Characteristics Of Magnetic Tunnel Junctions Research Articles

Magnetization dynamics driven by displacement currents across a magnetic tunnel junction

Understanding the high-frequency transport characteristics of magnetic tunnel junctions (MTJs) is crucial for the development of fast-operating spintronics memories and radio frequency devices. Here, we present the study of a frequency-dependent capacitive current effect in CoFeB/MgO-based MTJs and its influence on magnetization dynamics using a time-resolved magneto-optical Kerr effect technique. In our device, operating at gigahertz frequencies, we find a large displacement current of the order of mA, which does not break the tunnel barrier of the MTJ. Importantly, this current generates an Oersted field and spin-orbit torque, inducing magnetization dynamics. Our discovery holds promise for building robust MTJ devices operating under high current conditions, also highlighting the significance of capacitive impedance in high-frequency magnetotransport techniques. Published by the American Physical Society 2024

Half-metallic Heusler alloy/AlP based magnetic tunnel junction

Exploring the spin transport characteristics of magnetic tunnel junctions based on half-metallic Heusler holds significance, not only in unraveling the fundamental physics at play but also in advancing applications of spintronic devices. Here, density functional theory in conjunction with non-equilibrium Green’s functions has been systematically employed to investigate two prominent Heusler alloys, Co2CrAl and CoFeCrAl, which are of direct interest to the candidates of magnetic tunnel junction. The electronic structures of two Hesler alloys reveal that both exhibit characteristics of half-metallic ferromagnets, featuring a substantial spin-down bandgap and achieving 100% spin polarization. The tunneling magnetoresistance ratios obtained for Co2CrAl/AlP/Co2CrAl and CoFeCrAl/AlP/CoFeCrAl magnetic tunnel junctions are determined to be 173% and 59%, respectively, with the former exhibiting superior device characteristics. Therefore, the Co2CrAl/AlP-based magnetic tunnel junction demonstrates ideal performance, providing new opportunities for two-dimensional spintronics.

Tunnel Magnetoresistance Based Passive Resistance Replacement in Hybrid MTJ-CMOS Integration

Previous theoretical and experimental works revealed the novel factors that Magnetic tunnel junction (MTJ) can be integrated into novel hybrid circuits except for memory applications. This paper makes exploitation of tunnel magnetoresistacne based replacement in diminishing layout penalty of on-chip passive component during circuit design and takes sigma-delta analog-to-digital converter (SD-ADC), resistor-based temperature sensor (-55 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{\circ }$</tex-math></inline-formula> C <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sim 125^{\circ }$</tex-math></inline-formula> C) as two case study where large resistance is needed and restricts the scaling down. Considering the application in MRAM, the mainstream field of MTJ process and dealing with the problems of MRAM in wide temperature write operation, two temperature adaptive write schemes of MRAM are also proposed as the further applications of the proposed MTJ-based temperature sensor. The research of these circuits covers major characteristics of MTJ as the passive component, including area, variation and temperature characteristics. Large CMOS resistance in SD-ADC and bridge transducer in resistor-based temperature sensor are replaced by MTJ-based resistors. Simulation results reveal that the layout area of passive resistors in resistor-capacitor (RC) integrator was greatly reduced by the tunnel magnetoresistacne based replacement, specifically by 94.52% in comparison with fully 28nm CMOS design or 94.13% for wide temperature use. As for other figures of merit (FoM), Effective Numbers of Bits (ENOB) is improved by 0.06 bit along with higher thermal stability, and Signal-to-Noise Ratio (SNR) is decreased by 1.1 dB as design trade-off. In addition, the MTJ based bridge transducer in resistor-based temperature sensor can reduce the resistance layout area by over 90% with better linearity comparing with general CMOS resistor-based temperature sensor designs. Based on the MTJ-based temperature sensor, the two different adaptive write circuits help reduce write power consumption and delay of MRAM respectively for wide temperature use.

Non-Equilibrium Green’s Function Modeling of the Transport Characteristics of Magnetic Tunnel Junctions Containing Anderson Impurities

A nonequilibrium Green’s function (NEGF) formulation to study the transport characteristics of magnetic tunnel junctions (MTJs) that contains impurities at the barrier region and many-body interaction at the electrode region is presented. The formulation makes use of the already developed NEGF method for MTJs without any impurities and the impurity Green’s function is obtained using Haldane’s approach that explicitly takes into account the on-site Coulomb interaction ([Formula: see text]) of the impurity. The formulation is used to obtain the spin-dependent tunnel current of model MTJs as a function of the applied bias for different values of [Formula: see text] corresponding to both the parallel and antiparallel configuration of the end electrodes of the MTJ. The tunnel currents are used to obtain the tunnel magnetoresistance (TMR) of the MTJ and we observed from our study that the TMR is strongly influenced by the impurities. The TMR is found to be reduced compared to that of the MTJ without impurities and the bias dependence of the TMR is found to be strongly influenced by [Formula: see text]. Our studies revealed that the MTJ can exhibit almost completely spin polarized current at certain values of [Formula: see text].

Low power device design application by magnetic tunnel junctions in Magnetoresistive Random Access Memory (MRAM)

The spintronics techniques combines the spin degree and charge which gives rise to state of the art devices like magnetic tunnel junctions which are ultra-low power devices. It is proposed in the paper to evaluate magnetic tunnel junctions (MTJ) at the device level. The paper present an overview of fabrication process, while considering different aspects of the characteristics of MTJs, performance, challenges and a wide range of applications which are different from conventional CMOS technology. The MTJ devices are CMOS compatible for non-volatility, high stability and reliability. The MTJ device is a promising candidate for building non-volatile circuits for ultra- low power operation and fast switching speeds. The current approaches to design logic functionalities and memory devices with MTJs w include hybrid CMOS and MTJ circuits are also discussed in the paper. The results on the most robust magnetic tunnel junction with Resistance Area = 8 Ω-µm2 demonstrates that it satisfies the statistical requirements in parallel with high magnetic tunnel resistance greater than 15σ(Rp), breakdown-of voltage to edge more than 0.5 V, read-induced rate beneath 10−9, and with adequate write endurance, free of undesirable write endurance magnetic inversion. Also this paper demonstrates on the most vigorous MTJ with Resistance Area = 8 Ω-µm2 that satisfies the statistical requirements in parallel with high magnetic tunnel resistance greater than 15σ(Rp),breakdown-of voltage to edge more than 0.5 V, read-induced rate less than 10−9, and with adequate write endurance, and is free of undesirable write endurance magnetic inversion.

In Situ Stochastic Training of MTJ Crossbars With Machine Learning Algorithms

Owing to high device density, scalability, and non-volatility, magnetic tunnel junction (MTJ)-based crossbars have garnered significant interest for implementing the weights of neural networks (NNs). The existence of only two stable states in MTJs implies a high overhead of obtaining optimal binary weights in software. This article illustrates that the inherent parallelism in the crossbar structure makes it highly appropriate for in situ training, wherein the network is taught directly on the hardware. It leads to significantly smaller training overhead as the training time is independent of the size of the network, while also circumventing the effects of alternate current paths in the crossbar and accounting for manufacturing variations in the device. We show how the stochastic switching characteristics of MTJs can be leveraged to perform probabilistic weight updates using the gradient descent algorithm. We describe how the update operations can be performed on crossbars implementing NNs and restricted Boltzmann machines, and perform simulations on them to demonstrate the effectiveness of our techniques. The results reveal that stochastically trained MTJ-crossbar feed-forward and deep belief nets achieve a classification accuracy nearly the same as that of real-valued weight networks trained in software and exhibit immunity to device variations.

Modeling and Analysis of Magnetic Field Induced Coupling on Embedded STT-MRAM Arrays

Spin transfer torque magnetic random access memory (STT-MRAM) is an emerging memory technology which exhibits nonvolatility, high density, high endurance, and nano-second read and write times. These characteristics make STT-MRAM suitable for last-level cache and other embedded applications. The STT-MRAM bit-cell consists of a magnetic tunnel junction (MTJ) which is composed of two ferromagnetic layers (free and fixed layers) and one insulating layer in between. As STT-MRAM arrays become denser to meet cost and requirements of high performance computing, the distance between adjacent MTJ bits reduces. This aggravates the magnetic coupling from free and fixed layer of one MTJ bit to its neighbors. Even though magnetic coupling is expected to become stronger as MTJ scales down, its impacts on static and dynamic properties on MTJ is relatively unexplored. In this paper, we present a model of the magnetic field coupling in high-density MTJ arrays for three different types of MTJ stacks and evaluate the effect of magnetic field induced coupling on static and dynamic properties. Lastly, we show how process induced variations in MTJ characteristics affect the magnitude of magnetic coupling and their effect on the electrical characteristics of the STT-RAM arrays.

A Comparative Study Between Spin-Transfer-Torque and Spin-Hall-Effect Switching Mechanisms in PMTJ Using SPICE

The spin transfer torque magnetoresistive random access memory (STT-MRAM) is the leading candidate for spin-based memories. Nevertheless, the high write energy and read disturbance of the STT-MRAM motivated researchers to find other solutions. The spin Hall effect (SHE)-based MRAM is an alternative for the STT-MRAM, which also provides nonvolatility, zero leakage, and competitive area per bit, but with a lower write current. This paper focuses on a systematic performance analysis of these two proposed memory solutions. The SHE requires an external field to deterministically switch perpendicular magnetic anisotropy magnetic tunnel junction (MTJ). A previous experiment showed that the SHE can switch composite MTJ containing an in-plane layer without any field. In this paper, both traditional and composite MTJ structures are modeled in SPICE, which can reproduce realistic MTJ characteristics with user-defined input parameters. This self-contained model is used to compare the write energy and delay of the STT-MRAM and the SHE magnetoresistive random access memory (SHE-MRAM) for various write schemes including thermal fluctuation. Our simulations show, compared with the STT-MRAM, that the SHE-MRAM improves the write delay and the energy by eight times and seven times, respectively. Based on our extensive analysis incorporating the latest advances in magnetic materials and device technology, we predict that the SHE-MRAM is a feasible low-energy memory solution for future computing systems.

Nanoscale Spin-Transfer Torque MRAM Etching Using Various Gases

As the volatile dynamic random access memory (DRAM) device faces a physical limitation, new memory devices are actively investigated to solve this problem. Among them, spin transfer torque magnetic random access memory (STT-MRAM) has received a lot of attention because of its very high speed of device, endurance, low power consumption of device, and high density memory cell in addition to the non-volatility of information. Especially, the MTJ memory elements which consist of CoxFeyBz/MgO/CoxFeyBz multilayer structure are the critical building blocks for high density magnetic random access memory with high tunneling magnetoresistance(TMR). However, the dry etching of MTJ stack with sub 70 nm is a challenging step in the fabrication of STT-MRAM. The etching of nanoscale MTJ induces oxidation in magnetic layer, pattern degradation and damage or sidewall redeposition at patterned MTJ sidewall especially. In this study, for successful etching of MTJ and MTJ related materials, RIB (reactive ion beam) etching using NF3, CH3OH, and CO/NH3 gas mixture and ICP etching using Ar, He, CO/NH3 and so on were conducted. Etch characteristics of magnetic tunnel junction (MTJ) stacks masked with W were investigated precisely using field emission scanning electron microscopy (FE-SEM) and TEM (Transmission electron microscopy). Simulation of MTJ material and hardmask material was conducted with SRIM simulation to observe tendency with etch process of blank unit thin film samples. In addition, the magnetic hysteresis curves were analyzed by a vibrating sample magnetometer (VSM). The results showed that, among the various gas combinations, the use of He gas improved etch selectivity of MTJ materials over W and also improved the etch profile of MTJ features. Also the He etch gas could be beneficial in reducing damage. The detailed etch mechanism and results on the etching of MTJ materials will be presented.

A Smaller, Faster, and More Energy-Efficient Complementary STT-MRAM Cell Uses Three Transistors and a Ground Grid: More Is Actually Less

Spin-transfer torque magnetoresistance random access memory is a major contender for static random access memory replacement in embedded caches at advanced fin field effect transistor nodes. It suffers, however, from the low resistance difference between the bistable states of the magnetic tunnel junction (MTJ). Variability on MTJ resistance and access transistors makes reliable read-out even more challenging. This triggered the use of complementary cells for low level caches needing high performance. This paper, focusing on the lower level caches, shows an improved 3T 2MTJ cell with a ground grid and a novel three transistor read and write operation to improve area density, sense margin, write performance, and write energy consumption. Despite the cell’s three transistors, the improved array configuration reduces the cell area by 22% as compared with the 2T 2MTJ cell, making it only 55% larger than a 1T 1MTJ cell. The novel mismatch tolerant read operation uses all three transistors and increases the sense margin by up to 88%. The novel variation resilient write operation also uses all three transistors and takes advantage of the inherent MTJ characteristics and complementary operation of the cell. This increases the write performance by $2\times $ and reduces the write energy by $3\times $ compared with the 2T 2MTJ cell and by $1.5\times $ compared with the 1T 1MTJ cell.

Enhanced Reliability of Top-pinned Perpendicular Magnetic Tunnel Junction by Post-oxidation of Sputtered MgO Barrier

ABSTRACTWe proposed an MgO barrier which is fabricated by combination of rf-sputter deposition of MgO film and subsequent in-situ post oxidation (PO). We found that the perpendicular magnetic anisotropy (PMA) of the CoFeB layer formed on this MgO barrier with PO was improved. We also found that a short error rate reduced drastically and a magnetoresistance (MR) ratio increased about 20% for the magnetic tunnel junction (MTJ) with this MgO barrier with PO. In addition, we showed that this MgO barrier with PO has long endurance life compared with conventional sputtered MgO barriers, and has a potential to operate over 1016 write cycles.Furthermore, we have observed that the PO could suppress the Fe diffusion into the MgO barrier and form Fe-O bonding at MgO/CoFeB interface using electron energy-loss spectroscopy (EELS). The obtained results might be involved to the improvement of PMA and MTJ characteristics.

Scalable Adaptive Spintronic Reconfigurable Logic Using Area-Matched MTJ Design

Spin-transfer torque (STT) random access memory has been researched as a promising alternative for static random access memory in reconfigurable fabrics, particularly in lookup tables (LUTs), due to its nonvolatility, low standby and static power, and high integration density features. In this brief, we leverage physical characteristics of magnetic tunnel junctions (MTJs) to design a unique reference MTJ which has a calibrated resistance matching the STT-based LUT (STT-LUT) circuit requirements to provide optimal reading operation. Results obtained show 42% and 70% power–delay product (PDP) improvement over previous MTJ-based LUT designs. Moreover, a four-input adaptive STT-based LUT (A-LUT) is proposed based on the developed STT-LUT, which is configurable to function in seven independent modes. An $n$ -input A-LUT exhibits PDP which can be a fraction of $n$ -input STT-LUT PDP, when performing two-input to $(n-1)$ -input Boolean logic functions.

Temperature dependence of reliability characteristics for magnetic tunnel junctions with a thin MgO dielectric film

Temperature dependence of the reliability characteristics of magnetic tunnel junctions (MTJs) with a thin (∼1 nm thick) MgO dielectric film were investigated by numerical analyses based on the E-model, 1/E-model, and power-law voltage V-model, as well as by measuring time-dependent dielectric breakdown (TDDB) degradation. Although the tunneling process giving rise to TDDB is still under debate, the temperature dependence of TDDB was much weaker using the 1/E model than the E-model or power-law model. The TDDB data measured experimentally in CoFeB/MgO/CoFeB MTJ devices also showed rather weak temperature dependence, in good agreement with the numerical results obtained from the 1/E-model considering the self-heating effect in MTJ devices. Moreover, we confirmed by interval voltage stress tests that some degradation in the MgO dielectric layer occurred. Based on our findings, we suggest that to characterize the reliability of MTJs, combined temperature measurements of TDDB and 1/E-model analyses taking the self-heating effect into account should be performed.

Energy-Efficient Nonvolatile Reconfigurable Logic using Spin Hall Effect-based Lookup Tables

In this paper, we leverage magnetic tunnel junction (MTJ) devices to design an energy-efficient nonvolatile lookup table (LUT), which utilizes a spin Hall effect (SHE) assisted switching approach for MTJ storage cells. SHE–MTJ characteristics are modeled in Verilog-A based on precise physical equations. Functionality of the proposed SHE–MTJ-based LUT is validated using SPICE simulation. Our proposed SHE—MTJ-based LUT (SHE–LUT) is compared with the most energy-efficient MTJ-based LUT circuits. The obtained results show more than 6%, 37%, and 67% improvement over three previous MTJ-based designs in term of read energy consumption. Moreover, the reconfiguration delay and energy of the proposed design is compared with that of the MTJ-based LUTs which utilize the spin transfer torque (STT) switching approach for reconfiguration. The results exhibit that SHE–LUT can operate at 78% higher clock frequency while achieving at least 21% improvement in terms of reconfiguration energy consumption. The operation-specific clocking mechanisms for managing the SHE–LUT operations are introduced along with detailed analyses concerning tradeoffs. Results are extended to design a 6-input fracturable LUT using SHE–MTJs.

A Survey on the Modeling of Magnetic Tunnel Junctions for Circuit Simulation

Spin-transfer torque-based magnetoresistive random access memory (STT-MRAM) is a promising candidate for universal memory that may replace traditional memory forms. It is expected to provide high-speed operation, scalability, low-power dissipation, and high endurance. MRAM switching technology has evolved from the field-induced magnetic switching (FIMS) technique to the spin-transfer torque (STT) switching technique. Additionally, material technology that induces perpendicular magnetic anisotropy (PMA) facilitates low-power operation through the reduction of the switching current density. In this paper, the modeling of magnetic tunnel junctions (MTJs) is reviewed. Modeling methods and models of MTJ characteristics are classified into two groups, macromodels and behavioral models, and the most important characteristics of MTJs, the voltage-dependent MTJ resistance and the switching behavior, are compared. To represent the voltage dependency of MTJ resistance, some models are based on physical mechanisms, such as Landau-Lifshitz-Gilbert (LLG) equation or voltage-dependent conductance. Some behavioral models are constructed by adding fitting parameters or introducing new physical parameters to represent the complex switching behavior of an MTJ over a wide range of input current conditions. Other models that are not based on physical mechanisms are implemented by simply fitting to experimental data.

Enhancement of Spin-transfer torque switching via resonant tunneling

We propose the use of resonant tunneling as a route to enhance the spin-transfer torque switching characteristics of magnetic tunnel junctions. The proposed device structure is a resonant tunneling magnetic tunnel junction based on a MgO-semiconductor heterostructure sandwiched between a fixed magnet and a free magnet. Using the non-equilibrium Green's function formalism coupled self consistently with the Landau-Lifshitz-Gilbert-Slonczewski equation, we demonstrate enhanced tunnel magneto-resistance characteristics as well as lower switching voltages in comparison with traditional trilayer devices. Two device designs based on MgO based heterostructures are presented, where the physics of resonant tunneling leads to an enhanced spin transfer torque thereby reducing the critical switching voltage by up to 44%. It is envisioned that the proof-of-concept presented here may lead to practical device designs via rigorous materials and interface studies.

Extracting Physically Unclonable Function From Spin Transfer Switching Characteristics in Magnetic Tunnel Junctions

Hardware security devices that include physically unclonable function (PUF) hold significant advantages over software-based secure identification and key storage. We propose a PUF for magnetic tunnel junctions (MTJs) and a method for extracting PUF based on intrinsic properties of spin transfer switching (STS). The STS characteristics of MTJs with perpendicular layers were measured and analyzed to determine how to extract PUF properties. Given the experimental intrinsic stochastic properties combined with the proposed sequence, we discuss the PUF properties extracted from MTJs. To realize secure hardware key in non-volatile devices, our methods and results are promising for constructing MTJ-PUF based on spintronic LSI devices.

Magnetic tunnel junctions with single-layer-graphene tunnel barriers

We report on the fabrication and characterization of magnetic tunnel junctions consisting of a single layer of graphene as the tunnel barrier, sandwiched between two metallic ferromagnetic electrodes. We employ a fabrication process chosen to minimize oxidation of the electrode materials at the ferromagnet/graphene interfaces. The devices have low resistance-area products of 1.5--6 \textohm{} \ensuremath{\mu}m${}^{2}$, with low-temperature magnetoresistances of 1.5--3.4%. The temperature and bias dependencies of the resistance confirm that transport is dominated by tunneling processes rather than by any unintended pinholes.

Determination of probe spacings for the precise evaluation of electrical characteristics of a magnetic tunnel junction by in-house current-in-plane tunneling measurements

Magnetic tunnel junctions have not been easily accessible for research because of not only their complicated fabrication processes but also side effects induced during the fabrication. The method utilizing arrays composed of in-line four-point probes with various spacings is promising as an alternative to the fabrication method. We found in the current-in-plane tunneling measurement that the determination of the probe spacing is the most important to evaluate the characteristics of magnetic tunnel junctions. Our simulation indicates that if one would choose at least more than three sets of an array with probe spacings centered at the spacing at which the maximum current-in-plane tunneling magnetoresistance is observed, the statistics should become improved resulting in the accurate evaluation of the properties of tunnel junctions. We also found that the suitable probe spacings with a change in the resistance of electrodes are not as sensitive as those with a change in the RA of the tunnel junction. Our results alert that the failure of selecting suitable probe spacings observes no tunneling signals because tunneling is very sensitive to the resistances of the tunnel junction and electrodes, which makes the current-in-plane tunneling method useless.

Reliability-Based Optimization of Spin-Transfer Torque Magnetic Tunnel Junction Implication Logic Gates

Recently, magnetic tunnel junction (MTJ)-based implication logic gates have been proposed to realize a fundamental Boolean logic operation called material implication (IMP). For given MTJ characteristics, the IMP gate circuit parameters must be optimized to obtain the minimum IMP error probability. In this work we present the optimization method and investigate the effect of MTJ device parameters on the reliability of IMP logic gates. It is shown that the most important MTJ device parameters are the tunnel magnetoresistance (TMR) ratio and the thermal stability factor Δ. The IMP error probability decreases exponentially with increasing TMR and Δ.