Magnetic Tunnel Junction Sensors Research Articles

Non-orthogonal two-step annealing method for linearized magnetic tunnel junction sensors

The orthogonal two-step annealing process is an effective strategy to linearize the response of magnetic tunnel junctions for magnetic field sensors. However, the response after the orthogonal annealing is inevitably modulated by the Neel effect from the reference layer, which results in an unexpected shift of the linear interval and a disappointing sensitivity deterioration in the weak field. Here, a non-orthogonal two-step annealing method is proposed to suppress the shift by compensating for the Neel coupling field. Experimental results show that the curve shift of junctions annealed in the non-orthogonal direction of 120° is 47.6% lower than that in the orthogonal one, with a significant sensitivity promotion in the weak field and little hysteresis increment. A simple energy minimization model is introduced to explain the results. Based on the model, the suppression of the curve shift is fulfilled with the effective field compensation for the Neel coupling field, modulated by the non-orthogonal annealing. Finally, Wheatstone bridge devices are constructed, and the bridge with non-orthogonally annealed junctions is found to have an increased sensitivity of 46.8% in the major loop along the sensing axis. Additionally, the non-orthogonal annealing method is also effective in suppressing the cross sensitivity, which is important for further application to three-axis magnetic sensors.

Equivalent Noise Analysis and Modeling for a Magnetic Tunnel Junction Magnetometer with In Situ Magnetic Feedback

Magnetic tunnel junction (MTJ) sensors have been one of the excellent candidates for magnetic field detection due to their high sensitivity and compact size. In this paper, we design a magnetometer with in situ magnetic feedback consisting of an MTJ sensor. To analyze and evaluate the detectivity of the MTJ magnetometer, a noise model of the MTJ sensor in the magnetometer without magnetic feedback is first developed. Then, the noise model of the MTJ magnetometer with in situ magnetic feedback is also established, including the noises of the MTJ sensor and the signal conditioning circuit, as well as the feedback circuit. The equivalent noise model of the MTJ magnetometer with in situ magnetic feedback is evaluated through nonlinear fitting for the noise voltage spectrum. Although the noise generated by the MTJ sensor is much greater than that of the signal conditioning circuit, the noise introduced by the feedback coils into the MTJ sensor is slightly more than twice that generated by the MTJ sensor itself. The measurement results show that the detectivity of the MTJ magnetometer with in situ magnetic feedback reaches 526 pT/Hz1/2 at 10 Hz. The equivalent noise analysis method presented in this paper is suitable for the detectivity analysis of magnetometers with magnetic feedback.

Engineering buffer layers to improve temperature resilience of magnetic tunnel junction sensors

Improving the thermal resilience of magnetic tunnel junctions (MTJs) broadens their applicability as sensing devices and is necessary to ensure their operation under harsh environments. In this work, we are address the impact of temperature on the degradation of the magnetic reference in field sensor stacks based on MgO-MTJs. Our study starts by simple MnIr/CoFe bilayers to gather enough insights into the role of critical morphological and magnetic parameters and their impact in the temperature dependent behavior. The exchange bias coupling field (H ex), coercive field (H c), and blocking temperature (T b) distribution are tuned, combining tailored growth conditions of the antiferromagnet and different buffer layer materials and stackings. This is achieved by a unique combination of ion beam deposition and magnetron sputtering, without vaccum break. Then, the work then extends beyond bilayers into more complex state-of-the-art MgO MTJ stacks as those employed in commercial sensing applications. We systematically address their characteristic fields, such as the width of the antiferromagnetic coupling plateau ΔH, and study their dependence on temperature. Although, [Ta/CuN] buffers showed higher key performance indications (e.g. H ex) at room temperature in both bilayers and MTJs, [Ta/Ru] buffers showed an overall wider ΔH up to 200 °C, more suitable to push high temperature operations. This result highlights the importance of properly design a suitable buffer layer system and addressing the complete MTJ behavior as function of temperature, to deliver the best stacking design with highest resilience to high temperature environments.

Noise measurement and system calibration on magnetoresistive sensors

PurposeThe noise measurement on magnetoresistive (MR) sensors is generally conducted by techniques including single-channel data sampling and fast Fourier transform (FFT) analysis as well as two-channel cross-correlation. The single-channel method is easy to implement and is widely used in the noise measurement on MR sensors, whereas the two-channel method can only eliminate part of the system noise. This study aims to address two key issues affecting measurement accuracy: calibration of the measurement system and the elimination of system noise.Design/methodology/approachThe system is calibrated by using a low-noise metal film resistor in that the system noise is eliminated through power spectrum subtraction. Noise measurement and analysis are conducted for both thermal noise and detectivity of magnetic tunnel junction (MTJ) sensor.FindingsThe thermal noise measurement error is less than 2%. The detectivity of the MTJ sensor reaches 27 pT/Hz1/2 at 2 kHz.Originality/valueThis study provides a more practical solution for noise measurement and system calibration on MR sensors with a bias voltage and magnetic field.

Computing Resistance-Style Image Sensors for Artificial Neural Networks

Today, machine vision experiences large latency due to big data processing, which is a barrier to time-critical applications. To address this issue, in-sensor computing was presented in the past. Here, we present a scheme of computing in a magnetic tunneling junction (MTJ) sensor array for proof-of-principle. Using the MTJ sensor array, the functions of artificial neural network (ANN) classifiers and autoencoders were verified. The time for correct classification of one picture was less than <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$9~\mu \text{s}$ </tex-math></inline-formula> . The power consumed in the sensor array can be decreased according to the square law without affecting the results. Our work shows universal circuits and algorithms to compute in resistance-style ANN image sensors with promising energy efficiency.

CoFeBX layers for MgO-based magnetic tunnel junction sensors with improved magnetoresistance and noise performance

Magnetoresistive sensors have been enthusiastically selected for applications requiring magnetic field detection with small footprint sensors. The optimisation of the sensor response includes using soft magnetic free layers, based on CoFeB and NiFe alloys. Here we report the TMR and noise performance of magnetically saturated in-plane MTJ sensors including CoFeBTa and CoFeSiB soft magnetic films as free layers (FL). Assessing magneto-crystalline anisotropy μ0Hk values of 2.1 and 0.7 mT in CoFeB 2.5 (nm)/Ru 0.2/CoFeBTa 4 and CoFeB 3/Ru 0.2/CoFeSiB 4 compared to 1.7 mT in CoFeB 2.5/Ru 0.2/NiFe 4, together with an improved magnetoresistance of 230% in CoFeBSi comparing with 170% (NiFe) with superior noise characteristics, with Hooge parameter of αH = 7 × 10−11 μm2.

Tailoring of magnetic anisotropy by ion irradiation for magnetic tunnel junction sensors

Magnetic Tunnel Junctions (MTJ) are employed in a range of technologies such as data storage, sensing, and so on due to their compact nature and high field sensitivity. In magnetic films and structures, ion irradiation is capable of modifying the crystal structure, phases and magnetic properties including magnetic anisotropy, which enables tunability of the sensing axis of devices such as MTJ sensors. Irradiation can also adjust the magnetic properties of specific layers without affecting the others, making it one of the most effective methods to post-process multi-layered devices. In this article we review the literature of light ion irradiation effects on MTJs and their component materials, focusing on the effects most relevant to MTJ sensors, such as magnetic anisotropy and magnetization. We first briefly review the effects of the ion-solid interaction, then discuss the causes of magnetic anisotropy, and lastly, we summarize the state of the field of ion-induced modification of magnetic properties in ferromagnetic thin films, antiferromagnetic structures, and MTJs.

Control of sensitivity in vortex-type magnetic tunnel junction magnetometer sensors by the pinned layer geometry

The tuning of sensitivity and dynamic range in linear magnetic sensors is required in various applications. We demonstrate the control and design of the sensitivity in magnetic tunnel junction (MTJ) sensors with a vortex-type sensing layer. In this work, we develop sensor MTJs with NiFe sensing layers having a vortex magnetic configuration. We demonstrate that by varying the pinned layer size, the sensitivity to magnetic field is tuned linearly. We obtain a high magnetoresistance ratio of 140%, and we demonstrate a controllable sensitivity from 0.85% Oe−1 to 4.43% Oe−1, while keeping the vortex layer fixed in size. We compare our experimental results with micromagnetic simulations. We find that the linear displacement of vortex core by an applied field makes the design of vortex sensors simple. The control of the pinned layer geometry is an effective method to increase the sensitivity, without affecting the vortex state of the sensing layer. Furthermore, we propose that the location of the pinned layer can be used to realize more sensing functionalities from a single sensor.

RF Analog Hardware Trojan Detection Through Electromagnetic Side-Channel

With the advent of globalization, hardware trojans provide an ever-present threat to the security of devices. Much of the research to date has centered around documenting and providing detection methods for digital trojans. Few, however, have explored the space of trojans in the RF/analog front end. Two hardware trojans, an analytical analysis of the trojan impacts on two different types of amplifiers, and an unsupervised ML detection method for edge IOT applications using magnetic tunnel junction sensors for side-channel monitoring are explored. A classification autoencoder for anomaly detection is presented with an accuracy of greater than 90% with both single tone and BLE data is presented.

Corrections to “Normalization and Correction Factors for Magnetic Tunnel Junction Sensor Performances Comparison”

In the above article <xref ref-type="bibr" rid="ref1" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">[1]</xref> , unfortunately, errors were present in <xref ref-type="table" rid="table1" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Table I</xref> , for the correction factors of the 1/4 Wheatstone bridge regarding the thermal noise, the shot noise, and the Hooge parameter entry. The errors have no influence on the experimental results, neither on the other table nor on the discussion and conclusions of the article. <xref ref-type="table" rid="table1" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Table I</xref> contained an improper formula for the corrections factor (CF) in the case of the 1/4 Wheatstone bridge configuration (case c).

Normalization and Correction Factors for Magnetic Tunnel Junction Sensor Performances Comparison

In this manuscript we propose a calculation method where the magneto-resistive elements are modelled as fluctuating resistances to correct the output voltage noise of magnetic tunnel junction (MTJ) from standard electronic circuits. This method is validated on single elements, partial and full Wheatstone bridge circuits, giving rise to a correction factor affecting the output voltage noise as well as sensitivity values. Combining the correction factor and a normalization by the number of MTJs pillars and the pillar surface, we show that the performances extracted by this method allow universal comparison between any results from literature.

Magnetoresistive sensor detectivity: A comparative analysis

We report on the noise performance characteristics of magnetic sensors using both magnetic tunnel junction (MTJ) and giant magnetoresistance (GMR) elements. Each sensor studied has a notably different noise and detectivity. Of the sensors we measured, those based on GMR multilayers have the lowest noise and detectivity. However, the GMR sensor also has a significantly smaller linear range. To make a direct comparison between sensors, we scale the linear operating ranges of each sensor to be the same. This is the phenomenological equivalent of modifying the flux concentration. Upon scaling, the low frequency detectivity of the tunneling magnetoresistance (TMR) sensors becomes essentially equal to that of the GMR sensor. Using the scaling approach, we are able to place the detectivity in the context of other key parameters, namely, size and power consumption. Finally, we use this technique to examine the upper limit for magnetoresistive sensor performance based on a notional MTJ sensor using present record setting TMR values.

Nonhysteretic Vortex Magnetic Tunnel Junction Sensor with High Dynamic Reserve

Multiple geometrical conditions for different magnetic vortex states to appear in micrometer-sized magnetic tunnel junctions (MTJs) are investigated in experiment and simulation. Both results match up well, providing clear images of vortex behaviors. We pattern a compact array of single-vortex MTJ elements and perform magnetotransport and noise measurements. This sensor features nonhysteretic behavior, a small size of 0.5 \ifmmode\times\else\texttimes\fi{} 0.5 ${\mathrm{mm}}^{2}$, a low normalized noise of ${10}^{\ensuremath{-}13}\phantom{\rule{0.25em}{0ex}}{\mathrm{Hz}}^{\ensuremath{-}1}$, a detectability of $18\phantom{\rule{0.1em}{0ex}}nT/\sqrt{Hz}$ at 1 Hz, and a large dynamic range of 100 Oe. Its 115 dB broad dynamic reserve and superior stability against temperature and environmental stray fields are ideally suited for magnetic sensing applications.

Detection of HIV-1 antigen based on magnetic tunnel junction sensors**Project supported by President’s Fund of CUHKSZ, Longgang Key Laboratory of Applied Spintronics, at The Chinese University of Hong Kong, the National Natural Science Foundation of China (Grant Nos. 11974298 and 61961136006), the Shenzhen Fundamental Research Fund, China (Grant No. JCYJ20170410171958839), and Shenzhen Peacock Group Plan, China (Grant No. KQTD20180413181702403).

We report a p24 (HIV disease biomarker) detection assay using an MgO-based magnetic tunnel junction (MTJ) sensor and 20-nm magnetic nanoparticles. The MTJ array sensor with sensing area of 890 × 890 μm2 possessing a sensitivity of 1.39 %/Oe was used to detect p24 antigens. It is demonstrated that the p24 antigens could be detected at a concentration of 0.01 μg/ml. The development of bio-detection systems based on magnetic tunnel junction sensors with high-sensitivity will greatly benefit the early diagnosis of HIV.

High-Temperature Magnetic Tunnel Junction Magnetometers Based on L1$_0$-PtMn Pinned Layer

Magnetometer sensors with high sensitivity based on magnetic tunnel junctions (MTJs) were developed to operate at high temperatures of more than 250 °C. We replaced the commonly used IrMn antiferromagnetic pinning layer with an optimized PtMn antiferromagnet, which has a high magnetic thermal stability due to its L1 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> crystal structure. We investigate the effect of measurement temperature on magnetoresistance (MR), exchange bias, and sensitivity in sensor MTJ stacks using either IrMn or PtMn pinning layers. Upon increasing the temperature up to 250 °C, the MR ratio decreases from 146% to 50% for IrMn, and 133% to 80% for PtMn pinning layers. The PtMn-based MTJ sensors sustain a constant sensitivity (4-5 %/Oe) up to 250 °C, compared with IrMn-based sensors, which decrease drastically. This high-temperature performance links to the thermal stability of exchange bias in the pinned layer by using PtMn.

Design and fabrication of integrated magnetic field sensing system with enhanced sensitivity

Weak magnetic field detection is always a challenging topic and sensing systems face a great challenge to be very sensitive in response to magnetic field, in the meantime, to maintain very low overall system noise. Some external auxiliary system could provide significant enhancement to overall signal-to-noise ratio (SNR) such as a magnetic flux concentrator made with soft magnetic material. In this paper, we have systematically studied and built an integrated magnetic tunnel junction (MTJ) sensing system to meet these challenges. MTJ sensor design is optimized to achieve high zero-field sensitivity of 5% MR/Oe, which is defined by the slope of the device’s transfer loops at zero field, equivalent to 20 mV/Oe with 10 mA driving current. Both deposition conditions and fabrication techniques are optimized for magnetic flux concentrator (MFC) made of soft magnetic material to achieve a magnetic susceptibility of χ = 30,800 and a gain of 4 for a 100 nm thick prototype.

Controlling domain configuration of the sensing layer for magnetic tunneling junctions by using exchange bias

The control of magnetic domain formation and fluctuations in the sensing layer is important to progress for low noise in magnetic tunnel junction sensors. We studied the effect of exchange bias on the domain structure in micro-patterned Permalloy (Py: Ni80Fe20) sensing layer. We deposited single Py films, and Pt48Mn52/Py films, where the latter showed exchange bias. By controlling the thickness of Py, Pt48Mn52 (15nm)/Py (t=235 nm) showed a small coercivity and exchange bias of 7 Oe. After micro-fabrication into circular pillars 80 µm in diameter, we measured the domain structure by Magneto Optical Kerr Effect (MOKE) microscopy. MOKE images showed that single Py pillars have a simple closure domain, where the domain wall at the center moved with the applied field. The exchange-biased Py pillars exhibited a more complicated structure, but with fixed domains at the center region due to the exchange bias overcoming the magnetostatic energy. The uniform rotation of magnetization at the center of the sample is promising for decreasing the domain hopping magnetic noise.

Practical Challenges of Magnetic Sensors Based on Magnetic Tunnel Junctions for Power Grid Applications

We provide a review of the status and challenges for utilizing magnetic tunneling junction (MTJ) based magnetic sensors for power grid applications. We show that, with both modeling and experimental measurement, an optimized MTJ-based magnetic sensor can be utilized to monitor grid current, especially for individual grid lines. From the perspective of the sensor, the sensitivity, signal-to-noise ratio, and linearity can all meet the needs in the field. Unlike traditional measurements, this measurement can be based on a contactless or “remote” sensing setup, where the sensor is placed away from the grid line. One of the challenges is that a complex topology and multiple grid lines may exist with different current levels. The other challenge is that, with the improved sensing capability provided by these MTJ sensors, a significantly larger amount of data can be collected, so the system bottleneck shifts from sensing to data transfer.

Effect of second-order magnetic anisotropy on nonlinearity of conductance in CoFeB/MgO/CoFeB magnetic tunnel junction for magnetic sensor devices

We studied the effect of second-order magnetic anisotropy on the linear conductance output of magnetic tunnel junctions (MTJs) for magnetic-field-sensor applications. Experimentally, CoFeB/MgO/CoFeB-based MTJs were fabricated, and the nonlinearity, NL was evaluated for different thicknesses, t of the CoFeB free layer from the conductance. As increasing t from 1.5 to 2.0 nm, maximum NL, NLmax was found to decrease from 1.86 to 0.17% within the dynamic range, Hd = 1.0 kOe. For understanding the origin of such NL behavior, a theoretical model based on the Slonczewski model was constructed, wherein the NL was demonstrated to be dependent on both the normalized second-order magnetic anisotropy field of Hk2/|Hkeff| and the normalized dynamic range of Hd/|Hkeff|. Here, Hkeff, Hk2, are the effective and second-order magnetic anisotropy field of the free layer in MTJ. Remarkably, experimental NLmax plotted as a function of Hk2/|Hkeff| and Hd/|Hkeff|, which were measured from FMR technique coincided with the predictions of our model. Based on these experiment and calculation, we conclude that Hk2 is the origin of NL and strongly influences its magnitude. This finding gives us a guideline for understanding NL and pioneers a new prospective for linear-output MTJ sensors to control sensing properties by Hk2.

Multistep Parametric Inversion of Scanning Magnetic Microscopy Data for Modeling Magnetization of Multidomain Magnetite

AbstractDevelopment in instrumentation and technology now allows for mapping of magnetic anomalies, caused by spatial variations in magnetization in the source material, from the mineral to the crustal anomalies scale. High‐resolution magnetic mapping techniques allow for accurate investigation of the magnetization in natural rock samples and particularly of their remanence carriers, which can record geologically meaningful information. Multidomain magnetic grains are expected to retain a remanence that is susceptible to change by exposure to magnetic fields or by changes in temperature. Although this makes multidomain grains less reliable remanence carriers for paleomagnetic studies, their magnetization contributes to rocks bulk magnetization and to induced anomalies in the Earth crust. Here, we investigate the fine‐scale magnetization of a sample that exhibits a multidomain behavior. We used a scanning magnetic microscope, equipped with a room temperature magnetic tunnel junction sensor, to map the magnetic field over a petrographic thin section of the sample containing large magnetite grains (&gt;100 μm) surrounded by serpentine and carbonate. We modeled the fine‐scale remanent magnetization of the magnetite by inverting the magnetic scans data acquired in near‐field‐free conditions. We applied a multistep inversion, a priori tested on a synthetic model, with a controlled range on the intensity of the magnetization. Modeling results on the study sample suggest homogenously magnetized regions within the magnetite grains with variable remanent magnetization intensities and directions coherent with the multidomain behavior inferred from bulk measurements.