Magnetic Tunnel Junctions Research Articles

Spin Coupling in the Initial Stages of the Zinc‐Blende MnN Growth on the CrN (111) Surface

AbstractSpin‐polarized first‐principles calculations are carried out to study the structural, electronic, and magnetic properties of the MnN growth on the CrN (111) surface. Different magnetic configurations between Mn‐deposited atoms and the substrate are considered. The H3 site is the most favorable site for the adoption and incorporation cases and the Mn spins switch from paramagnetic to antiferromagnetic arrangement. The minimum energy pathway for the Mn diffusion is calculated, showing magnetic modifications in the substrate. A structural transition from tetragonal rock‐salt to zinc‐blende (ZB) structure as the number of MnN deposited layers increases is noticed. Thermodynamic analysis demonstrates that the growth of MnN on CrN is feasible. The density of states for an MnN layer with an extra nitrogen layer shows a half‐metallic behavior and the remaining structures exhibit a metallic character. The ZB‐MnN structures have an antiferromagnetic behavior with a metallic character. Magnetic anisotropy energies reveal a switching of easy magnetization axis, from in‐plane to out‐of‐plane with the formation of MnN on the surface. These results suggest that the CrN/ZB‐MnN system may be employed in antiferromagnetic spintronics applications as the fabrication of perpendicular magnetic tunnel junctions.

TMR-high-temperature superconductor composite magnetic sensor and its performance optimization

Tunnel magnetoresistance (TMR), recognized for its high sensitivity and low power consumption, holds significant promise in the domain of weak magnetic field detection. Using superconducting materials as magnetic concentrators can achieve several hundred to even a thousandfold amplification of magnetic fields, making it one of the most effective approaches to enhance the magnetic field resolution of TMR sensors. This paper utilized the flip-chip bonding process to integrate TMR with the high-temperature superconductor YBCO (YBa2Cu3O7-δ), and successfully developed TMR-YBCO composite magnetic sensor. Building upon this foundation, through optimization of the fabrication process and the pioneering use of a structural design incorporating the filling of annular holes with superconducting concentrators, the sensitivity of the sensor was further enhanced. Finally, the magnetic field resolution was increased by 1030 times compared to TMR sensors, reached to 2.9pT/Hz1/2 at 1 Hz. Simultaneously, results from frequency band testing indicated excellent frequency band characteristics, with a frequency response range exceeding kHz.

Switching characteristics of MgO-based MTJ with intermediate state

The resistance change of magnetic tunnel junctions (MTJs) is used to store digital information in spin-transfer-torque magnetoresistive random-access memory. In some MTJs, intermediate (IM) states occur between two stable resistance states just after the application of a switching pulse. In this study, the effect of IM states on switching is investigated. IM states increase the switching voltage, and their probability is significantly high at a narrow pulse width. For no-error switching, MTJs with IM states require higher voltages and wider pulse widths compared with MTJs without IM states. A 300 nm MTJ has multiple IM states, whereas a 120 nm MTJ has only one IM state. Overall, a large MTJ’s free layer has multiple regions with different switching characteristics, and switching occurs stepwise through IM states.

Multistate Compound Magnetic Tunnel Junction Synapses for Digital Recognition.

The quest to mimic the multistate synapses for bioinspired computing has triggered nascent research that leverages the well-established magnetic tunnel junction (MTJ) technology. Early works on the spin transfer torque MTJ-based artificial neural network (ANN) are susceptible to poor thermal reliability, high latency, and high critical current densities. Meanwhile, work on spin-orbit torque (SOT) MTJ-based ANN mainly utilized domain wall motion, which yields negligibly small readout signals differentiating consecutive states and has designs that are incompatible with technological scale-up. Here, we propose a multistate device concept built upon a compound MTJ consisting of multiple SOT-MTJs (number of MTJs, n = 1-4) on a shared write channel, mimicking the spin-based ANN. The n + 1 resistance states representing varying synaptic weights can be tuned by varying the voltage pulses (±1.5-1.8 V), pulse duration (100-300 ns), and applied in-plane fields (5.5-10.5 mT). A large TMR difference of more than 13.6% is observed between two consecutive states for the 4-cell compound MTJ, a 4-fold improvement from reported state-of-the-art spin-based synaptic devices. The ANN built upon the compound MTJ shows high learning accuracy for digital recognition tasks with incremental states and retraining, achieving test accuracy as high as 95.75% in the 4-cell compound MTJ. These results provide an industry-compatible platform to integrate these multistate SOT-MTJ synapses directly into neuromorphic architecture for in-memory and unconventional computing applications.

Exchange Bias Modulated by Antiferromagnetic Spin-Flop Transition in 2D Van der Waals Heterostructures.

Exchange bias is extensively studied and widely utilized in spintronic devices, such as spin valves and magnetic tunnel junctions. 2D van der Waals (vdW) magnets, with high-quality interfaces in heterostructures, provide an excellent platform for investigating the exchange bias effect. To date, intrinsic modulation of exchange bias, for instance, via precise manipulation of the magnetic phases of the antiferromagnetic layer, is yet to be fully reached, owing partly to the large exchange fields of traditional bulk antiferromagnets. Herein, motivated by the low-field spin-flop transition of a 2D antiferromagnet, CrPS4, exchange bias is explored by modulatingthe antiferromagnetic spin-flop phase transition in all-vdW magnetic heterostructures. The results demonstrate that undergoing the spin-flop transition during the field cooling process, the A-type antiferromagnetic ground state of CrPS4 turns into a canted antiferromagnetic one, therefore, it reduces the interfacial magnetic coupling and suppresses the exchange bias. Via conducting different cooling fields, one can select the exchange bias effect switching among the "ON", "depressed", and "OFF" states determined by the spin flop of CrPS4. This work provides an approach to intrinsically modulate the exchange bias in all-vdW heterostructures and paves new avenues to design and manipulate 2D spintronic devices.

Sub-10 nm magnetic sensor for gigahertz operation using lateral spin valve structure

We present a new magnetic sensor design using a lateral spin valve (LSV) structure that features a sensing end less than 10 nm thick, making it suitable for integration with nanoscale devices. Shot noise reduction and signal-to-noise ratio (SNR) improvement under gigahertz operation are expected for our design. The spin detection part of the LSV forms a magnetic tunnel junction (MTJ) beneath the aluminum channel. We propose to replace the single layer ferromagnet in the spin detection part of the LSV with a synthetic antiferromagnetic structure: this facilitates spin-transfer torque (STT) induced magnetization switching. Macrospin simulation shows that interlayer magnetic dipole coupling helps STT switching while interlayer exchange coupling hurts, owing to their different precession symmetries in the out-of-plane direction. It also shows that SNR > 20 dB can be achieved for a pseudorandom binary sequence with a minimum bit length 0.5 ns.

A novel TMR sensor based on digital lock-in amplifier technology for portable oil multipollutant detection system

Metal particles identification in lubrication oil has become a popular technology to perform the fault diagnosis and health monitoring of machinery equipment. Unfortunately, the magnetic signals induced by particles are generally weak or even submerged in heavy noise. The sensitivity of conventional inductive sensors for detecting non-ferrous particles can still be improved to better distinguish between different non-ferrous particles. Based on the above issues, this paper proposed a novel tunnel magnetoresistance (TMR) sensor based on digital lock-in amplifier (DLIA) technology to detect ferrous and multiple nonferrous metallic particles in oil under noisy conditions. Moreover, compared to conventional inductive sensor structures, the proposed method can effectively reduce the sensor volume, which is particularly necessary when considering the design of portable oil multipollutant detection system. Owing to the high resolution of TMR and outstanding versatility of DLIA for extracting weak signal overwhelmed in noise, the detection sensitivity can be improved tremendously. Extensive simulation and experimental results validated that the proposed approach was practical and effective. Based on the experimental results, the minimum values of detectable diameter could be computed as 24.76 µm for iron particles, 81.34 µm and 82.32 µm for copper and aluminium particles respectively.

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.

Dimensional scaling effects on critical current density and magnetization switching in CoFeB-based magnetic tunnel junction

In this work, we theoretically investigate the size dependence of the magnetization reversal behavior in CoFeB-MgO-CoFeB magnetic tunnel junctions (MTJs) by employing an atomistic spin model coupled with the spin accumulation model. The former and the latter are used to construct the magnetic structure and to model the spin transport behavior, respectively. The accuracy of the approach is confirmed by investigating the dependence of the magnetic properties on the size of the MTJ. Perpendicular magnetic anisotropy (PMA) is observed for thickness less than 1.3 nm, which is in an excellent agreement with experiment. To investigate the magnetization dynamics induced by spin-polarized current, a charge current is injected into the MTJ structure perpendicular to the stack leading to a spin-transfer torque acting on the magnetization of the CoFeB layer. The results show that the critical current density to reverse the magnetization is lower for PMA-MTJ and in addition for the same injected current density the time required to switch the magnetization is shorter than for an in-plane MTJ. The results can be used as a guideline to optimize the design of high performance MTJs for STT-MRAM applications.

Fabrication and performance enhancement of TMR-Superconducting composite magnetic sensors for weak magnetic field detection

The tunneling magnetoresistance (TMR) effect, utilizing the CoFeB/MgO/CoFeB magnetic tunnel junction (MTJ) structure, has seen a surge in research interest owing to its pronounced sensitivity to magnetic fields. However, advancements in TMR technology have plateaued in recent times. In response, this study explores the integration of TMR with superconductivity to form a TMR-superconducting composite magnetic sensor, aiming to amplify the external magnetic field effect and improve sensor performance. Such sensors offer significant promise for applications in detecting feeble magnetic fields due to their compact dimensions, minimal energy consumption, and exceptional magnetic field resolution. Despite their potential, manufacturing challenges persist. This paper focuses on the meticulous fabrication of the TMR's critical thin film layer and its subsequent patterning. Moreover, we propose an innovative technique utilizing magnetron sputtering to deposit a niobium (Nb) layer atop the TMR structure, followed by intricate patterning to successfully construct a TMR-superconducting composite film. This novel approach importantly diminishes the interspace between TMR and superconducting layers compared to traditional bonding methods, thereby enhancing device performance. Experimental assessments of the device constructed superconducting loops, contrasted with traditional bonding process, demonstrate that sputtering-based sensor exhibit better magnetic field resolution. Consequently, TMR-superconducting composite sensors realized via the sputtering technique hold distinct advantages in weak magnetic field detection.

Impact of net magnetization on spin–orbit torque switching of synthetic ferromagnets in magnetic tunnel junctions

Spin–orbit torque (SOT)-induced reversal of a ferromagnet in a magnetic tunnel junction (MTJ) is promising for next-generation novel magnetic memory and spin-logic devices. However, inherent limitations of single ferromagnetic-based free layer (FL) systems are low thermal stability and susceptibility to external magnetic stray fields. To overcome these challenges, synthetic antiferromagnets (SAFs) are integrated into MTJ devices to form hybrid free layers (HFLs); this FL system enables tunability of thermal stability and stray field experienced by the FL, simultaneously lowering the SOT-induced switching currents. Here, we investigate the impact of net magnetization of the SAF-based HFL on the field and current-driven switching behavior. We observe that nearly compensated SAF-HFL systems demonstrate a higher proportion of switching failures at the device level than the uncompensated SAF-HFL systems in both current and field-driven schemes. Micro-magnetic simulations and thin film characterization of SAF-HFL show that the device-level stochasticity arises due to a dynamic decoupling within the nearly compensated SAF systems. We show that optimizing the SAF-HFL systems to have uncompensated magnetization mitigates this stochasticity and improves the SOT switching current for Pt-based SOT channel by at least 20% across the entire tested pulse width range down to 300 ps, thus leading to reliable switching of SOT-MTJs with SAF-HFL stacks.

Impact of a ferromagnetic insulating barrier in perovskite-based magnetic tunnel junctions

Impact of a ferromagnetic insulating barrier in perovskite-based magnetic tunnel junctions

An all-Heusler magnetic tunnel junction model which provides a probability to realize maximal tunneling magnetoresistance at room temperature

The CoFeTiSi compound is an experimentally existing half-metallic ferromagnetic Heusler compound with a Curie temperature higher than room temperature. By replacing the Si atom with Al, we can obtain a narrow bandgap semiconductor CoFeTiAl. The lattice mismatch between CoFeTiSi and CoFeTiAl is very small, which serves as motivation for constructing a CoFeTiSi/CoFeTiAl/CoFeTiSi magnetic tunnel junction. In this junction, CoFeTiSi and CoFeTiAl are used as electrode and barrier layers, respectively. The optimal device structure can be achieved simply by replacing the Si atoms in the barrier region with Al atoms. Through the use of abinitio atomic thermodynamics, we have found that this device structure is more stable compared to a junction formed by the same two compounds with an alternative interface. Transport calculations reveal that this junction can achieve a high tunneling magnetoresistance ratio at room temperature and exhibit a large transmission coefficient. These characteristics make the CoFeTiSi/CoFeTiAl/CoFeTiSi magnetic tunnel junction an ideal candidate for future spintronic applications.

Direct Observation of Room-Temperature Magnetic Skyrmion Motion Driven by Ultra-Low Current Density in Van Der Waals Ferromagnets.

The recent discovery of room-temperature ferromagnetism in 2D van der Waals (vdW) materials, such as Fe3GaTe2 (FGaT), has garnered significant interest in offering a robust platform for 2D spintronic applications. Various fundamental operations essential for the realization of 2D spintronics devices are experimentally confirmed using these materials at room temperature, such as current-induced magnetization switching or tunneling magnetoresistance. Nevertheless, the potential applications of magnetic skyrmions in FGaT systems at room temperature remain unexplored. In this work, the current-induced generation of magnetic skyrmions in FGaT flakes employing high-resolution magnetic transmission soft X-ray microscopy is introduced, supported by a feasible mechanism based on thermal effects. Furthermore, direct observation of the current-induced magnetic skyrmion motion at room temperature in FGaT flakes is presented with ultra-low threshold current density. This work highlights the potential of FGaT as a foundation for room-temperature-operating 2D skyrmion device applications.

Influence of surface acoustic wave (SAW) on nanoscale in-plane magnetic tunnel junctions

The use of voltage induced strain to switch magnetic tunnel junctions (MTJs) is a promising solution for reducing the switching energy in MRAM technologies. The MTJ is integrated with a piezoelectric layer to generate the strain. A very thin layer is needed to switch with small voltages and small energy dissipation. It is challenging to synthesize ultrathin piezoelectric layers that retain a high degree of piezoelectricity. An alternate approach is to use time-varying strain generated by a surface acoustic wave (SAW). This approach does not require a thin piezoelectric layer since the SAW is confined to the surface of the layer. In this study, we fabricated in-plane MTJs on piezoelectric LiNbO3 substrates and used IDTs to generate the SAW signal within the substrate. Our results showed that the SAW signal had a significant influence on the resistance and the tunneling magnetoresistance (TMR) ratio of the MTJs. The influence was much less significant in nanometer size MTJs than in micrometer sized ones. Most surprisingly, the SAW signal caused the tunneling magnetoresistance ratio (TMR) to drop below zero for the micrometer size MTJ, meaning that the antiparallel resistance RAP is temporarily less than the parallel resistance RP under SAW excitation. Our results provide insight into the dynamic behavior of MTJs under periodic strain and the dependence of this behavior on the device dimensions as they are scaled down to nanometer sizes.

Implementing bidirectional logic with backhopping in magnetic tunnel junctions

A bidirectional logic gate has been designed based on the backhopping phenomenon observed in magnetic tunnel junctions (MTJ) at high bias. The magnetization dynamics of each magnetic layer of the MTJ—having materials and geometry of a standard spin-transfer torque magnetic random access memory device—is calculated using the coupled Landau–Lifshitz–Gilbert equation-based theoretical framework. A circuit design interconnecting the MTJs has been proposed to simulate a two-input NAND gate. The results in both forward and reverse directions agree well with those found from the Boltzmann distribution, thereby demonstrating the equiprobability of all valid states.

Towards fully electrically controlled domain-wall logic

Utilizing magnetic tunnel junctions (MTJs) for write/read and fast spin-orbit-torque (SOT)-driven domain-wall (DW) motion for propagation, enables non-volatile logic and majority operations, representing a breakthrough in the implementation of nanoscale DW logic devices. Recently, current-driven DW logic gates have been demonstrated via magnetic imaging, where the Dzyaloshinskii-Moriya interaction (DMI) induces chiral coupling between perpendicular magnetic anisotropy (PMA) regions via an in-plane (IP) oriented region. However, full electrical operation of nanoscale DW logic requires electrical write/read operations and a method to pattern PMA and IP regions compatible with the fabrication of PMA MTJs. Here, we study the use of a Hybrid Free Layer (HFL) concept to combine an MTJ stack with DW motion materials, and He+ ion irradiation to convert the stack from PMA to IP. First, we investigate the free layer thickness dependence of 100-nm diameter HFL-MTJ devices and find an optimal CoFeB thickness, from 7 to 10 Å, providing high tunneling magnetoresistance (TMR) readout and efficient spin-transfer torque (STT) writing. We then show that high DMI materials, like Pt/Co, can be integrated into an MTJ stack via interlayer exchange coupling with the CoFeB free layer. In this design, DMI values suitable for SOT-driven DW motion are measured by asymmetric bubble expansion. Finally, we demonstrate that He+ irradiation reliably converts the coupled free layers from PMA to IP. These findings offer a path toward the integration of fully electrically controlled DW logic circuits.

Temperature and bias voltage dependences of magnetic tunnel junction with FeAlSi electrode

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.

Size dependence of domain wall mediated switching dynamics of perpendicular magnetic tunnel junctions in the presence of reference layer stray field

Recent investigations on spin-transfer-torque-induced switching dynamics of perpendicular magnetic tunnel junctions (MTJ) have revealed different switching anomalies. Here, the influence of stray field from a synthetic anti-ferromagnet (SAF) based reference layer on the domain wall (DW) mediated switching of the free-layer magnet is studied via finite temperature micro-magnetic simulations for varying MTJ diameters. For larger diameters (∼80 – 120 nm), a stray field gives rise to persistent back-and-forth oscillation of the unswitched domain, delaying the switching process and causing increased write errors. For smaller diameters (∼30 – 56 nm), quasi-coherent switching occurs, as expected. For the intermediate ranges of MTJ diameters (∼60 – 70 nm), another switching mode emerges where a bubble-like feature is observed to evolve during the switching process, causing a very rapid change in magnetization. These paths are observed to originate from DWs partly in Bloch and partly in Néel configuration. We find that at the intermediate ranges of device size, the stray field becomes stronger for a given SAF configuration, and the energy difference between the Néel and Bloch configurations is also lowered. Hence, a stronger stray field could easily distort the DW propagation in the Walker breakdown regime, leading to such magnetization behavior. Our findings present interesting insights into DW-mediated switching modes in perpendicular MTJ structures caused by unoptimized stray fields from the SAF.

L10 FePd-based perpendicular magnetic tunnel junctions with 65% tunnel magnetoresistance and ultralow switching current density

L10 FePd is increasingly recognized as a potential candidate for magnetic tunnel junctions (MTJs), yet there remains room for enhancing device performance. In this work, we fabricated fully-integrated L10 FePd-based perpendicular MTJ devices and achieved a significant increase in tunnel magnetoresistance, reaching ∼65%, compared to the previous record of 25%. Notably, we observed bi-directional switching with a low switching current density of about 1.4 × 105 A/cm2, which outperforms the typical spin-transfer torque (STT) MTJ by about one order of magnitude. We propose two possible mechanisms to elucidate the switching process and associated device performance: (1) The voltage-controlled exchange coupling-driven switching of the bottom CoFeB layer; (2) The STT-driven switching of the exchange-coupled L10 FePd–CoFeB composite. While additional research is necessary, these findings may further advance the integration of L10 FePd into spintronic devices, potentially enabling low-energy memory and logic technologies.