All-spin Logic Devices Research Articles

The effect of output-input isolation on the scaling and energy consumption of all-spin logic devices

All-spin logic (ASL) is a novel approach for digital logic applications wherein spin is used as the state variable instead of charge. One of the challenges in realizing a practical ASL system is the need to ensure non-reciprocity, meaning the information flows from input to output, not vice versa. One approach described previously, is to introduce an asymmetric ground contact, and while this approach was shown to be effective, it remains unclear as to the optimal approach for achieving non-reciprocity in ASL. In this study, we quantitatively analyze techniques to achieve non-reciprocity in ASL devices, and we specifically compare the effect of using asymmetric ground position and dipole-coupled output/input isolation. For this analysis, we simulate the switching dynamics of multiple-stage logic devices with FePt and FePd perpendicular magnetic anisotropy materials using a combination of a matrix-based spin circuit model coupled to the Landau–Lifshitz–Gilbert equation. The dipole field is included in this model and can act as both a desirable means of coupling magnets and a source of noise. The dynamic energy consumption has been calculated for these schemes, as a function of input/output magnet separation, and the results show that using a scheme that electrically isolates logic stages produces superior non-reciprocity, thus allowing both improved scaling and reduced energy consumption.

Spin Transport in Ge Nanowires for Diluted Magnetic Semiconductor-Based Nonvolatile Transpinor

Spintronic devices, in particular spin field-effect transistors (spinFETs), have been studied for decades as a promising candidate to replace Si CMOS devices. Several variants of spinFETs have been proposed to use the spin of electrons as another degree of freedom for information processing. However, all the previous proposals still use charge current, which generates power dissipation. To eliminate the charge current, we propose a novel spin-based transistor (transpinor) based on diluted magnetic semiconductor (DMS) with electric field-controlled paramagnetism-to-ferromagnetism phase transition. One promising prototype is to integrate a MnxGe1-x DMS nanowire, whose ferromagnetism can be effectively controlled by a gate electrode, with Mn5Ge3ferromagnetic contacts for spin injection (Figure 1). Another variant of the ferromagnetic contacts could be epitaxial Fe/MgO tunnel junctions. In this transpinor device, the magnetic moments are transferred from the source to the channel, and then to the drain through spin injection and exchange interaction. As the gate electrode manipulates the ferromagnetism of the DMS channel, it controls the communication between the source and drain.In order to realize the proposed transpinor device, we first studied the spin transport in Ge nanowires and demonstrated the electrical spin injection and detection using both ferromagnetic Mn5Ge3 Schottky contacts and Fe/MgO tunnel junctions (Figure 2). The Mn5Ge3 contacts were fabricated by germanidation through rapid thermal annealing. This convenient approach produced single-crystalline Mn5Ge3 contacts with a controlled channel length and atomically clean interfaces to the Ge nanowire, which helped alleviate Fermi level pinning at the interface. Subsequently, electrical spin injection and detection was successfully demonstrated in the Mn5Ge3/Ge/Mn5Ge3 nanowire transistor. A spin diffusion length of λsf = 480 nm and a spin flip lifetime of τsf = 244 ps are revealed in p-type Ge nanowires at T = 10 K, which were much larger than those reported for bulk p-type Ge. Meanwhile, spin injection into Ge nanowires with epitaxial Fe/MgO tunneling contacts was performed in nonlocal spin valve measurements. The spin diffusion length of λsf = 2.57 μm and the spin lifetime of τsf = 7.2 ns were obtained in n-type Ge nanowires at T = 40 K, which are again much larger than those in bulk n-type Ge. The significant enhancement in the spin lifetime and diffusion length in nanowires suggested that the spin relaxation was suppressed in nanowires compared with bulk materials.Furthermore, we successfully demonstrated the pattern-assisted growth of MnxGe1-x DMS nanowires (Figure 3), which serve as the transpinor channel. The growth was performed using molecular beam epitaxy (MBE) chamber with SiO2 nanotrenches as the mask. The ferromagnetic MnxGe1-xnanowires showed a Curie temperature above 400 K without any precipitate. The ferromagnetism can be further modulated by a gate voltage at 100 K in a metal-oxide-semiconductor capacitor structure, demonstrating electric field control of ferromagnetism. This result, along with the electrical spin injection into Ge nanowires, lays the foundation to build the transpinor device as a possible realization of all-spin logic devices with built-in memory for low-power applications. Acknowledgement: This work was in part supported by Western Institution of Nanoelectronics (WIN) and National Science Foundation through grant ECCS 1308358.

Circuit Simulation of Magnetization Dynamics and Spin Transport

In this paper, compact circuit models for spintronic devices have been developed by manipulating the underlying physical equations. We have simulated, via circuit simulation: 1) the magnetization dynamics governed by the Landau-Lifshitz-Gilbert (LLG) equation and 2) the spin transport physics governed by the spin drift-diffusion equation. The models have been validated using numerical and analytical solutions of the LLG equation and the spin drift-diffusion equations, respectively. Simulations of an all-spin logic device demonstrate the applications of the developed models in device and circuit simulation.

All-Spin Logic Device With Inbuilt Nonreciprocity

The need for low power alternatives to digital electronic circuits has led to increasing interest in logic devices where information is stored in nanomagnets. This includes both nanomagnetic logic (NML) where information is communicated through magnetic fields of nanomagnets and all-spin logic (ASL) where information is communicated through spin currents. A key feature needed for logic implementation is non-reciprocity, whereby the output is switched according to the input but not the other way around, thus providing directed information transfer. The objective of this paper is to draw attention to possible ASL-based schemes that utilize the physics of spin-torque to build in non-reciprocity similar to transistors that could allow logic implementation without the need for special clocking schemes. We use an experimentally benchmarked coupled spin-transport/ magnetization-dynamics model to show that a suitably engineered single ASL unit indeed switches in a non-reciprocal manner. We then present heuristic arguments explaining the origin of this directed information transfer. Finally we present simulations showing that individual ASL devices with inbuilt directionality can be cascaded to construct circuits.

Switching energy-delay of all spin logic devices

A recent proposal called all spin logic (ASL) proposes to store information in nanomagnets that communicate with spin currents in order to construct spin based digital circuits. We present a coupled magnetodynamics/spin-transport model for ASL devices that is based on established physics and is benchmarked against available experimental data. This model is used to show the linear dependence of switching energy and quadratic dependence of energy-delay of ASL devices on the number of Bohr magnetons comprising a nanomagnet. A scaling scheme that could lower the energy-delay of spin-torque switching while maintaining thermal stability is discussed.

Proposal for an all-spin logic device with built-in memory

The possible use of spin rather than charge as a state variable in devices for processing and storing information has been widely discussed, because it could allow low-power operation and might also have applications in quantum computing. However, spin-based experiments and proposals for logic applications typically use spin only as an internal variable, the terminal quantities for each individual logic gate still being charge-based. This requires repeated spin-to-charge conversion, using extra hardware that offsets any possible advantage. Here we propose a spintronic device that uses spin at every stage of its operation. Input and output information are represented by the magnetization of nanomagnets that communicate through spin-coherent channels. Based on simulations with an experimentally benchmarked model, we argue that the device is both feasible and shows the five essential characteristics for logic applications: concatenability, nonlinearity, feedback elimination, gain and a complete set of Boolean operations.