Tunneling Magnetoresistance Devices Research Articles

Perspective on nanoscale magnetic sensors using giant anomalous Hall effect in topological magnetic materials for read head application in magnetic recording

Recent advances in the study of materials with topological electronic band structures have revealed magnetic materials exhibiting giant anomalous Hall effects (AHEs). The giant AHE has not only attracted the research interest in its mechanism but also opened up the possibility of practical application in magnetic sensors. In this article, we describe simulation-based investigations of AHE magnetic sensors for applications to read head sensors (readers) of hard disk drives. With the shrinking of magnetic recording patterns, the reader technology, which currently uses multilayer-based tunnel magnetoresistance (TMR) devices, is associated with fundamental challenges, such as insufficient spatial resolution and signal-to-noise ratio (SNR) in sensors with dimensions below 20 nm. The structure of an AHE-based device composed of a single ferromagnetic material is advantageous for magnetic sensors with nanoscale dimensions. We found that AHE readers using topological ferromagnets with giant AHE, such as Co2MnGa, can achieve a higher SNR than current TMR readers. The higher SNR originates from the large output signal of the giant AHE as well as from the reduced thermal magnetic noise, which is the dominant noise in TMR readers. We highlight a major challenge in the development of AHE readers: the reduction in the output signal due to the shunting of the bias current and the leakage of the Hall voltage through the soft magnetic shields surrounding the AHE reader. We propose reader structures that overcome this challenge. Finally, we discuss the scope for future research to realize AHE readers.

Improvement of magnetic field detectivity in electrical 1/f noise-dominated tunnel magnetoresistive sensors by AC magnetic field modulation technique

Suppression of 1/f noise in tunnel magnetoresistance (TMR) sensors is a central issue in the realization of magnetic field sensors with ultrafine magnetic field detectivity. Although AC modulation with an external magnetic field has been proposed as a method to shift the operating frequency of a sensor to a high frequency and substantially suppress 1/f noise, its effects on the two types of 1/f noise, that is, magnetic and electrical 1/f noise, are not well understood. In this study, we investigated the noise characteristics and signal detection performance of TMR sensors with an even-function resistance-magnetic field curve operated by the AC modulation method. For one TMR device in which the magnetic 1/f noise was dominant, AC modulation degraded the magnetic field detectivity owing to the additional noise induced by the AC modulation field. However, in another TMR device, in which the electrical 1/f noise was artificially enhanced by introducing lattice defects in the MgO tunnel barrier, AC modulation effectively suppressed the 1/f noise and improved the magnetic field detectivity by one order. This demonstrates that the AC modulation method using an external magnetic field is effective for magnetic field sensors in which electrical 1/f noise is dominant.

Design of Interface Circuits and Lightweight PUF for TMR Sensors

The micro-magnetometers with high-resolution digital output are widely used in military and civilian fields. We proposed a novel high-precision interface circuit with optimized chopper technique and switched-capacitor modulator for tunneling magnetoresistance (TMR) sensors. This work also proposes a novel method to create lightweight physically unclonable function (PUF) by using existing TMR devices. The sigma-delta modulator converts the sensor signal into a robust digital output and maintain the signal-to-noise ratio (SNR) of the front-end circuit. We also take advantage of inherent variations of TMR sensors to generate PUF responses that are similarly unique and unclonable. The interface circuit is fabricated by 0.35μm CMOS process from Shanghai Huahong foundry. The active area of ASIC is only about 3 mm×2.7 mm. The interface circuit can achieve a SFDR of 120dB and a SNR of 98 dB at a sampling frequency of 200kHz. The TMR magnetometers were tested in an environment of three-layer magnetic shielding. Our proposed PUF is also tested in terms of uniqueness and reliability.

Tunneling magnetoresistive devices as read heads in hard disk drives

Hard Disk Drive read heads moved from GMR to Tunneling Magnetoresistive (TMR) Devices as a way to enhance head SNR in order to keep up with advanced linear densities. TMR technology was further extended by the transition from amorphous alumina tunneling barriers to crystalline MgO barriers lattice matched to adjacent BCC magnetic layers resulting in much higher MR ratios. Enhanced side shielding using Junction Shield and reduced shield to shield spacing were additional enablers that increased track and linear density capabilities of TMR heads. TMR heads gained additional momentum and complexity by the introduction of two dimensional recording heads that boosted head SNR by up to 3 dB. As the RA of the tunneling barriers is reduced, overcoming spin torque noise presents itself as a further challenge to be addressed. As device dimensions are scaled down, magnetic thermal noise can only be overcome by increasing the moment of the free layer which may already start establishing the final density point achievable by TMR heads.

Tunnel magnetoresistance sensors with symmetric resistance-field response and noise properties under AC magnetic field modulation

Magnetoresistance (MR) devices with symmetric (even-function) resistance change to an external magnetic field is useful for various magnetic sensor applications, such as encoders for position and rotation detection and ultra-sensitive magnetic field sensors, when combined with a frequency modulation method. In this study, we developed tunnel magnetoresistance (TMR) sensors with even-function resistance–field (R–H) responses and investigated their noise properties. The developed TMR sensors achieved a higher MR ratio (∼120%) than those of conventional giant magnetoresistive multilayers exhibiting even-function R–H responses. The TMR sensors were realized by stabilizing the magnetization of the magnetic free layer (FL) in an anti-parallel configuration with respect to that of the reference layer. Orange-peel ferromagnetic coupling between the FL and a pinned layer through an AgSn spacer layer was used to control the strength of the unidirectional magnetic anisotropy of the FL, by which even-function R–H responses with negligible hysteresis were obtained. The sensors enabled us to study the intrinsic noise properties of TMR devices, particularly under an external AC modulation magnetic field. We found that the magnetic 1/f noise originating from thermal magnetization fluctuation was the dominant noise of the TMR sensors at low frequencies. We observed that the application of an AC modulation field to the TMR sensors increased the noise owing to the magnetic domain fluctuation and the transfer of magnetic 1/f noise to the modulation frequency.

Type-Y magnetic tunnel junctions with CoFeB doped tungsten as spin current source

Spin–orbit torque magnetic tunnel junctions (SOT-MTJs) with high tunneling magnetoresistance (TMR) ratio and high energy-efficiency are crucial for the development of SOT-magnetic random-access memory and other SOT devices. Here, the SOT-MTJs doped with an ultrathin layer of 0.2 nm CoFeB in the W writing line are fabricated, and the TMR ratio of the updated MTJs is up to 179%. Meanwhile, the SOT efficiency of the W layer doped with magnetic atoms (∼0.149) is weakly dependent on the doping, manifesting the intrinsic mechanism of the W layer in generating the spin Hall effect. This study shows promise of the magnetic-atom doped W/CoFeB/MgO/synthetic antiferromagnetic stacks to achieve high TMR and efficient type-Y SOT devices.

Laser induced crystallization of Co\u2013Fe\u2013B films

Local crystallization of ferromagnetic layers is crucial in the successful realization of miniaturized tunneling magnetoresistance (TMR) devices. In the case of Co–Fe–B TMR devices, used most successfully so far in applications and devices, Co–Fe–B layers are initially deposited in an amorphous state and annealed post-deposition to induce crystallization in Co–Fe, thereby increasing the device performance. In this work, first direct proof of locally triggered crystallization of 10 nm thick Co–Fe–B films by laser irradiation is provided by means of X-ray diffraction (XRD) using synchrotron radiation. A comparison with furnace annealing is performed for benchmarking purposes, covering different annealing parameters, including temperature and duration in the case of furnace annealing, as well as laser intensity and scanning speed for the laser annealing. Films of Co–Fe–B with different stoichiometry sandwiched between a Ru and a Ta or MgO layer were systematically assessed by XRD and SQUID magnetometry in order to elucidate the crystallization mechanisms. The transformation of Co–Fe–B films from amorphous to crystalline is revealed by the presence of pronounced CoFe(110) and/or CoFe(200) reflexes in the XRD θ-2θ scans, depending on the capping layer. For a certain window of parameters, comparable crystallization yields are obtained with furnace and laser annealing. Samples with an MgO capping layer required a slightly lower laser intensity to achieve equivalent Co–Fe crystallization yields, highlighting the potential of laser annealing to locally enhance the TMR ratio.

Magnetic, magnetoresistive and low-frequency noise properties of tunnel magnetoresistance sensor devices with amorphous CoFeBTa soft magnetic layers

Magnetic field sensors using the tunnel magnetoresistance (TMR) effect require linear resistance–magnetic field (R–H) response curves with small hysteresis, for which the soft magnetic property of the free layer (FL) is critical. In this work, we investigated amorphous CoFeBTa (CFBT) as a soft magnetic layer of the FL of CoFeB/MgO/CoFeB-based magnetic tunnel junctions in view of magnetic, TMR, and low-frequency noise properties. A two-step annealing process enabled an orthogonal magnetization configuration between the FL and the reference layer, by which linear R–H curves with small hysteresis were realized. The change in the shape of the R–H curve depending on annealing temperature is explained by the Stoner–Wohlfarth model. The highest TMR ratio of ∼160% and sensitivity of ∼70%/mT were obtained with a CFBT (20 nm)/Ta (0.3 nm)/CoFeB (3 nm) FL. The noise of the TMR devices are dominated by 1/f noise below ∼10 kHz in frequency, which limits the detectivity (D) of the magnetic field of the sensor. The sensor devices patterned to 50 µm diameter circular shapes showed a minimum D of ∼2 nT/ at 10 Hz, which is superior to the previously reported values for the TMR sensors with NiFe soft magnetic layers.

Rectified Tunnel Magnetoresistance Device With High On/Off Ratio for In-Memory Computing

Low on/off ratio gravely hinders the application of magnetoresistance (MR) devices for the accurate and reliable data fetch. In this paper, a rectified tunnel MR (R-TMR) device is fabricated by integrating perpendicular-magnetic-anisotropy magnetic tunnel junction (PMA MTJ) and Schottky diode. High on/off ratio, intrinsic non-volatility and multi-dimensional regulation capabilities can be obtained, which makes this emerging spintronic device suitable for in-memory computing (IMC). By reversing the rectifying direction of the diode and tuning the proportions of alternating current (AC) and direct current (DC), reconfigurable logic operations have been achieved. Two proofs of concept, i.e. “NOR” and “NAND”, are experimentally performed.

Microstructure and Tunneling Magnetoresistive Characteristics of Laser-Irradiated Magnetic Data Recording Head

Heat-assisted magnetic recording is a future hard disk drive (HDD) technology for extremely increasing areal density. The tunneling magnetoresistive (TMR) device structure, located inside the magnetic read-write head of a hard drive, is the most complex and sensitive component, which contains sensitive components, such as a touchdown sensor and a near-field transducer. The laser used in the writing process might cause degradation of magnetic head components. The effects of laser irradiation on the TMR device have been investigated using a quasi-static test measurement to observe the characteristic changes of the magnetic response sensitivity function. The results showed only the amplitude degradation, as the resistance and asymmetry parameters were unchanged. The nanostructure of the degradation TMR devices was investigated by using scanning transmission electron microscopy and X-ray energy-dispersive spectrometry. It was found that the manganese depletion in the antiferromagnetic layer causes a nonactive zone due to the poor magnetic coupling pinning function and reduces the functioning of the HDD. The ratio of active and nonactive zone elements has a strong correlation to the amplitude degradation of individual TMR devices. The TMR structure with shorter strip height can have a higher risk of device degradation.

Integrated Magnetic Sensor Probe and Excitation Wire for Nondestructive Detection of Submillimeter Defects

Eddy current testing with excitation and pickup coils is widely used to detect defects in metals. The detection of submillimeter defects is difficult because of limitations on pickup coil size. We have developed an integrated magnetic probe for detecting submillimeter defects in both nonmagnetic and ferromagnetic materials. The sensor probe consists of a tunneling magnetoresistance (TMR) device and an excitation wire. This probe can apply small, localized magnetic fields to a sample that is near the TMR sensor. The direction of the field excitation is perpendicular to the sensing direction of the TMR sensor to minimize magnetic coupling between the excitation wire and the sensor. A change in the line-scanned signal was detected when measuring pits from 0.5 mm to 1.0 mm in diameter at depths of more than 0.1 mm in both steel and aluminum plates. Signal changes depended on defect size. As the frequency was increased from 1 kHz to 50 kHz, the defect signal change for the aluminum plate increased due to an increase in eddy currents. A clear defect signal for the steel plate was obtained even at a low frequency of 1 kHz because the detected signal for steel consists of a frequency-dependent eddy-current component and a magnetization component even at low frequencies.

High Tc TMR-Superconducting Mixed Sensor: Fabrication and Performance

Giant magneto-resistive (GMR)-superconducting mixed magnetic sensors are expected to be widely used because of their excellent sensitivity, stability, band characteristics, and low cost. If a tunnel magnetoresistance (TMR)-superconducting mixed magnetic sensor is prepared using a higher sensitivity and lower power TMR instead of GMR, the performance of the device will be further improved. We have fabricated a TMR-superconducting mixed sensor by a bonding process, and the feasibility was verified. The sensitivity was 21 times higher than that of the TMR device alone, reaching 18.1%/Oe. The noise level reached 6.6 pT/Hz1/2 at 77 K. At the same time, the influence of different constriction width of the superconducting magnetic flux concentrator on the device performance was compared at 4.5–77 K.

Device Modeling of MgO-Barrier Tunneling Magnetoresistors for Hybrid Spintronic-CMOS

Spintronic sensors, which are based on the tunneling-magnetoresistive (TMR) effect, have been utilized in detecting low-magnetic fields. However, still no computer-based model of these devices is available to integrated circuit designer to implement them in a hybrid spintronic-CMOS system. We developed a finite-element method (FEM)-based model of a MgO-barrier TMR device in COMSOL Multiphysics . The parameters of this model were extracted from the state-of-the-art fabrication and experimental data. Results were compared with respect to the model geometry and the used material. The proposed TMR sensor model offers a linear response with a high TMR ratio of 233% at 10-mV power supply. The model was exported to Cadence Spectre to create a compact model using Verilog-A language. The developed sensor model was simulated with its analog front end in the same environment. This model provided a reliable benchmark for modeling of the future hybrid spintronic-CMOS developments.

Design of a novel lower-noise tunneling magnetoresistance micromachined gyroscope

The core technology behind micromachined gyroscopes is the detection of the weak Coriolis force, the noise level of which by conventional methods has reached its limit. In this paper, an in-plane gyroscope structure based on the tunneling magnetoresistance (TMR) effect is designed providing a means to detect this force in the MEMS vibratory gyroscope. TMR is used so that a high sensitivity is achieved. From a theoretical analysis of the designed gyroscope structure, the structural sensitivity of the detection Coriolis force is obtained. A permanent magnet was designed and simulated for the TMR device and a magnet sensitivity was obtained. To obtain the magnetoresistance sensitivity, an experiment was performed to test a TMR device. A TMR gyroscope system model is implemented in the SIMULINK environment using parameter values obtained from calculations and some obtained from the experimental tests of the total sensitivity. Combining our separate results, a total sensitivity for the TMR gyroscope of 1.18 mV/°/s was obtained along with a theoretical noise floor of 9.48 × 10−5 °/s/√Hz. Therefore, the designed TMR gyroscope provides high sensitivity while achieving a lower noise level.

Interfacial properties of Hg2CuTi-type Heusler alloy Ti2NiAl/GaAs(100) heterojunction

For Hg2CuTi-type Inverse-Heusler alloy Ti2NiAl/GaAs(100) tunnel heterojunction, the magnetism, density of states and spin polarization of atoms at the interface were investigated systematically based on the first-principle calculation within the density functional theory (DFT). The calculated results reveal that the interface states seriously destroy the structural half-metallicity and lead to the spin polarization less than 60%. Among all of calculational hetero-structures, only the heterojunction with TA-ATII structure still retains nearly 60% spin polarization, which is expected for further application in Tunnel Magneto resistance (TMR) devices.

Thermal Stress Evaluation of Tunneling Magnetoresistive Structures in Data Storage Devices

The conventional read sensor technology used in hard disk drives is the tunneling magnetoresistive (TMR) device. The technology evolution of the TMR device toward higher signal-to-noise performance has been achieved through aggressive scaling of the thin film layered structures in the device to the point that thermal stability and thermally induced degradation have come to limit reliability. In this study, a thermal stress has been applied to TMR devices between 150–250 °C, and the resistance, amplitude, and asymmetry parameters, both before and after the thermal stress, were measured using a quasi-static test (QST). The results reveal a temperature dependence of the TMR device performance. The microstructure of the annealed devices was further studied using transmission electron microscopy and energy dispersive X-ray analysis, revealing structural defects that are related to the QST parametric changes. Both atomic misalignment of the MgO layer and Ir depletion are proposed as origins for the instability of the magnetic response in the device after thermal stress.

Etching behaviors of tunneling magnetoresistive (TMR) materials by ion beam etching system

In data storage manufacturing, ion etching process is commonly chosen for the preparation of surface patterns, for example, under the read-write head of the hard disk drive (HDD) to form an air bearing surface (ABS). This read-write head consists of a multilayer thin film of different materials to form a tunneling magnetoresistive (TMR) device. In this work, we applied the Monte Carlo-based simulation package to calculate the etching yield of major materials in the HDD head’s structure. The plasma characteristic in the industrial-size ion beam etching (IBE) system has been studied by the special plasma diagnostic probes. Effects of input parameters in IBE system such as incident beam angle, ion beam current and gas flow in the system were considered. Plasma characteristic can be obtained from plasma I-V curve characterization. The sputtering yields of materials were found maximum when an incident angle is about 70° to the normal surface. The floating potential of plasma in the system with the ion/electron compensation from PBN is calculated.

Effect of Finite Tunneling Magnetoresistance for the Switching Dynamics in the Spin Transfer Torque Magnetic Tunneling Junctions

We investigate the effect of tunneling magnetoresistance (TMR) on the spin transfer-torque (STT) switching behaviors in magnetic tunneling junctions. In most of the micromagnetic simulations for STT switching, a uniform current density has been assumed, which is not realistic in the high TMR devices. The local STT is proportional to the local current density, and the local current density will be determined by the local resistivity. Since higher than 150% of TMR values is required in the real STT-magnetoresistive random access memory devices, the local resistance is dramatically changed as a function of the relative spin orientation between the fixed and free layers under the constant voltage operation mode. By employing non-uniform current density in STT switching simulations using the “embedded object-oriented micromagnetic framework” scheme, we found that the details of switching behaviors such as switching time and critical current density are significantly influenced by the TMR values.

Tunnel magnetoresistance of a molecular chain homo-catenated by group IV elements

Spin conduction in a molecular chain homo-catenated by group IV elements is calculated using the first-principle density functional theory. In particular, the characteristics of the tunnel magnetoresistance (TMR) are remarked upon. The germanium and tin molecular chains, which have σ-conjugation in the main chain, exhibit relatively high TMR ratios and retain these values up to a bias voltage of 0.3 V. It is found that the considerable difference in TMR effects between the carbon molecule and the σ-conjugated molecular chain is dependent on the spin polarization of the linking group between the nickel electrode and the main chain. The σ-conjugated molecular chains would thus be suitable for application to TMR devices.

터널링 자기저항 소자의 교류 전압 및 주파수 의존성 연구

터널링 자기저항 소자의 교류 전압 및 주파수 의존성 연구