The rapid development of information technology has led to urgent requirements for high efficiency and ultralow power consumption. In the past few decades, neuromorphic computing has drawn extensive attention due to its promising capability in processing massive data with extremely low power consumption. Here, we offer a comprehensive review on emerging artificial neuromorphic devices and their applications. In light of the inner physical processes, we classify the devices into nine major categories and discuss their respective strengths and weaknesses. We will show that anion/cation migration-based memristive devices, phase change, and spintronic synapses have been quite mature and possess excellent stability as a memory device, yet they still suffer from challenges in weight updating linearity and symmetry. Meanwhile, the recently developed electrolyte-gated synaptic transistors have demonstrated outstanding energy efficiency, linearity, and symmetry, but their stability and scalability still need to be optimized. Other emerging synaptic structures, such as ferroelectric, metal–insulator transition based, photonic, and purely electronic devices also have limitations in some aspects, therefore leading to the need for further developing high-performance synaptic devices. Additional efforts are also demanded to enhance the functionality of artificial neurons while maintaining a relatively low cost in area and power, and it will be of significance to explore the intrinsic neuronal stochasticity in computing and optimize their driving capability, etc. Finally, by looking into the correlations between the operation mechanisms, material systems, device structures, and performance, we provide clues to future material selections, device designs, and integrations for artificial synapses and neurons.

A comprehensive review on emerging artificial neuromorphic devices

A review of recent works on inclusions

Conventional MOS integrated circuits and systems suffer serve power and scalability challenges as technology nodes scale into ultra-deep-micron technology nodes (e.g., below 40nm). Both static and dynamic power dissipations are increasing, caused mainly by the intrinsic leakage currents and large data traffic. Alternative approaches beyond charge-only-based electronics, and in particular, spin-based devices, show promising potential to overcome these issues by adding the spin freedom of electrons to electronic circuits. Spintronics provides data non-volatility, fast data access, and low-power operation, and has now become a hot topic in both academia and industry for achieving ultra-low-power circuits and systems. The ITRS report on emerging research devices identified the magnetic tunnel junction (MTJ) nanopillar (one of the Spintronics nanodevices) as one of the most promising technologies to be part of future micro-electronic circuits. In this review we will give an overview of the status and prospects of spin-based devices and circuits that are currently under intense investigation and development across the world, and address particularly their merits and challenges for practical applications. We will also show that, with a rapid development of Spintronics, some novel computing architectures and paradigms beyond classic Von-Neumann architecture have recently been emerging for next-generation ultra-low-power circuits and systems.

Spintronics: Emerging Ultra-Low-Power Circuits and Systems beyond MOS Technology

In this paper, the minimum mean-square error (MMSE) technique has been used to equalize the recording channel in order to facilitate the application of the Viterbi detector. The resulting performance has been compared with that of the optimal equalization system which yields the minimum probability of error at the output of the Viterbi detector. The results indicate that depending on the constraint used in the MMSE design, the amount of noise correlation varies significantly at the equalizer output, which in turn makes a large difference in the performance of the Viterbi detector. In particular, in the jitter-dominant channel where unconditioned channel noise samples are highly correlated, the monic constraint on the equalizer target response tends to whiten the noise samples at the equalizer output. This results in a significant performance improvement of the monic constraint upon the fixed-energy constraint as well as the popular partial response targets of the form (1-D)(1+D)/sup n/. >

Equalization For Maximum Likelihood Detectors

Spin transfer torque magnetic random access memory (STT-MRAM) has been widely regarded as a potential nonvolatile memory candidate in the next-generation computer architectures. Nevertheless, the write energy consumption and delay are two significant concerns for STT-MRAM, blocking its applications for working memories. Recently, magnetic tunnel junction (MTJ) based on voltage-controlled magnetic anisotropy (VCMA) effect shows tremendous superiority in terms of dynamic write energy and delay over the STT-based one, attracting much attention for advanced low-power and high-speed MRAM designs. In this paper, we evaluate the prospects and challenges of the VCMA-MTJ devices for advanced MRAM applications. First, the magnetization dynamics of the free layer of VCMA-MTJ devices are studied by solving a modified Landau–Lifshitz–Gilbert equation. Afterward, a VCMA-MTJ electrical model is built by integrating the VCMA effect, Slonczewski STT model, Brinkman resistance model, and tunnel magnetoresistance model. Finally, three MTJ switching strategies, including precessional VCMA, STT-assisted precessional VCMA and STT-assisted thermally-activated VCMA, are studied for MRAM applications. Our results show that the STT-assisted precessional VCMA strategy is the most potential one for high-speed and low-power VCMA-MRAM design. This paper provides models, strategies, and guidelines for VCMA-MRAM design and application.

Modeling and Exploration of the Voltage-Controlled Magnetic Anisotropy Effect for the Next-Generation Low-Power and High-Speed MRAM Applications

A screw dislocation interacting with a coated fiber

On the interaction between an edge dislocation and a coated inclusion

Stress investigation for the interaction problem between a coated circular inclusion and a near-by line crack has been carried out. The crack and the coated inclusion (a coated fiber) are embedded in an infinitely extended isotropic matrix, with the crack being along the radial direction of the inclusion. Two loading conditions, namely, the tensile and shear loading ones are considered. During the solution procedure, the crack is treated as a continuous distribution of edge dislocations. By using the solution of an edge dislocation near a coated fiber as the Green's function, the problem is formulated into a set of singular integral equations which are solved by Erdogan and Gupta (1972) method. The expressions for the stress intensity factors of the crack are then obtained in terms of the asymptotic values of the dislocation density functions evaluated from the integral equations. Several numerical examples are given for various material and geometric parameters. The solutions obtained from the integral equations have been checked and confirmed by the finite element analysis results.

Stress intensity factor for a Griffith crack interacting with a coated inclusion

Stress analysis for a Zener–Stroh crack interacting with a coated inclusion

Leveraging on interfacial Dzyaloshinskii-Moriya interaction (DMI) induced intrinsic magnetization tilting in nanostructures, a parametric window enabling field-free spin-orbit torque (SOT) magnetization switching in a perpendicular ferromagnet is established. The critical current density (Jc) bounds for SOT switching are highly dependent on the DMI, producing a distorted diamond-shaped region bounded by the Jc-DMI curves. The widest Jc interval is found for DMI values between 0.5 mJ/m2 and 0.8 mJ/m2. Geometrical modulation, of the ferromagnetic layer, reveals that the circular structure is optimum for minimizing the switching energy while maximizing the parametric window. For all the structures investigated, the SOT induced reversal process is via domain wall nucleation and propagation, and the switching is practical at room temperature.

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Bingjin Chen

Papers

A screw dislocation interacting with a coated fiber

On the interaction between an edge dislocation and a coated inclusion

Stress intensity factor for a Griffith crack interacting with a coated inclusion

Stress analysis for a Zener–Stroh crack interacting with a coated inclusion

Field-free spin-orbit torque switching of a perpendicular ferromagnet with Dzyaloshinskii-Moriya interaction