Magnetic Tunnel Junctions Research Articles

Application of tunneling magnetoresistance in electromagnetic tomography system construction.

Tunneling magnetoresistance (TMR) is a magnetic sensor that measures the magnetic induction component toward a sensitive axis. It is used in electromagnetic tomography (EMT) systems for its high sensitivity, frequency independence, and small size. TMR-based EMT (TMR-EMT) system construction and performance enhancement require a more comprehensive understanding of TMR characteristics. This paper introduces the construction principle of a TMR-EMT system, studies effect of TMR quantity on imaging results, compares the difference between TMR-EMT and coil-based EMT (Coil-EMT) in imaging results, analyzes the influence of TMR characteristics on the measurements, and gives the corresponding solutions. Simulations and experiments indicate that adding more TMRs could enhance imaging quality, obtaining better results than Coil-EMT imaging quality. The experimental results show that dynamically adjusting the excitation amplitude and downward zero-crossing switching can effectively improve the problems of output saturation and deteriorating consistency of measurements caused by TMR nonlinear characteristic.

Influence of physical and material parameters on switching current density in perpendicular STT-MTJ: a micromagnetic study

Abstract Switching in magnetic tunnel junctions (MTJs) is considered to be coherent according to the macrospin model but above a critical characteristic length (Rc) this process becomes incoherent. As a result, switching becomes a complex process and affects the switching current density (Jc). We designed a spin transfer torque (STT) based single barrier perpendicular MTJ (SMTJ) and observed the influence of the junction size and exchange stiffness constant (Aex) on the switching process through micromagnetic simulations performed on Object Oriented Micromagnetic Framework (OOMMF). It was found that coherent switching occurred only for junction diameter ≤ 20nm and showed dependence on Aex as well. The influence of damping constant and anisotropy on Jc is studied and the mechanism of magnetic reversal through domain formation is revisited in this work. Furthermore, Double barrier MTJ (DBMTJ) stack was designed, which showed increased STT efficiency in switching time with a requirement of Jc lower by 42.86% compared to SMTJ.

A neuromorphic radial-basis-function net using magnetic bits for time series prediction

Magnetic tunnel junctions (MTJs) are considered strong candidates for constructing neuromorphic systems owing to their low power consumption and high integrability. However, research on MTJ-based local approximation network is still lacking. In this work, we propose an MTJ-based radial basis function (RBF) network and numerically investigate its time-series prediction capability. The results demonstrate that the MTJ-based RBF network can enhance its prediction performance by utilizing increased environmental temperatures, achieving performance better than traditional software artificial neural networks.

Spin photo detector by using a CoFeB magnetic tunnel junction

Abstract Ultra-fast photoelectric conversion devices are a crucial element in photonics applications and are the subject of intense research and development. In conventional high-speed photo detectors, photoelectric charge generation in semiconductors is rooted in a mechanism that directly outputs current. Owing to the dilemma of the amount of generated charge and high-speed output, it is being faced with fundamental difficult to achieve a higher response. Spin order, rather than charge generation, also exists as a phenomenon that enables ultrafast photoresponses, and magnetic tunnel junction (MTJ) are well-known high-speed electrical detection elements for magnetic states. In this study, we experimentally confirmed the magnetic switching of photoresponsive MTJ using a practical single CoFeB free layer with a high MR ratio of 80% by irradiation with laser light. Furthermore, we experimentally confirmed the operation of the reversible photo detector, and its rise time reached an ultrafast speed of 20 ps. We named it spin photo detector and it has huge potential applications that require ultrafast photo detection.

Gate-tunable high tunneling magnetoresistance induced by hybrid interface states in Ni-electrode single-molecule junctions

Based on the density functional theory and non-equilibrium Green's function method, the modulation effects of gate voltage on the spin transport properties of Ni/triphenylene/Ni single-molecule junctions are investigated. The plane-, platform-, and tip-shaped electrode configurations are considered in the calculations. The numerical results show that the electrode configuration and gate voltage significantly influence the hybrid interface states (HIS) and further the spin transport properties of the molecular junctions. The molecular junctions with plane- and platform-shaped electrode configurations show weak spin polarization (SP) and tunneling magnetoresistance (TMR). Still, they exhibit excellent field effect transistor behaviors with the modulation of the gate voltage. Due to the delocalized spin-down HIS located at the Fermi level, the molecular junction with tip-shaped electrode configuration presents high and controllable SP and TMR behaviors through the gate voltage modulation. The calculations demonstrate that high spin-polarized HIS located at the Fermi level are extremely significant for the generation of excellent spin transport properties in molecular junctions composed of non-magnetic molecule and magnetic electrodes.

Nearly perfect spin polarization of noncollinear antiferromagnets

Ferromagnets with high spin polarization are known to be valuable for spintronics—a research field that exploits the spin degree of freedom in information technologies. Recently, antiferromagnets have emerged as promising alternative materials for spintronics due to their stability against magnetic perturbations, absence of stray fields, and ultrafast dynamics. For antiferromagnets, however, the concept of spin polarization and its relevance to the measured electrical response are elusive due to nominally zero net magnetization. Here, we define an effective momentum-dependent spin polarization and reveal an unexpected property of many noncollinear antiferromagnets to exhibit nearly 100% spin polarization in a broad area of the Fermi surface. This property leads to the emergence of an extraordinary tunneling magnetoresistance (ETMR) effect in antiferromagnetic tunnel junctions (AFMTJs). As a representative example, we predict that a noncollinear antiferromagnet Mn3GaN exhibits nearly 100% spin-polarized states that can efficiently tunnel through low-decay-rate evanescent states of perovskite oxide SrTiO3 resulting in ETMR as large as 104%. Our results uncover hidden functionality of material systems with noncollinear spin textures and open new perspectives for spintronics.

Double-barrier magnetic tunnel junctions with enhanced tunnel magnetoresistance

Tunnel magnetoresistance (TMR) ratio is a key parameter characterizing the performance of a magnetic tunnel junction (MTJ), and a large TMR ratio is essential for the practical application of it. Generally, the traditional solutions to increasing the TMR ratio are to choose different material combinations as the ferromagnetic (FM) leads and nonmagnetic tunnel barrier. In this work, we study an architecture of MTJs of “FM/barrier/FM/barrier/FM” with double barriers, in contrast to the traditional single barrier structure “FM/barrier/FM.” We first analytically show that double barrier MTJ will generally have much higher TMR ratio than the single barrier MTJ and then substantiate it with the well-known example of “Fe/MgO/Fe” MTJ. Based on density functional calculations combined with nonequilibrium Green's function technique for quantum transport study, in the single barrier “Fe/MgO/Fe” MTJ, the TMR ratio is obtained as 122%, while in the double barrier “Fe/MgO/Fe/MgO/Fe” MTJ, it is greatly increased to 802%, suggesting that double barrier design can greatly enhance the TMR and can be taken into consideration in the design of MTJs.

High Electrical Conductance in Magnetic Emission Junction of Fe3GeTe2/ZnO/Ni Heterostructure via Selective Spin Emission through ZnO Ohmic Barrier.

The insulator is essential for magnetic tunneling junction (MTJ) that increases magnetoresistance (MR) by decoupling magnetization directions between two ferromagnets. However, wide bandgap tunnel barrier blocks the thermionic emission of electrons, significantly reducing electrical conductance through MTJ. Here, a magnetic emission junction (MEJ) is demonstrated for the first time using an Fe3GeTe2 (FGT)/ZnO/Ni heterostructure with very high electrical conductance. The conduction band of ZnO (electron affinity 4.6eV) aligns with Fermi levels (EF) of FGT (4.47eV) and Ni (4.58eV) ferromagnets and forms an Ohmic barrier, enabling free spin-electron emission through ZnO barrier and high electrical conductance. In contrast to the typical positive MR in MTJ by majority spin tunneling, negative MR is observed in FGT/ZnO/Ni MEJ. The minority spin electrons of Ni, with maximum states near the EF, are dominantly emitted to FGT over the ZnO barrier, while majority spin electrons of Ni, with maximum states below the EF, are blocked by it. In the FGT/FGT/ZnO/Ni heterostructure, the MR ratio is further increased by combining positive and negative MR at the MTJ (FGT/FGT) and MEJ (FGT/ZnO/Ni), respectively. As a result, FGT-MEJ exhibits 10-1000 orders higher conductance than other 2D-MTJs, while MR ratio remains similar to other 2D-MTJs.

First principles electron transport in magnetoelectric SrRuO3/BaTiO3/SrTiO3/SrRuO3interfaces.

Recently, all-oxide ferroelectric tunnel junctions, with single or composite potential barriers based on SrRuO3/BaTiO3/SrTiO3(SRO/BTO/STO) perovskites, have drawn a particular interest for high density low power applications, due to their highly tunable transport properties and device scaling down possibility to atomic size. Here, using first principles calculations and the NEGFs formalism, we explore the electronic structure and tunneling transport properties in magnetoelectric SRO/BTO/mSTO/SRO interfaces, (m= 0, 2, or 4 unit cells), considering both the RuO6octahedra tilts and magnetic SRO electrodes. Our main results may be summarized as follows: i) The band alignment schemes predict that polarization direction may determine both Schottky barrier or Ohmic contacts form(STO)=0, but only Schottky contacts form(STO)=2 and 4 junctions; ii) The tunnel electroresistance and tunnel magnetoresistance ratios are evaluated at 0 and 300 K; iii) The most magnetoelectric responsive interfaces are obtained for them(STO)=2 heterostructure, this system also showing co-existent giant tunnel electroresistance and tunnel magnetoresistance effects; iv) The interfacial magnetoelectric coupling is not strong enough to control the tunnel magnetoresistance by polarization switching, in spite of significant SRO ferromagnetism.

Field‐Free Perpendicular Magnetic Memory Driven by Out‐of‐Plane Spin‐Orbit Torques

AbstractMagnetic memory based on spin‐orbit torque (SOT) is a promising candidate for the next‐generation storage devices. Materials that generate out‐of‐plane spin polarization (σz) are highly desired and actively pursued as the SOT generator, due to its ability to achieve field‐free SOT switching of perpendicular magnetization. However, integrating σz into perpendicular magnetic tunnel junction for efficient data writing is not realized. Here, utilizing σz from antiferromagnetic spin Hall effect in Mn2Au, σz‐enabled field‐free SOT switching of perpendicular magnetic tunnel junctions is realized demonstrating a magnetic memory with both writing and reading electrically. The tunnel magnetoresistance ratio achieves 66% with a critical current density of 5.6 × 106 A cm−2 at room temperature. Such field‐free fully SOT switching is further directly confirmed by domain imaging via magneto‐optical Kerr effect microscopy. In addition to enabling field‐free switching, σz is proposed to assist ultrafast and more efficient switching of perpendicular magnetization compared with conventional in‐planes ones, based on simulations. This research advances the application of out‐of‐plane SOTs and paves the way for high‐density, high‐speed, and low‐power magnetic memory.

Dynamics of spin oscillation in double barrier synthetic antiferromagnet based magnetic tunnel junction in presence of spin-transfer torque

In this work, we have studied the spin dynamics of a synthetic antiferromagnet (AFM)/heavy metal/ferromagnet double barrier magnetic tunnel junction in the presence of Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction, interfacial Dzyaloshinskii–Moriya (iDM) interaction, Néel field, and Spin–Orbit Coupling (SOC) with different Spin-Transfer Torque (STT). We employ the Landau–Lifshitz–Gilbert–Slonczewski equation to investigate the AFM dynamics of the proposed system. We found that the system exhibits a transition from regular to damped oscillations with the increase in strength of STT for systems with a weaker strength of iDM interaction than RKKY interaction while displaying sustained oscillations for systems having the same order of RKKY and iDM interactions. On the other hand, the systems with sufficiently strong iDM interaction strength exhibit self-similar but aperiodic patterns in the absence of the Néel field. In the presence of the Néel field, the RKKY interaction dominating systems exhibit chaotic oscillations for low STT but display sustained oscillations under moderate STT. Our results suggest that the decay time of oscillations can be controlled via SOC. The system can work as an oscillator for low SOC but displays non-linear characteristics with the rise in SOC for systems having weaker iDM interaction than RKKY interactions. In contrast, opposite characteristics are noticed for iDM interaction dominating systems. We found periodic oscillations under low external magnetic fields in RKKY interaction dominating systems. However, moderate fields are necessary for sustained oscillation in iDM interaction dominating systems. Moreover, the system exhibits saddle-node bifurcations and chaos under moderate Néel field and SOC with suitable RKKY and iDM interactions. In addition, our results indicate that the magnon lifetime can be enhanced by increasing the strength of iDM interaction for both optical and acoustic modes.

Thermal Characterization of Ultrathin MgO Tunnel Barriers.

Magnetic tunnel junctions (MTJs) with ultrathin MgO tunnel barriers are at the heart of magnetic random-access memory (MRAM) and exhibit potential for spin caloritronics applications due to the tunnel magneto-Seebeck effect. However, the high programming current in MRAM can cause substantial heating which degrades the endurance and reliability of MTJs. Here, we report the thermal characterization of ultrathin CoFeB/MgO multilayers with total thicknesses of 4.4, 8.8, 22, and 44 nm, and with varying MgO thicknesses (1.0, 1.3, and 1.6 nm). Through time-domain thermoreflectance (TDTR) measurements and thermal modeling, we extract the intrinsic (∼3.6 W m-1 K-1) and effective (∼0.85 W m-1 K-1) thermal conductivities of annealed 1.0 nm thick MgO at room temperature. Our study reveals the thermal properties of ultrathin MgO tunnel barriers, especially the role of thermal boundary resistance, and contributes to a more precise thermal analysis of MTJs to improve the design and reliability of MRAM technologies.

Decision making module based on stochastic magnetic tunnel junctions

Decision making module based on stochastic magnetic tunnel junctions

Substantially Enhanced Spin Polarization in Epitaxial CrTe2 Quantum Films.

2D van der Waals (vdW) magnets, which extend to the monolayer (ML) limit, are rapidly gaining prominence in logic applications for low-power electronics. To improve the performance of spintronic devices, such as vdW magnetic tunnel junctions, a large effective spin polarization of valence electrons is highly desired. Despite its considerable significance, direct probe of spin polarization in these 2D magnets has not been extensively explored. Here, using 2D vdW ferromagnet of CrTe2 as a prototype, the spin degrees of freedom in the thin films are directly probed using Mott polarimetry. The electronic band of 50 ML CrTe2 thin film, spanning the Brillouin zone, exhibits pronounced spin-splitting with polarization peaking at 7.9% along the out-of-plane direction. Surprisingly, atomic-layer-dependent spin-resolved measurements show a significantly enhanced spin polarization in a 3 ML CrTe2 film, achieving 23.4% polarization even in the absence of an external magnetic field. The demonstrated correlation between spin polarization and film thickness highlights the pivotal influence of perpendicular magnetic anisotropy, interlayer interactions, and itinerant behavior on these properties, as corroborated by theoretical analysis. This groundbreaking experimental verification of intrinsic effective spin polarization in CrTe2 ultrathin films marks a significant advance in establishing 2D ferromagnetic atomic layers as a promising platform for innovative vdW-based spintronic devices.

First principle atomistic modelling of magnetic tunnel junction (MTJ) to enhance its tunnel magneto-resistance (TMR) ratio

Abstract This paper investigates the impact of electrode materials on the Tunnel Magneto-Resistance (TMR) ratio of Magnetic Tunnel Junction (MTJ) device. Four different structures of MTJ have been simulated by using cobalt (Co), Nickel (Ni), Iron (Fe) and two alloy materials of nickel-iron (NiFe) and cobalt-iron (CoFe). These materials have been used as ferromagnetic electrodes. Mulliken population and transmission spectrum observed in both parallel and antiparallel configurations of these devices to understand the spin transport properties and Tunnel Magneto-Resistance (TMR) ratio has been estimated. The first principal study was performed based on density function theory (DFT) and Non-equilibrium Green’s Function (NEGF) computational methods using the QuantumATK simulation tool to study properties such as band structure, the density of states (DOS), Spin Transfer Torque (STT), I-V characteristics and TMR. This study explores how different electrode materials affect the Tunnel Magneto-Resistance (TMR) ratio in Magnetic Tunnel Junction (MTJ) devices. With these results, it is observed that cobalt-based MTJ devices (that is Co-MgO-Fe and CoFe-MgO-CoFe) exhibit higher TMR ratio as compared to Nickel- and Iron-based MTJ devices (that is NiFe-MgO-NiFe and Ni-MgO-Fe). As Cobalt has a high spin polarization this property makes it suitable for use in spintronics devices like MTJs, where the manipulation of electron spins is essential for data storage and information processing. These findings can be employed to improve the performance characteristics of the MTJ-based Random Access Memory (MRAM) in the field of spintronics.

Impact of Process Variation in Spin–Orbit Torque-Based Magnetic Tunnel Junctions on the Performance of Spiking Neural Networks

Impact of Process Variation in Spin–Orbit Torque-Based Magnetic Tunnel Junctions on the Performance of Spiking Neural Networks

Magnetic proximity effect and spin transport properties of MnPc/CuPc/MnPc molecular spintronic device

Molecular-scale spintronic devices show great application potential in information storage due to tiny size and rich controllability. Spin transport of molecular devices relies heavily on the magnetic interaction between magnetic metal atom and molecule, which is the core of determining the performance of spintronic devices. In this study, the magnetic proximity effect in MnPc/CuPc/MnPc molecular device and the influence on spin transport were investigated using non-equilibrium Green’s function method in combination with density functional theory. Mn atoms in MnPc molecules not only endow magnetism with system, but also trigger magnetic proximity effect which works in synergy with the spin filtering of phthalocyanine molecules to achieve a spin injection efficiency of 100 % in molecular device, and this efficiency remains stable under bias voltage. The MnPc/CuPc/MnPc molecular device shows a tunneling magnetoresistance ratio of up to 4165 % at 5 mV, indicating potential application in multifunctional spintronic devices. This work elucidates the unique physical properties and excellent performance of MnPc/CuPc/MnPc molecular device, providing valuable insights for spintronic applications.

Going deeper on magneto-optical Faraday effect analysis to detect fatigue crack with high-spatial resolution for non-destructive inspection

In this study, we introduce a high-resolution technique for defect detection that uses the magneto–optical (MO) Faraday effect. Our system combines a portable polarized microscope and an MO sensor for the high spatial observation of a magnetic domain structure. We accurately localized and characterized the fatigue defects through integrated image pre-processing and cross-power spectral density analysis. This approach enhances our analysis of the magnetic domain structure, increasing the spatial capabilities of the microscope to detect fatigue defects. We conducted experiments on fatigue cracks with defect depth angles of 0°, 30°, and 60°, analyzing their relationship through signal amplitude and the tendency of the signal shift with increasing defect angle. Our findings were validated using a tunnel magnetoresistance sensor. Future research will focus on the optimization of feature extraction complexity, considering the limited computational power available for portable non-destructive testing devices.

Spin-Tunneling Magnetoresistive Effects in Bottom-Up-Grown Ni/Graphene/Ni Nanojunctions.

Two-dimensional (2D) materials embedded in magnetic tunnel junctions (MTJs) provide a platform to increase the control over spin transport properties by the proximity spin-filtering effect. This could be harnessed to craft spintronic devices with low power consumption and high performance. We explore the spin transport in the 2D MTJs based on graphene, which is uniformly grown on Ni(111) substrates using the chemical vapor deposition technique. After the Ni thin film is deposited bottom-up on the well-grown Ni(111)/graphene surface in an e-beam evaporation system by the physical vapor deposition method, the Ni/graphene/Ni nanojunction array devices are successfully prepared by using nanography technology. Evidence of the emergence of tunneling magnetoresistance (TMR) effects with ultrasmall resistance × area products in graphene-based nanojunctions is observed by the exclusion of anisotropic magnetoresistance. The theoretical analysis shows that this TMR is mainly attributed to the strong spin-filtering effect at the perfect Ni(111)/graphene interface. Besides, earlier findings indicate that the TMR would be promoted more effectively if the short-circuit effect formed in the process of nanographic etching by an ion beam can be further eliminated. Overall, this study provides a path to harness the full potential of graphene-based MTJ array devices with a high efficiency and performance.

Coherent Driving of a Single Nitrogen Vacancy Center by a Resonant Magnetic Tunnel Junction.

Nitrogen vacancy (NV) centers, atomic spin defects in diamond, represent an active contender for advancing transformative quantum information science (QIS) and innovations. One of the major challenges for designing NV-based hybrid systems for QIS applications results from the difficulty of realizing local control of individual NV spin qubits in a scalable and energy-efficient way. To address this bottleneck, we introduce magnetic tunnel junction (MTJ) devices to establish coherent driving of an NV center by a resonant MTJ with voltage controlled magnetic anisotropy. We show that the oscillating magnetic stray field produced by a resonant micromagnet can be utilized to effectively modify and drive NV spin rotations when the NV frequency matches the corresponding resonance conditions of the MTJ. Our results present a new pathway to achieve all-electric control of an NV spin qubit with reduced power consumption and improved solid-state scalability for implementing cutting-edge QIS technological applications.