TMR sensor has dimensions of 178 µm (L) × 0.1 µm (W) × 100 µm (H), consisting of two TMR devices. Magnetic images were obtained for a vertically magnetized Hawaii basalt thin section in two sensor configurations, with the sensor length aligned parallel to the X- (lift-off = 174 μm) and Y-axes (lift-off = 200 μm), without introducing anisotropic distortion in the magnetic images. Although the magnetic images obtained with a scanning SQUID microscope (SSM) were similar, slight discrepancies were observed in the high-spatial-resolution region. A magnetic point source (50 μm × 50 μm) with a perpendicular magnetization film was prepared for evaluation. The SSM measurements showed a clear magnetic dipole at an angle of approximately 1° from the vertical direction. The FWHMs for both the SSM and TMR sensors increased linearly with lift-off. However, the peak magnetic fields, magnetic moments, and dipole tilts of the TMR sensor were significantly larger than those of the SSM sensor. This discrepancy may be due to the vertical extent of the active region of the TMR sensor, as well as due to sensor noise and drift.

We measured nanometer-scale displacements induced by a magnetic field gradient by using geometry-optimized tunnel magnetoresistive (TMR) sensors and a magnetic thin film. A comparison using free layers with different geometries revealed that the free-layer geometry affects both the sensor sensitivity and the effective magnetic field gradient, suggesting its influence on the displacement resolution. We demonstrated that a 35 mu m wide free layer provided a displacement resolution of 145 pm/root Hz at 368.7 Hz, with an estimated resolution of 27 pm/root Hz at 5 kHz. These results indicate that fine TMR sensors with high displacement resolution are applicable to industrial and navigation systems.

We deposited epitaxial thin films of typical ferromagnetic Fe-Co-Ni alloys with various compositions on MgO (001) single crystal substrate by sputtering and then systematically investigated their magnetic anisotropy and magnetic damping constant alpha. The results showed that the magneto-crystalline anisotropy constant K-1 roughly reproduced the composition dependence of bulk single crystals, but the region with K-1 > 0 extended further on the Ni-rich side compared to the bulk's. Additionally, uniaxial magnetic anisotropy on the order of 10(3) erg/cc was manifested in all the films. Regarding alpha, larger values were observed on the Ni-rich alloys, with a tendency to decrease with the addition of Fe or Co. Furthermore, the compositional dependence of alpha was found to replicate the behavior predicted by the Kambersk & yacute; model.

We performed first-principles calculations at finite temperatures to investigate the temperature dependence of the magnetic properties, such as exchange stiffness constants and Curie temperatures, of Co2MnZ (Z= Si, Al) assuming L21 and B2 structures. In L21 structures, we confirmed a relatively high Curie temperature for Co2MnAl, compatible with that of Co2MnSi; however, its exchange stiffness constant and single site magnetic excitation energy at zero temperature are much smaller than those of Co2MnSi. This might indicate that the Curie temperature of itinerant magnets cannot be determined by the exchange interaction at zero temperature. We also investigated the temperature dependence of the exchange stiffness constants of both alloys, and we found robustness in the temperature dependence of the exchange stiffness constant for Co2MnAl, assuming an L21 structure. This might lead to a high Curie temperature, contrary to the small exchange stiffness constant. Finally, we examined the temperature dependence of the electronic structure to investigate the origin of the behavior of the exchange stiffness constant at finite temperatures. We confirmed that the spin polarization at chemical potential effectively increases with an increasing temperature due to the altered electronic structure induced by the spin disorder. This might contribute to the robustness of the exchange stiffness constant at finite temperatures. Our results might indicate that renomarization of the electronic structure due to spin disorder at finite temperature influences the exchange interactions of Co2MnAl.

Tunnel-magnetoresistance (TMR) sensors based on magnetic tunnel junctions are emerging spintronic devices that are promising for applications to wearable bio-magnetic-field monitoring systems. Targeting bio-magnetic fields from the human heart and brain requires TMR sensors with sub-pT detectivity at frequencies of 1–1000 Hz. In this article, technical strategies for achieving such detectivity from the viewpoints of thin-film materials and sensor configurations are reviewed. Recent demonstrations of magnetocardiography and magnetoencephalography using our TMR sensors are also reviewed, and potentially effective techniques to further optimize detectivity are proposed.

We propose a voltage-controlled MRAM structure with bipolar switching, where opposite voltage polarities write different bit states. A synthetic antiferromagnet (SAF) free layer has the same symmetry as the simultaneous application of electric and magnetic fields. Micromagnetic simulations validate the proposal, and show self-limiting switching.

We have developed a novel nondestructive inspection technique for infrastructures using spintronics technologies. Since the tunnel magneto-resistance (TMR) effect has become dramatically more sensitive in recent years, TMR sensors can be applied to the highly sensitive nondestructive inspection named magnetic hammering test (MHT). The proposed MHT technique is based on the principle of detecting slight fluctuations in the spatial magnetic field caused by the vibration of steel materials. Since the fluctuations in the magnetic field occur with the natural vibration frequency of the steel materials, their condition can be detected from the change of natural frequency using highly sensitive TMR sensors. In this work, we have demonstrated that the size of steel plates was determined with high accuracy using the MHT technique.

The magnetic field resolution of the tunnel magneto-resistive (TMR) sensors has been improving and it reaches below 1.0 pT/Hz0.5 at low frequency. The real-time measurement of the magnetocardiography (MCG) and the measurement of the magnetoencephalography (MEG) have been demonstrated by developed TMR sensors. Although the MCG and MEG have been applied to diagnosis of diseases, the conventional MCG/MEG system using superconducting quantum interference devices (SQUIDs) cannot measure the signal by touching the body, the body must be fixed, and maintenance costs are huge. The MCG/MEG system with TMR sensors operating at room temperature have the potential to solve these problems. In addition, it has the great advantage that it does not require a special magnetic shielded room. Further developments are expected to progress to maximize these unique features of TMR sensors.

We have developed a highly sensitive tunnel magneto-resistive (TMR) sensor. A sensitivity has been dramatically improved by development of soft-magnetic materials and optimization of structure of TMR devices. In addition, 1/f noise was also decreased by improvement of integration techniques and reduction in defects of tunneling barrier layer. Because of dramatic improvement of the sensitivity and 1/f noise, we have successfully achieved the magnetic field resolution of 0.4 pT/Hz0.5 at 1 Hz. We have also demonstrated bio-magnetic field measurement at RT using the improved TMR sensors.

AbstractScanning magnetic microscopes enable high-sensitivity mapping of magnetic fields in thin geological sections, facilitating submillimeter- to submicrometer-scale studies of paleomagnetism and rock magnetism. Magnetic fields of geological samples have been mapped using various sensors, including Hall-effect devices, magneto-impedance devices, superconducting quantum interference devices (SQUIDs), quantum diamond devices, and tunnel magneto-resistance (TMR) devices. This study proposes magnetic microscopy using high-sensitivity room-temperature TMR sensors developed for biomagnetic applications. The goal was to create high-performance magnetic microscopes that do not require labor-intensive techniques, such as cryogenic technology. An XYZ stage developed for a scanning SQUID microscope (SSM) was used to demonstrate and evaluate magnetic microscopy with TMR sensors. The original TMR sensors developed for biomagnetic sensing composed of serially connected TMR elements with a total length of 2684 μm were shortened to 1073 μm (Sensor #1) and 357 μm length (Sensor #2). Background measurements at 50 Hz show magnetic field sensitivities better than 200 nT/√Hz and 600 nT/√Hz at 1 Hz for Sensor #1 and Sensor #2, respectively. By averaging 10 points of the original 50 Hz sampling, magnetic field sensitivities are better than 30 nT/√Hz and 90 nT/√Hz at 1 Hz for Sensor #1 and Sensor #2, respectively. To demonstrate TMR sensors as magnetic microscopes, a vertically magnetized Hawaii basalt thin section was measured and compared with a SQUID-acquired magnetic field map. Magnetic scanning images obtained with TMR sensors on a 0.1-mm grid were compared with those of SSM after adjusting the lift-off by upward continuation and integrated along the length of the sensors. The results demonstrated that magnetic images for 1073-μm-long (357 μm-long) TMR sensors aligned along the y-axis and x-axis are consistent with those after upward continuation to 0.3 mm (0.25 mm) and 0.4 mm (0.25 mm) and convolution by 1 × 10 (1 × 4) and 10 × 1 (4 × 1) matrix, respectively. Overall, the high-sensitivity TMR sensors exhibited promising performance. Further improvements can be made by optimizing the sensors, preamplifiers, and measurement systems for magnetic microscopy to achieve an optimum target resolution. Graphical Abstract

In high-sensitivity magnetic sensors, micro-sized magnetically-soft islands function as magnetic field detectors, featuring magnetic configurations composed of multiple domains, potentially containing cross-tie domain walls. Employing micromagnetics simulations, we examine the chaotic dynamics of a vortex within a cross-tie domain wall. We find that the vortex is confined within the domain wall, and it is continuously in core-reversal mode during relaxation or when driven by thermal excitation. This behavior contrasts with the 2D-free vortices which are more robust.

A tunnel magnetoresistance (TMR) sensor is a highly sensitive magnetic sensor workable at room temperature. To achieve smaller magnetic field detection, both a high TMR ratio and small magnetic anisotropy of the free layer are required. In this study, we developed single crystalline Co-based Heusler alloy Co2FeAlxSi1-x films as the free layer of TMR sensors and systematically investigated their crystalline and magnetic properties. The magnetic tunnel junction (MTJ) with these alloys is a promising device for highly sensitive TMR sensors due to their half-metallicity. We evaluated the B2 and L21 ordering parameters SB2 and SL21. SB2 was above 80 % for all the samples and SL21 was 48 % at a maximum, which indicates that all the samples exhibit half-metallicity. In addition, we evaluated their first magneto-crystalline anisotropy constant K1 from magnetization curves. K1 changed from positive to negative as Al component x increases and K1 was about 1200 erg/cc at x = 0.33. These results indicate that Co2FeAlxSi1-x films around x = 0.33 have a good candidate for the free layer of TMR sensors because of their half-metallicity and small magnetic anisotropy.

A new nondestructive inspection method, the magnetic hammer test (MHT), which uses a compact and highly sensitive tunnel magnetoresistance (TMR) sensor, is proposed. This method complements the magnetic flux leakage method and eliminates the issues of the hammer test. It can therefore detect weak magnetic fields generated by the natural vibration of a pipe with a high signal-to-noise ratio. In this study, several steel pipes with different wall thicknesses were measured using a TMR sensor to demonstrate the superiority of MHT. The results of the measurement show that wall thickness can be evaluated with the accuracy of several tens of microns from the change in the natural vibration frequency of the specimen pipe. The pipes were also inspected underwater using a waterproofed TMR sensor, which demonstrated an accuracy of less than 100 μm. The validity of these results was by simulating the shielding of magnetic fields and vibration of the pipes with the finite element method (FEM) analysis. The proposed noncontact, fast, and accurate method for thickness testing of long-distance pipes will contribute to unmanned, manpower-saving nondestructive testing (NDT) in the future.

We fabricated magnetic tunnel junctions (MTJs) with FeAlSi free layers and investigated the tunnel magnetoresistance (TMR) properties. We found that the temperature and bias voltage dependences of the TMR effect in FeAlSi-MTJs were almost the same as MTJs with Fe free layers despite the low Curie temperature of FeAlSi. In the inelastic electron tunneling spectroscopy measured at low temperatures, the relatively large cutoff energy of magnon excitation at the FeAlSi and MgO interface was confirmed. In addition, we studied for the first time the exchange stiffness constant of FeAlSi films by Brillouin light scattering. The determined value of the stiffness constant of FeAlSi was 14.3 (pJ/m), which was similar to that of Fe. Both the large magnon cutoff at the interface and the stiffness constant of FeAlSi are considered to be the reason for the good temperature and voltage dependences of FeAlSi-MTJs.

Using Machine Learning, the possibility of predicting small K 1 materials and exhaustive search, which is thought difficult with existing theoretical calculations, was verified for FeCoNi alloy systems. At the K 1 ≠ 0 point for FeCoNi alloys of about 10 3~5 (erg/cc), the results show that K 1 can be predicted with an error of 0.5 order of magnitude for virtually unknown compositions and temperatures, indicating the possibility of predicting small K 1 materials using machine learning. The results also show that it is possible to successfully predict how the K 1 = 0 line changes by structure-ordering (annealing samples), and exhaustively search for points where the sign of K 1 changes.

The magnetic hammer test (MHT) technique was developed using tunnel magnetoresistive (TMR) sensors that detect very weak magnetic signals generated by the natural vibration of steel plates. For the non-destructive testing (NDT) of infrastructures, which requires high accuracy and energy conservation, TMR sensors with high sensitivity have the potential to meet the NDT requirements. TMR sensors detected weak signals of nanotesla due to natural vibration and accurately evaluated the size of steel plates from the natural frequency. In addition, a finite element method (FEM) simulation analysis was conducted to elucidate the mechanism of the magnetic signals generated by the MHT. We concluded that the MHT technique using TMR sensors can be applied as a novel NDT method for detecting small fractures and corrosion in steel materials.