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Reliability and packing density concerns are the two major shortcomings of spin transfer torque and spin orbit torque based magnetic memory, respectively. Voltage controlled magnetic anisotropy (VCMA) becomes energy efficient and fast, showing transcendence for the writing mechanism in the magnetic tunnel junction. Deterministic switching cannot be achieved by VCMA alone in the out of plane nanomagnet. It requires an external in-plane magnetic field, but the use of an external field is inconvenient for on-chip applications. We exploit stress and exchange bias provided by an antiferromagnetic material to mitigate the external magnetic field requisite. We perform macro-spin simulations using the Landau–Lifshitz–Gilbert equation at room temperature. We use the VCMA effect cum stress effect to investigate field free switching performance, and this improves the write error rate (WER) to 5 × 10 − 5 against WER of 0.1 with the VCMA effect alone. We studied the effects of applied voltage (amplitude and pulse width), exchange bias field, and VCMA coefficient on the switching performance in detail. This proposed two-terminal device can be helpful in achieving high cell density to implement nonvolatile magnetic memory. [ABSTRACT FROM AUTHOR]

The current-induced magnetization precession dynamics provoked by the spin transfer torque (STT) in a spin valve device i.e. tri-layer device (commonly spin torque nano oscillator (STNO)) is investigated numerically by solving the governing Landau–Lifshitz–Gilbert–Slonczewski (LLGS) equation. In this study, we have devised an STNO device made of EuO-based ferromagnetic alloy in free and fixed magnetic layers. The copper acts as a nonmagnetic spacer. Here, we have introduced the current induced Oesterd field (CIOF), which is generated when a spin-polarized current passes through the STNO device. In the device, we have tuned the free layer angle θ from 3 0 ∘ to 9 0 ∘ as an increment of 3 0 ∘ . For every individual θ ranging from 3 0 ∘ to 9 0 ∘ , the generated Oersted field's strength can be altered by increasing the STNO device's diameter. Henceforth, it is apparent that the frequency tunability is achieved in the device for all the values of θ. The frequency and power of the device depend entirely on the material's saturation magnetization, which inherently reflects the current density and coherence of spin-polarized DC. From the results, it is apparent that for a particular θ , the frequency keeps increasing with the eventual decrease in power when we increase the strength of the Oersted field from 10 kA/m to 50 kA/m. By doing so, the maximum frequency can be tuned up to 212 GHz for θ = 9 0 ∘ with H oe as 50 kA/m. The high frequency emitted by the device acts as a linchpin ingredient, as well as a launch pad element in much of scientific and technological point of view. It paves way for a new route in the areas such as high capacity, high precision, high density as well as the sensing applications. [ABSTRACT FROM AUTHOR]

The dynamics of a spin torque-driven ferrimagnetic (FiM) system is investigated using the two-sublattice macrospin model. We demonstrate ultrafast switching in the picosecond range. However, we find that the excessive current leads to magnetic oscillation. Therefore, faster switching cannot be achieved by unlimitedly increasing the current. By systematically studying the impact of thermal fluctuations, we find that the dynamics of FiMs can also be distinguished into the precessional region, the thermally activated region, and the crossover region. However, in the precessional region, there is a significant deviation between FiM and ferromagnet (FM), i.e., the FM is insensitive to thermal fluctuations since its switching is only determined by the amount of net charge. In contrast, we find that the thermal effect is pronounced even when a very short current pulse is applied to the FiM. We attribute this anomalous effect to the complex relation between the anisotropy and overdrive current. By controlling the magnetic anisotropy, we demonstrate that the FiM can also be configured to be insensitive to thermal fluctuations. This controllable thermal property makes the FiM promising in many emerging applications such as the implementation of tunable activation functions in the neuromorphic computing. [ABSTRACT FROM AUTHOR]

In the future, DW memory will replace conventional storage memories with high storage capacity and fast read/write speeds. The only failure in DW memory arises from DW thermal fluctuations at pinning sites. This work examines, through calculations, the parameters that might help control DW thermal stability at the pinning sites. It is proposed to design a new scheme using a stepped area of a certain depth (d) and length (λ). The study reveals that DW thermal stability is highly dependent on the geometry of the pinning area (d and λ), magnetic properties such as saturation magnetization (Ms) and magnetic anisotropy energy (Ku), and the dimensions of the nanowires. For certain values of d and λ, DWs remain stable at temperatures over 500 K, which is beneficial for memory applications. Higher DW thermal stability is also achieved by decreasing nanowire thickness to less than 10 nm, making DW memories stable below 800 K. Finally, our results help to construct DW memory nanodevices with nanodimensions less than a 40 nm width and less than a 10 nm thickness with high DW thermal stability. [ABSTRACT FROM AUTHOR]

Magnetic tunnel junctions (MTJs), a key component of spin transfer torque magnetic random access memory, are typically fabricated using two main processes: plasma etching and in situ protective cap layer deposition. It has been found that while the etching process predominantly affects MTJ performance, the cap layer process can further enhance electrical and magnetic properties. In this study, we achieved performance improvements in MTJs by optimizing the cap layer deposition process through various experimental methods, such as modifying the gas mixtures used in the deposition process and incorporating a novel post-plasma treatment. During the deposition of the silicon nitride (SiNx) cap layer, N-rich dissociated compounds can induce passivation of the MTJ layer, leading to additional loss of tunneling magnetoresistance (TMR) and coercive field (Hc). To circumvent this challenge, we prioritized modifying the gas ratio in the SiNx deposition process. Additionally, hydrogen introduced during SiNx deposition can penetrate the MTJ pillars and degrade their properties. To mitigate this, we developed a novel post-nitrogen plasma treatment in a plasma-enhanced chemical vapor deposition chamber, which effectively desorbed the excess hydrogen from the MTJ film stack. As a result of these optimized processes, the TMR loss, compared to a blanket wafer, was reduced from 25% to 8%, and Hc increased by up to 33% for the same stack, achieving significant performance enhancements. [ABSTRACT FROM AUTHOR]

The manipulation of spin textures by spin currents is of fundamental and technological interest. A particularly interesting system is the 2D van der Waals ferromagnet Fe3GeTe2, in which Néel-type skyrmions have recently been observed. The origin of these chiral spin textures is of considerable interest. Recently, it was proposed that these derive from defects in the structure that lower the symmetry and allow for a bulk vector Dzyaloshinsky-Moriya interaction. Here, we demonstrate current-induced domain wall motion in Fe3GeTe2 flakes, in which the maximum domain wall velocity is an order of magnitude higher than those reported in previous studies. In heterostructures with Pt or W layers on top of the Fe3GeTe2 flakes, domain walls can be moved via a combination of spin transfer and spin-orbit torques. The competition between these torques leads to a change in the direction of domain wall motion with increasing magnitude of the injected current. In this work, the researchers realize the current-induced motion of Néel type chiral domain walls via spin-transfer-torque in the pristine van der Waals ferromagnet Fe3GeTe2 and via spin-orbit-torques in heterostructures with platinum or tungsten. [ABSTRACT FROM AUTHOR]

Current-induced domain wall motion (CIDWM) in magnetic wires can be driven by spin transfer torque (STT) originating from transferring angular momentums of spin-polarized conducting electrons to the magnetic DW and can be driven by spin orbit torque (SOT) originating from the spin Hall effect (SHE) in a heavy metal layer and Dzyaloshinsky Moriya (DMI) generated at an interface between a heavy metal layer and a magnetic layer. In this work, we carried out a comparative study of CIDWM driven by STT and by SOT in ferrimagnetic GdFeCo wires with magnetic perpendicular anisotropy based on structures of SiN (10 nm)/GdFeCo (8 nm)/SiN (10 nm) and Pt (5 nm)/GdFeCo (8 nm)/SiN (10 nm). We found that CIDWM driven by SOT exhibited a much lower critical current density (JC), and much higher DW mobility (µDW). Our work might be useful for the realization and the development of low-power and high-speed memory devices. [ABSTRACT FROM AUTHOR]

AbstractHybrid magnetic tunnel junction (MTJ)/CMOS circuits based on the computation-in-memory (CIM) architecture are contemplated as the future generation of digital integrated circuits. It overcomes the limitations of von-Neumann architecture by offering solutions to problems such as memory wall and standby power dissipation. In this work, we have developed hybrid logic gates, such as AND/NAND, OR/NOR, and XOR/XNOR, for CIM architecture by integrating three terminal spin-Hall effect assisted spin transfer torque (SHE + STT) MTJs with standard CMOS. To write the MTJs an auto-write-stopping (AWS) circuit is adopted, whereas to perform the logic operations and produce the corresponding outputs, an improved sense amplifier circuit (ISA) is employed. All the hybrid logic gates are investigated for key performance indicators such as power, delay, device count, and power delay product (PDP). The results are compared with their conventional counterparts. The comparison reveals that the ISA + AWS-based hybrid gates dissipate 50.52% lower total power. The worst-case read delay of ISA + AWS hybrid AND/NAND, OR/NOR, and XOR/XNOR gates are 27.41%, 13.4%, and 21.28% lower. Meanwhile, the reduction of read PDP (write PDP) is 47.64% (37.09%), 25.78% (36.29%), and 39.31% (35.48%) observed with ISA + AWS hybrid AND/NAND, OR/NOR, and XOR/XNOR gates in comparison with the conventional counterparts. Hence the ISA + AWS gates are superior in terms of total power dissipation, worst read delay, and read/write PDP. Further, we have conducted Monte-Carlo simulations on all the logic circuits to study the parameter variations during fabrication.

The tri-layer magnetic tunnel junction (MTJ) has surfaced as a building block for engineering next-generation integrated circuits while combining the attributes of non-volatility and meager energy consumption. Nevertheless, the perceptible switching energy (≈ 20 – 50 fJ/bit) and sub-optimal tunnelmagnetoresistance (TMR) (≈ 200 % – 300 %) have acted as major hindrances, concealing its potential to supersede the capabilities of static and dynamic random access memories. In this work, we introduce a novel device that features a minimalistic non-uniform heterostructure/superlattice instead of the oxide layer in a conventional MTJ and analyze it in the premise of the self-consistent coupling of the Non-Equilibrium-Green's Function (NEGF) and the Landau-Liftshitz-Gilbert-Slonczewski (LLGS) equation. We ascertain that the coupling of the electrodes to the proposed heterostructure renders a highly spin-selective broadband transmittance, thereby enabling a towering TMR (%) of 3.7 × 104% along with a significant reduction in the spin transfer torque (STT) switching energy (≈1.96 fJ). Furthermore, the sizable slonczewski term (Is‖) originating from the heterostructure facilitates a swift STT-switching within the scale of a few hundred picoseconds (≈400 ps). [ABSTRACT FROM AUTHOR]

Ferrimagnets perform versatile properties, attributed to their antiferromagnetic sublattice coupling and finite net magnetization. Despite extensive research, the inhomogeneous dynamics in ferrimagnets, including domain walls and magnons, remain not fully understood. Therefore, we adopted a multi-spin model by considering the effect of the spin torques and explored the localized phase-dependent and inhomogeneous THz-oscillation dynamics in a ferrimagnetic spin-chain. Our results demonstrate that the exchange oscillation mode, induced by spin transfer torque, exhibits three typical phases, and the oscillation frequency is dominated by a joint effective field derived in the spin-chain. We also found that the localized spin configurations can be used to tune the bandwidth and sensitivity of the frequency response. Furthermore, we propose an anti-parallel exchange length to reveal the inhomogeneity in the ferrimagnetic spin-chain, which could serve as a valuable tool for characterizing the spin dynamics of these systems. Our findings offer understandings beyond uniform spin-dynamics in ferrimagnets. Ferrimagnets present advantages over their anitferromagnetic and ferromagnetic counterparts for applications in ultrafast spintronics but the dynamics of the non-uniform spin textures are not as well understood. Here, the authors investigate the inhomogeneous oscillation dynamics in the ferrimagnetic system via a multi-spin model. [ABSTRACT FROM AUTHOR]

Hybrid magnetic tunnel junction/complementary metal oxide semiconductor (MTJ/CMOS) circuits based on in-memory-computation (IMC) architecture is considered as the next-generation candidate for the digital integrated circuits. However, the energy consumption during the MTJ write process is a matter of concern in these hybrid circuits. In this regard, we have developed a novel write circuit for the contemporary three-terminal perpendicular-MTJs that works on the voltage-gated spin orbit torque (VG+SOT) switching mechanism to store the information in hybrid circuits for IMC architecture. Investigation of the novel write circuit reveals a remarkable reduction in the total energy consumption (and energy delay product) of 92.59% (95.81) and 92.28% (42.03%) than the conventional spin transfer torque (STT) and spin-Hall effect assisted STT (SHE+STT) write circuits, respectively. Further, we have developed all the hybrid logic gates followed by nonvolatile full adders (NV-FAs) using VG+SOT, STT, and SHE+STT MTJs. Simulation results show that with the VG+SOT NOR-OR, NAND-AND, XNOR-XOR, and NV-FA circuits, the reduction in the total power dissipation is 5.35% (4.27%), 5.62% (3.2%), 3.51% (2.02%), and 4.46% (2.93%) compared to STT (SHE+STT) MTJs respectively. [ABSTRACT FROM AUTHOR]

Under the presence of temperature gradient (TG) on a nanotrack, it is necessary to investigate the skyrmion dynamics in various magnetic systems under the combined effect of forces due to magnonic spin transfer torque (μ S T T) , thermal STT ( τ S T T), entropic difference (dS) , as well as thermal induced dipolar field (DF). Hence, in this work, the dynamics of skyrmions in ferromagnets (FM), synthetic antiferromagnets (SAF), and antiferromagnets (AFM) have been studied under different TGs and damping constants ( α G ). It is observed that α G plays a major role in deciding the direction of skyrmion motion either towards the hotter or colder side in different magnetic structures. Later, FM skyrmion based logic device is proposed that consists of a cross-coupled nanotrack, where the skyrmions on horizontal and vertical nanotrack are controlled by exploiting TG and electrical STT (eSTT), respectively by taking the advantages of thermal induced skyrmion Hall effect (SkHE). The proposed device performs AND and OR logic functionalities simultaneously, when the applied current density is 2 × 10 11 A m - 2. Moreover, the proposed device is also able to exhibit the half adder functionality by tuning the applied current density to 3 × 10 11 A m - 2. The total energy consumption for AND and OR logic operation and half adder are 33.63 fJ and 25.06 fJ, respectively. This paves the way for the development of energy-efficient logic devices with ultra-high storage density. [ABSTRACT FROM AUTHOR]

We present our findings on how orange peel coupling (OPC) between the ferromagnetic layers affects spin transfer torque-assisted magnetization reversal dynamics in CoPt, CoFeB, and EuO pentalayer nanopillar devices. We accomplish this by solving the Landau-Lifshitz-Gilbert-Slonczewski (LLGS) equation (dynamical equation) using the Runge-Kutta fourth-order numerical technique. To begin, we examine the influence of OPC on switching times by simulating the LLGS equation separately for each pentalayer device, both with and without OPC. The magnetization switching times for CoPt, CoFeB, and EuO devices in the absence of OPC are 58 ps, 82 ps, and 154 ps, respectively. When introducing OPC in the pentalayer alloys, we observe a reduction in switching times of 3.5%, 11%, and 40% for CoPt, CoFeB, and EuO devices, respectively. Furthermore, we calculate the current density required to reverse the magnetization of the free layer in CoPt, CoFeB, and EuO pentalayer devices, which is found to be 0.78 × 10 12 A / m 2 , 1.8 × 10 12 A / m 2 , and 4.4 × 10 12 A / m 2 , respectively. In addition, we investigate how the thicknesses of the free and spacer layers, as well as the wavelength and amplitude of interface roughness, affect the magnetization switching time. Understanding the effects of OPC paves the way for the practical implementation of spin transfer torque (STT)-based memory devices. [ABSTRACT FROM AUTHOR]

Based on the spinor Boltzmann equation, we obtain a temperature-dependent thermal spin–orbit torque in terms of the local equilibrium distribution function in a two-dimensional ferromagnet with Dresselhaus spin–orbit coupling. We also derive the continuity equation of spin accumulation and spin current—the spin diffusion equation in Dresselhaus ferromagnet, which contains the thermal spin–orbit torque under local equilibrium assumption. This temperature-dependent thermal spin–orbit torque originates from the temperature gradient applied to the system, it is also sensitive to temperature due to the local equilibrium distribution function therein. In the spin diffusion equation, we can single out the usual spin–orbit torque as well as the spin transfer torque, which is conceded to our previous results. Finally, we illustrate them by an example of spin-polarized transport through a ferromagnet with Dresselhaus spin–orbit coupling driven by a temperature gradient, those torques including thermal spin–orbit torque are demonstrated numerically. We study the thermal spin–orbit torque (TSOT) in a system with Dresselhaus Spin–Orbit Coupling. [ABSTRACT FROM AUTHOR]

Inevitable process variations (PVT) brought by both the magnetic tunneling junction (MTJ) and MOSFET based on the complementary metal-oxide semiconductor (CMOS) technology become a major obstacle for the mass production of spin transfer torque magnetic random access memory (STT-MRAM). The detriment of the process variations leads to a serious degradation in the fundamental yield with the shrinkage of the technology nodes. However, the conventional data-cell-variation-tolerant (DCVT) sense scheme cannot get the target read yield due to the limited sense margin (SM). To resolve this problem, a DCVT triple sampling non-destructive self-reference sensing scheme (TSNS) is proposed in the paper, which doubles the SM, with lower power consumption and better SM compared with the conventional DCVT sense scheme. Monte Carlo simulation with industry-compatible 65-nm model parameters results show that the proposed sensing scheme shows over 2.5 times higher SM and less power consumption compared to the previous self-reference circuit. The proposed sensing scheme can get the target read yield with lower power consumption. [ABSTRACT FROM AUTHOR]

In this study, we investigate the thermal pinning and depinning behaviors of vortex domain walls (VDWs) in constricted magnetic nanowires, with a focus on potential applications in storage memory nanodevices. Using micromagnetic simulations and spin transfer torque, we examine the impacts of device temperature on VDW transformation into a transverse domain wall (TDW), mobility, and thermal strength pinning at the constricted area. We explore how thermal fluctuations influence the stability and mobility of domain walls within stepped nanowires. The thermal structural stability of VDWs and their pinning were investigated considering the effects of the stepped area depth (d) and its length (λ). Our findings indicate that the thermal stability of VDWs in magnetic stepped nanowires increases with decreasing the depth of the stepped area (d) and increasing nanowire thickness (th). For th ≥ 50 nm, the stability is maintained at temperatures ≥ 1200 K. In the stepped area, VDW thermal pinning strength increases with increasing d and decreasing λ. For values of d ≥ 100 nm, VDWs depin from the stepped area at temperatures ≥ 1000 K. Our results reveal that thermal effects significantly influence the pinning strength at constricted sites, impacting the overall performance and reliability of magnetic memory devices. These insights are crucial for optimizing the design and functionality of next-generation nanodevices. The stepped design offers numerous advantages, including simple fabrication using a single electron beam lithography exposure step on the resist. Additionally, adjusting λ and d allows for precise control over the pinning strength by modifying the dimensions of the stepped areas.

Artificial intelligence and deep learning today are utilized for several applications namely image processing, smart surveillance, edge computing, and so on. The hardware implementation of such applications has been a matter of concern due to huge area and energy requirements. The concept of computing in-memory and the use of non-volatile memory (NVM) devices have paved a path for resource-efficient hardware implementation. We propose a dual-level spin–orbit torque magnetic random-access memory (SOT-DLC MRAM) based crossbar array design for image edge detection. The presented in-memory edge detection algorithm framework provides spin-based crossbar designs that can intrinsically perform image edge detection in an energy-efficient manner. The simulation results are scaled down in energy consumption for data transfer by a factor of 8x for grayscale images with a comparatively smaller crossbar than an equivalent CMOS design. DLC SOT-MRAM outperforms CMOS-based hardware implementation in several key aspects, offering 1.53x greater area efficiency, 14.24x lower leakage power dissipation, and 3.63x improved energy efficiency. Additionally, when compared to conventional spin transfer torque (STT-MRAM and SOT-MRAM, SOT-DLC MRAM achieves higher energy efficiency with a 1.07x and 1.03x advantage, respectively. Further, we extended the image edge extraction framework to spiking domain where ant colony optimization (ACO) algorithm is implemented. The mathematical analysis is presented for mapping of conductance matrix of the crossbar during edge detection with an improved area and energy efficiency at hardware implementation. The pixel accuracy of edge-detected image from ACO is 4.9% and 3.72% higher than conventional Sobel and Canny based edge-detection. [ABSTRACT FROM AUTHOR]

Achieving unidirectional spin wave (or magnon) transport in domain wall (DW) represents the key step for designing functional magnonic devices. Here, we theoretically investigate the propagation behavior of spin waves (SWs) in antiferromagnetic DW when the Dzyaloshinskii–Moriya interaction (DMI) and/or spin transfer torque (STT) are considered. On the one hand, we find that the DMI lifts the degeneracy of magnon bands, from which one can obtain pure right- or left-handed polarized SWs. On the other hand, the nonreciprocal attenuation of magnons induced by STT is identified. Interestingly, we realize the polarization-dependent unidirectional propagation of SWs when the nonadiabatic coefficient β exceeds a critical value. Moreover, the micromagnetic simulations verify the theoretical predictions with good agreement. Our work provides a simple method for achieving unidirectional magnons with desired polarity in antiferromagnetic DW, which is indispensable for future magnonic computing and communication. [ABSTRACT FROM AUTHOR]

Neural Networks (NN) can be efficiently accelerated in a neuromorphic fabric based on emerging resistive non-volatile memories (NVM), such as Spin Transfer Torque Magnetic RAM (STT-MRAM). Compared to other NVM technologies, STT-MRAM offers many benefits, such as fast switching, high endurance, and CMOS process compatibility. However, due to its low ON/OFF-ratio, process variations and runtime temperature fluctuations can lead to miss-quantizing the sensed current and, in turn, degradation of inference accuracy. In this article, we analyze the impact of the sensed accumulated current variation on the inference accuracy in Binary NNs and propose a design-time reference current generation method to improve the robustness of the implemented NN under different temperature and process variation scenarios (up to 125 °C). Our proposed method is robust to both process and temperature variations. The proposed method improves the accuracy of NN inference by up to 20.51% on the MNIST, Fashion-MNIST, and CIFAR-10 benchmark datasets in the presence of process and temperature variations without additional runtime hardware overhead compared to existing solutions. [ABSTRACT FROM AUTHOR]

Hybrid magnetic tunnel junction (MTJ)/CMOS circuits based on the computation-in-memory (CIM) architecture are contemplated as the future generation of digital integrated circuits. It overcomes the limitations of von-Neumann architecture by offering solutions to problems such as memory wall and standby power dissipation. In this work, we have developed hybrid logic gates, such as AND/NAND, OR/NOR, and XOR/XNOR, for CIM architecture by integrating three terminal spin-Hall effect assisted spin transfer torque (SHE + STT) MTJs with standard CMOS. To write the MTJs an auto-write-stopping (AWS) circuit is adopted, whereas to perform the logic operations and produce the corresponding outputs, an improved sense amplifier circuit (ISA) is employed. All the hybrid logic gates are investigated for key performance indicators such as power, delay, device count, and power delay product (PDP). The results are compared with their conventional counterparts. The comparison reveals that the ISA + AWS-based hybrid gates dissipate 50.52% lower total power. The worst-case read delay of ISA + AWS hybrid AND/NAND, OR/NOR, and XOR/XNOR gates are 27.41%, 13.4%, and 21.28% lower. Meanwhile, the reduction of read PDP (write PDP) is 47.64% (37.09%), 25.78% (36.29%), and 39.31% (35.48%) observed with ISA + AWS hybrid AND/NAND, OR/NOR, and XOR/XNOR gates in comparison with the conventional counterparts. Hence the ISA + AWS gates are superior in terms of total power dissipation, worst read delay, and read/write PDP. Further, we have conducted Monte-Carlo simulations on all the logic circuits to study the parameter variations during fabrication. [ABSTRACT FROM AUTHOR]

Spin torque nano-oscillators (STNOs) hold significant promise for communication and bio-inspired computing applications. However, their modulation capability is constrained by a dilemma between frequency window and linewidth reduction, particularly in hypercritical conditions like the presence of an external magnetic field. This poses a notable challenge in the practical application of STNOs. Here, we report a unique type of all-electrical compact STNOs that employ the tilted magnetic anisotropy (TMA), which can efficiently promote the linewidth Δf reduction and precisely modulate oscillation frequency ranging from 495 to 556 MHz. The developed STNOs consist of a ferromagnetic reference layer with tunable TMA, wherein the spin transfer torque along the tilted spin polarization direction elaborates a self-oscillation of magnetic moments in the free layer without application of magnetic field. The free layer equips in a magnetic droplet oscillation mode, and the oscillation frequency can be modulated either synergistically or independently by varying the current intensity and/or the TMA angle. Nevertheless, the TMA angle primarily governs the deformation of the magnetic droplet and the corresponding oscillation frequency and linewidth. Moreover, a unique 4 × 4 STNO array with optimized input current and TMA configuration is proposed to execute the reservoir computing hardware training based on nonlinear dynamic oscillation phase-coupling characteristics, promising a diverse synchronization map with high kernel quality and low generation rank for highly reliable pattern classification implementation. The developed STNOs possess a simple structure, nonlinearity, high frequency tunability, and compatibility with CMOS processes, enabling them a fundamental component for large-scale integration of advanced hardware in neuromorphic computing. [ABSTRACT FROM AUTHOR]

We investigate the torque field and skyrmion motion at an interface between a ferromagnet hosting a skyrmion and a material with a strong spin–orbit interaction. We analyze both semiconductor materials and topological insulators using a Hamiltonian model that includes a linear term. The spin torque-inducing current is considered to flow in the single band limit; therefore, a quantum model of current is used. Skyrmion motion due to spin transfer torque proves to be more difficult in the presence of a spin–orbit interaction in the case where only interface in-plane currents are present. However, edge effects in narrow nanowires can be used to drive the skyrmion motion and to exert a limited control on its motion direction. We also show the differences and similarities between torque fields due to electric current in the many and single band limits. [ABSTRACT FROM AUTHOR]

In the future, DW memory will replace conventional storage memories with high storage capacity and fast read/write speeds. The only failure in DW memory arises from DW thermal fluctuations at pinning sites. This work examines, through calculations, the parameters that might help control DW thermal stability at the pinning sites. It is proposed to design a new scheme using a stepped area of a certain depth (d) and length (λ). The study reveals that DW thermal stability is highly dependent on the geometry of the pinning area (d and λ), magnetic properties such as saturation magnetization (Ms) and magnetic anisotropy energy (Ku), and the dimensions of the nanowires. For certain values of d and λ, DWs remain stable at temperatures over 500 K, which is beneficial for memory applications. Higher DW thermal stability is also achieved by decreasing nanowire thickness to less than 10 nm, making DW memories stable below 800 K. Finally, our results help to construct DW memory nanodevices with nanodimensions less than a 40 nm width and less than a 10 nm thickness with high DW thermal stability.

Current-induced domain wall motion (CIDWM) in magnetic wires can be driven by spin transfer torque (STT) originating from transferring angular momentums of spin-polarized conducting electrons to the magnetic DW and can be driven by spin orbit torque (SOT) originating from the spin Hall effect (SHE) in a heavy metal layer and Dzyaloshinsky Moriya (DMI) generated at an interface between a heavy metal layer and a magnetic layer. In this work, we carried out a comparative study of CIDWM driven by STT and by SOT in ferrimagnetic GdFeCo wires with magnetic perpendicular anisotropy based on structures of SiN (10 nm)/GdFeCo (8 nm)/SiN (10 nm) and Pt (5 nm)/GdFeCo (8 nm)/SiN (10 nm). We found that CIDWM driven by SOT exhibited a much lower critical current density (JC), and much higher DW mobility (µDW). Our work might be useful for the realization and the development of low-power and high-speed memory devices.

A series of half-metallic XYZ half-Heusler alloys is combined with MgO to create Heusler–MgO junctions. The electronic and magnetic properties of these junctions are investigated. The strong oxidation between metal and oxygen atoms causes the systems with pure YY interfaces to be the most stable cases. We conclude that uniaxial anisotropy can be induced in Heusler layers adjacent to MgO. The type of interface layers determines the half-metallicity and anisotropy (in-plane or perpendicular) in the Heusler–MgO junctions. The capacity to retain both half-metallicity and perpendicular magnetic anisotropy in NiMnSb/MgO and CoTiSn/MgO junctions with a MnMn interface layer makes these structures potential candidates as electrode layers in spin transfer torque random access memory devices. [ABSTRACT FROM AUTHOR]

As the third-generation neural network, the spiking neural network (SNN) has become one of the most promising neuromorphic computing paradigms to mimic brain neural networks over the past decade. The SNN shows many advantages in performing classification and recognition tasks in the artificial intelligence field. In the SNN, the communication between the pre-synapse neuron (PRE) and the post-synapse neuron (POST) is conducted by the synapse. The corresponding synaptic weights are dependent on both the spiking patterns of the PRE and the POST, which are updated by spike-timing-dependent plasticity (STDP) rules. The emergence and growing maturity of spintronic devices present a new approach for constructing the SNN. In the paper, a novel SNN is proposed, in which both the synapse and the neuron are mimicked with the spin transfer torque magnetic tunnel junction (STT-MTJ) device. The synaptic weight is presented by the conductance of the MTJ device. The mapping of the probabilistic spiking nature of the neuron to the stochastic switching behavior of the MTJ with thermal noise is presented based on the stochastic Landau–Lifshitz–Gilbert (LLG) equation. In this way, a simplified SNN is mimicked with the MTJ device. The function of the mimicked SNN is verified by a handwritten digit recognition task based on the MINIST database. [ABSTRACT FROM AUTHOR]

We construct a nonlinear theory of electric resistance of chiral helimagnets, in which the shape changes and the magnetization spiral starts rotating during the passage of electric current due to the spin transfer torque effect. It is shown that the rotation of the spin spiral under the action of the passing current, the electric resistance of the helimagnet is always lower than the resistance of a helimagnet in which the spin spiral is stationary. It is found that the current–voltage characteristic of the helimagnet in the presence of the spin transfer torque from the conduction electron system to the system of localized electrons can be essentially nonlinear. The possibility of the spin electric bistability effect in helimagnets is predicted for the situation when the spin contribution to electric resistance of a helimagnet can take two different values for the same value of the current passing through it. The possibility of realization of states with a negative differential resistance in helimagnets is demonstrated. [ABSTRACT FROM AUTHOR]

Spiking Neural Networks (SNNs) that require synapse weight initialization using random numbers have been widely used in the neural morphological system. However, the random numbers generated by traditional digital circuits have certain repeatability, and the entire computing architecture has issues such as high resource consumption and low integration. In this letter, a hardware system for true random number generation is realized through integrating a magnetic tunnel junction, a memory cell of MRAM (magnetic random access memory) chips, with an interface circuit and using the same mechanism as writing data in spin transfer torque MRAM. The generated true random numbers are evaluated using NIST SP800-22 standard and are used for synapse weight initialization in an SNN system. The recognition rate of the system initialized by the generated true random numbers is about 84% for an MNIST handwritten digit dataset, which is 2%–3% higher than that using a traditional linear feedback shift register. The reported work provides a new approach for better SNN performance. [ABSTRACT FROM AUTHOR]

Manipulation of antiferromagnetic sublattice orientations, a key challenge in spintronic device applications, requires unconventional methods such as current induced torques including Spin Transfer Torque (STT) and Spin-Orbit Torque (SOT). In order to observe the deviation of the Néel vector from the anisotropy axis, one of the simplest approaches is the electrical detection, whereby one monitors the change in resistance as a function of applied current. In this work, we have investigated the conditions under which an ultra-thin metallic antiferromagnet, Ir20Mn80 becomes susceptible to SOT effects by studying antiferromagnetic layer structure and thickness dependence in antiferromagnetic metal (Ir20Mn80)/heavy metal (Pt) superlattices. Our electrical measurements reveal that in bilayer structures there exists a shallow range of Ir20Mn80 thicknesses (∼1–2 nm) for which SOT driven control of spins is apparent, whereas for lower thicknesses incomplete sublattice formation and for higher thicknesses improved thermal stability prohibits vulnerability to spin currents. Furthermore, in multilayers, structural changes in Ir20Mn80 layer quenches local torques due to stronger (111) magnetocrystalline anisotropy. These results suggest that an exhaustive optimization of the antiferromagnet parameters is crucial for the successful deployment of spintronic devices. [ABSTRACT FROM AUTHOR]

Currently, static circuit power is becoming a major concern, dominating the total power consumption due to the scaling down of CMOS technology. The smaller sizes drastically affect the leakage current, which integrated circuit designers attempt to overcome this issue. Hence, several methods and technologies have been proposed to prevail this phenomenon. One of these methods is using memory structures in logic designs. A Hybrid MTJ/CMOS circuit is one of these promising techniques to design low-power nonvolatile circuits with power gating ability and low overhead for reconfigurable possibilities. In this paper, we have proposed a fully nonvolatile, low-power Full-Adder based on MTJs that uses the spin transfer torque method assisted by the spin hall effect. Simulation results of these designs by HSPICE show that they can work fast with low-power consumption compared to other state-of-the-art nonvolatile full-adders. [ABSTRACT FROM AUTHOR]

Brain-inspired neuromorphic computing is a promising path towards next generation analogue computers that are fundamentally different compared to the conventional von Neumann architecture. One model for neuromorphic computing that can mimic the human brain behavior are spiking neural networks (SNNs), of which one of the most successful is the leaky integrate-and-fire (LIF) model. Since conventional complementary metal-oxide-semiconductor (CMOS) devices are not meant for modelling neural networks and are energy inefficient in network applications, recently the focus shifted towards spintronic-based neural networks. In this work, using the advantage of antiferromagnetic insulators, we propose a non-volatile magnonic neuron that could be the building block of a LIF spiking neuronal network. In our proposal, an antiferromagnetic domain wall in the presence of a magnetic anisotropy gradient mimics a biological neuron with leaky, integrating, and firing properties. This single neuron is controlled by polarized antiferromagnetic magnons, activated by either a magnetic field pulse or a spin transfer torque mechanism, and has properties similar to biological neurons, namely latency, refraction, bursting and inhibition. We argue that this proposed single neuron, based on antiferromagnetic domain walls, is faster and has more functionalities compared to previously proposed neurons based on ferromagnetic systems. [ABSTRACT FROM AUTHOR]

The combination of a low tunneling barrier height and a large tunneling magnetoresistance (TMR) ratio in MgO-class magnetic tunnel junctions (MTJs) has enabled next-generation information storage and bio-inspired computing solutions thanks to the spin transfer torque effect. Recent literature has proposed that this synergistic combination arises from the electronic properties of oxygen vacancies. To explicitly understand their impact on spin-polarized transport, we have computed the electronic and transport properties of single (F centers) and paired (M centers) oxygen vacancies using density functional theory and the projector augmented wave method. These point defects can generate energy level positions of 0.4 eV with respect to the Fermi level for FeCo electrodes irrespective of the defect's spatial position within the MgO barrier and of the orientation of the M center. These defects promote a strong decrease in the conductance of the spin up channel in the MTJ's parallel magnetic state that mainly accounts for an order-of-magnitude drop in TMR from ≈ 10 000 % in the ideal case toward values more in line with experiment. When placed in the middle layer of the MgO barrier, the F center introduces additional P ↑ transmission away from the Γ point. This scattering lowers TMR to 145%. In contrast, the M center merely broadens this transmission around Γ , thereby boosting TMR to 315%. Rotating a M center so as to partly point along the transmission direction sharpens transmission around Γ , further increasing TMR to 1423%. When these defects are placed at the MTJ interface, the transmission and ensuing TMR, which reaches ≈ 4000 %, suggest that such junctions behave as an ideal MTJ only with a much lower TMR. Our results, thus, theoretically reconcile the concurrent observations of high TMR and low barrier heights in line with experimental preparation techniques such as post-deposition oxidation of metallic Mg, which can generate oxygen vacancies at the lower MTJ interface, and annealing which can promote M centers over F centers. Our theory is also in line with an origin of perpendicular magnetic anisotropy in terms of oxygen vacancies at MTJ interfaces. The effective size of these vacancies sets a limit for both the barrier thickness, in line with experiment, as well as for the MTJ's lateral dimension. Our work provides a much-needed theoretical basis to move beyond the mostly unsuspected, fortuitous defect engineering of spintronic performance that has, thus, far propelled MgO-based spintronics and its applications. [ABSTRACT FROM AUTHOR]

Cellular-level neuron stimulation has attracted much attention in the areas of prevention, diagnosis, and treatment of neurological disorders. Herein, we propose a spintronic neurostimulator based on the domain wall movement inside stationary magnetic nanostructures driven by the spin transfer torques. The electromotive forces generated by the domain wall motion can serve as highly localized stimulation signals for neuron cells. Our simulation results show that the induced electric field from the domain wall motion in permalloy nanostructures can reach up to 14 V / m , which is well above the reported threshold stimulation signal for clinical applications. The proposed device operates on a current range of several microamperes that is 103 times lower than the current needed for the magnetic stimulation by microcoils. The duration and amplitude of the stimulating signal can be controlled by adjusting the applied current density, the geometry of the nanostructure, and the magnetic properties of the material. [ABSTRACT FROM AUTHOR]

In-memory computing (IMC) systems based on emerging nonvolatile memories (NVMs) provide a thorough solution for memory wall issues and von Neumann bottlenecks. Massive IMC schemes have been proposed by utilizing ingenious structures and additional auxiliary components. However, these schemes are not compatible with the basic cell (one memory unit and one transistor) of emerging random-access memory (RAM), which goes against low-power and high-density requirements. In this paper, we propose a logic implementation scheme based on one magnetic tunnel junction and one transistor (1MTJ-1T), which is the basic cell of spin-transfer-torque magnetic RAM (STT-MRAM). With no other assistance, complete 16 logic operations can be accomplished in two steps with their logic outputs in-situ stored in the MTJ. The area (0.2 µm2) and energy consumption per logic operation (1.1–2.6 pJ) of the logic gates under 14 nm process node are evaluated using SPICE simulations, indicating its excellent performance. Our work exhibits a 1MTJ-1T-based logic operation implementation, which can bridge the gap between STT-MRAM and high-performance IMC applications. [ABSTRACT FROM AUTHOR]

We prove via a Galerkin approximation the time local existence of regular solutions to a Landau–Lifshitz–Bloch equation with applied current in a bounded domain. The uniqueness of the solution is also established. Moreover, we show the global in time existence of a regular solution in dimension two. [ABSTRACT FROM AUTHOR]

Oscillation of physical parameters in materials can result in a peak signal in the frequency spectrum of the voltage measured from the materials. This spectrum and its amplitude/frequency tunability, through the application of bias voltage or current, can be used to perform neuron-like cognitive tasks. Magnetic materials, after achieving broad distribution for data storage applications in classical Von Neumann computer architectures, are under intense investigation for their neuromorphic computing capabilities. A recent successful demonstration regards magnetisation oscillation in magnetic thin films by spin transfer or spin orbit torques accompanied by magnetoresistance (MR) effect that can give a voltage peak in the frequency spectrum of voltage with bias current dependence of both peak frequency and amplitude. Here we use classical magnetoimpedance (MI) effect in a magnetic wire to produce such a peak and manipulate its frequency and amplitude by means of the bias voltage. We applied a noise signal to a magnetic wire with high magnetic permeability and owing to the frequency dependence of the magnetic permeability we got frequency dependent impedance with a peak at the maximum permeability. Frequency dependence of the MI effect results in different changes in the voltage amplitude at each frequency when a bias voltage is applied and therefore a shift in the peak position and amplitude can be obtained. The presented method and material provide optimal features in structural simplicity, low-frequency operation (tens of MHz-order) and high robustness at different environmental conditions. Our universal approach can be applied to any system with frequency dependent bias responses. [ABSTRACT FROM AUTHOR]

The inherent nonlinear magnetization dynamics in spintronic devices make them suitable candidates for neuromorphic hardware. Among spintronic devices, spin torque oscillators such as spin transfer torque oscillators and spin Hall oscillators have shown the capability to perform recognition tasks. In this paper, with the help of micromagnetic simulations, we model and demonstrate that the magnetization dynamics of a single spin Hall oscillator can be nonlinearly transformed by harnessing input pulse streams and can be utilized for classification tasks. The spin Hall oscillator utilizes the microwave spectral characteristics of its magnetization dynamics for processing a binary data input. The spectral change due to the nonlinear magnetization dynamics assists in real-time feature extraction and classification of 4-binary digit input patterns. The performance was tested for the classification of the standard MNIST handwritten digit data set and achieved an accuracy of 83.1% in a simple linear regression model. Our results suggest that modulating time-driven input data can generate diverse magnetization dynamics in the spin Hall oscillator that can be suitable for temporal or sequential information processing. [ABSTRACT FROM AUTHOR]

A magnetic skyrmionium can be perceived as an association of two magnetic skyrmions with opposite topological charges. In this work, we have investigated the transformation of skyrmionium into multi-skyrmionic states via domain wall pairs in three different devices with variable geometric configurations. The same device geometries are considered for single ferromagnetic layer and synthetic antiferromagnetic system. It is observed that by tuning the current density, deterministic generation of skyrmions is possible via the spin transfer torque. The proposed device is efficiently adjustable to change the number of skyrmions also at room temperature. The results may lead to development of skyrmion-based devices for neuromorphic and unconventional computing. [ABSTRACT FROM AUTHOR]

Magnetic skyrmions, as spintronic information carriers, are promising for next-generation spin logic and memory devices. For such skyrmion-based devices, effective control of skyrmion nucleation and controllable motion in the nanotrack are of great importance. The ion irradiation process can modify magnetic properties, such as perpendicular magnetic anisotropy (PMA) and Dzyaloshinskii–Moriya interaction (DMI), at the nanoscale, which can be used to reduce the design complexity of devices. In this study, a nanoregion without PMA in the nanotrack is adopted as a skyrmion nucleation seed and a current-driven highly efficient, in-line, and on-demand skyrmion nucleation schematic is presented. A key factor for realizing this concept is that the disappearance of PMA and the existence of DMI induce magnetization tilts and create a chiral perpendicular stripe domain within the nucleation region. This stripe domain allows the effective control of the spin transfer torque, and it is ejected from the PMA-modified region and propelled into the nanotrack, forming a stable skyrmion. Our proposed device allows the controlled nucleation and propagation of a series of skyrmions, which allows binary information to be written in a controlled manner, consequently, yielding simple devices with two terminals. This study provides an efficient route for designing tunable skyrmionics-mechanic memory devices. [ABSTRACT FROM AUTHOR]

We present a generalized potential theory for conservative as well as nonconservative forces for the Landau-Lifshitz magnetization dynamics. Importantly, this formulation makes possible an elegant generalization of the Behler-Parrinello machine learning (ML) approach, which is a cornerstone of ML-based quantum molecular dynamics methods, to the modeling of force fields in adiabatic spin dynamics of out-of-equilibrium itinerant magnetic systems. We demonstrate our approach by developing a deep-learning neural network that successfully learns the electron-mediated exchange fields in a driven s-d model computed from the nonequilibrium Green's function method. We show that dynamical simulations with forces predicted from the neural network accurately reproduce the voltage-driven domain-wall propagation. Our work also lays the foundation for ML modeling of spin transfer torques and opens a avenue for ML-based multi-scale modeling of nonequilibrium dynamical phenomena in itinerant magnets and spintronics. [ABSTRACT FROM AUTHOR]

In this study, we numerically investigate the spin transfer torque oscillation (STO) in a magnetic orthogonal configuration by introducing a strong biquadratic magnetic coupling. The orthogonal configuration consists of top and bottom layers with in-plane and perpendicular magnetic anisotropy sandwiching a nonmagnetic spacer. The advantage of an orthogonal configuration is the high efficiency of spin transfer torque leading a high STO frequency; however, maintaining the STO in a wide range of electric current is challenging. By introducing biquadratic magnetic coupling into the orthogonal structure of FePt/spacer/Co90Fe10, Ni80Fe20 or Ni, we were able to expand the electric current region in which the stable STO is realized, resulting in a relatively high STO frequency. For example, approximately 50 GHz can be achieved in an Ni layer at a current density of 5.5 × 107 A/cm2. In addition, we investigated two types of initial magnetic state: out-of-plane and in-plane magnetic saturation; this leads to a vortex and an in-plane magnetic domain structure after relaxation, respectively. The transient time before the stable STO was reduced to between 0.5 and 1.8 ns by changing the initial state from out-of-plane to in-plane. [ABSTRACT FROM AUTHOR]

The discovery of magnetization switching via spin transfer torque (STT) in PMA-based MTJs has led to the development of next-generation magnetic memory technology with high operating speed, low power consumption and high scalability. In this work, we theoretically investigate the influence of finite size and temperature on the mechanism of magnetization switching in CoFeB–MgO based MTJ to get better understanding of STT-MRAM fundamentals and design. An atomistic model coupled with simultaneous solution of the spin accumulation is employed. The results reveal that the incoherent switching process in MTJ strongly depends on the system size and temperature. At 0 K, the coherent switching mode can only be observed in MTJs with the diameter less than 20 nm. However, at any finite temperature, incoherent magnetization switching is thermally excited. Furthermore, increasing temperature results in decreasing switching time of the magnetization. We conclude that temperature dependent properties and thermally driven reversal are important considerations for the design and development of advanced MRAM systems. [ABSTRACT FROM AUTHOR]