https://iopscience.iop.org/article/10.3367/UFNe.2018.01.038279/pdf

Straintronics: a new trend in micro- and nanoelectronics and materials science

The field of spintronics has attracted tremendous attention recently owing to its ability to offer a solution for the present-day problem of increased power dissipation in electronic circuits while scaling down the technology. Spintronic-based structures utilize electron’s spin degree of freedom, which makes it unique with zero standby leakage, low power consumption, infinite endurance, a good read and write performance, nonvolatile nature, and easy 3D integration capability with the present-day electronic circuits based on CMOS technology. All these advantages have catapulted the aggressive research activities to employ spintronic devices in memory units and also revamped the concept of processing-in-memory architecture for the future. This review article explores the essential milestones in the evolutionary field of spintronics. It includes various physical phenomena such as the giant magnetoresistance effect, tunnel magnetoresistance effect, spin-transfer torque, spin Hall effect, voltage-controlled magnetic anisotropy effect, and current-induced domain wall/skyrmions motion. Further, various spintronic devices such as spin valves, magnetic tunnel junctions, domain wall-based race track memory, all spin logic devices, and recently buzzing skyrmions and hybrid magnetic/silicon-based devices are discussed. A detailed description of various switching mechanisms to write the information in these spintronic devices is also reviewed. An overview of hybrid magnetic /silicon-based devices that have the capability to be used for processing-in-memory (logic-in-memory) architecture in the immediate future is described in the end. In this article, we have attempted to introduce a brief history, current status, and future prospectus of the spintronics field for a novice.

/pdf/spintronic-devices-a-promising-alternative-to-cmos-devices-x2ekn7iv7a.pdf

Spintronic devices: a promising alternative to CMOS devices

A spin-torque oscillator with a fixed layer magnetization tilted out of the film plane is capable of strong microwave signal generation in zero magnetic field. Through numerical simulations, we study the microwave signal generation as a function of drive current for two realistic tilt angles. The tilted magnetization of the fixed layer can be achieved by using a material with high out-of-plane magnetocrystalline anisotropy, such as L10 FePt.

http://absimage.aps.org/image/MAR09/MWS_MAR09-2008-006300.pdf

Spin-torque oscillators with tilted fixed layer magnetization

The recent discovery that a spin-polarized electrical current can apply a large torque to a ferromagnet, through direct transfer of spin angular momentum, offers the possibility of manipulating magnetic-device elements without applying cumbersome magnetic fields. However, a central question remains unresolved: what type of magnetic motions can be generated by this torque? Theory predicts that spin transfer may be able to drive a nanomagnet into types of oscillatory magnetic modes not attainable with magnetic fields alone, but existing measurement techniques have provided only indirect evidence for dynamical states. The nature of the possible motions has not been determined. Here we demonstrate a technique that allows direct electrical measurements of microwave-frequency dynamics in individual nanomagnets, propelled by a d.c. spin-polarized current. We show that spin transfer can produce several different types of magnetic excitation. Although there is no mechanical motion, a simple magnetic-multilayer structure acts like a nanoscale motor; it converts energy from a d.c. electrical current into high-frequency magnetic rotations that might be applied in new devices including microwave sources and resonators.

Microwave Oscillations of a Nanomagnet Driven by a DC Spin-Polarized Current

We investigate the dynamics of a transverse domain wall (DW) in a bent nanostripe under an external field and spin-polarized current. Besides the standard Walker breakdown phenomenon, we show the emergence of a second Walker-like critical field, which depends on both the curvature of the nanostripe and its cross section geometry. At this field, DW can change its phase, i.e., can be re-oriented along another direction with respect to the nanostripe face. Additionally, we show that the amplitude and frequency of the DW oscillations above the Walker breakdown field also depend on the nanostripe geometry and can be controlled by external stimuli. Our results evidence that the inclusion of local curvatures in nanostripes is an important component for applications that demand an adequate control of the DW phase by the proper choice of external stimuli.

Curvature-induced emergence of a second critical field for domain wall dynamics in bent nanostripes

Realization of complete magnetization reversal is a major challenge for electrical control of magnetism. This paper focuses on voltage pulse induced magnetization reversal which is mediated by localized strain generated in a two-phase magnetostrictive/piezoelectric multiferroic heterostructure. This paradigm takes advantage of the damping and precessional property of magnetization dynamics, of which the magnetization reversal needs precise control of the voltage pulse. We have performed a micromagnetic study of varied-size elliptical single-domain Ni and Terfenol-D nanomagnets at T = 300 K. By using the actual ground states of the nanomagnets at T = 300 K as the initial states, we obtained the key parameters of the voltage pulse which can induce repeated magnetization reversal. Then we uncovered the mechanism of the size-dependent incoherent magnetization switching of Terfenol-D nanomagnets by a quantitative analysis of the interaction between the exchange energy, demagnetization energy and stress anisotropy energy. These results lay the foundation for the application of strain-mediated magnetization switching technology, and also provide a set of guidelines for signal design and the choice of nanomagnets with proper material and size in the design of specific straintronic circuits.

https://iopscience.iop.org/article/10.1088/1361-6463/aa7542/pdf

Voltage pulse induced repeated magnetization reversal in strain-mediated multiferroic nanomagnets: a size- and material-dependent micromagnetic study

Using micromagnetic simulations, we investigate the voltage control of magnetic vortex polarity based on a designed multiferroic heterostructure that contains two separate piezoelectric films beneath a magnetostrictive nanodisk. The results show that controllable switching of vortex polarity can be achieved by proper modulation of the phase difference between two sinusoidal voltage pulses V1 and V2, which are applied to the two separate piezoelectric films, respectively. The frequencies of V1 and V2 are set at the gyrotropic eigenfrequency fG of the nanodisk, and the vortex polarity switching is completed via the nucleation-annihilation process of the vortex-antivortex pair. Our findings provide an additional effective means for ultralow power switching of the magnetic vortex, which lays the foundation for voltage-controlled vortex random access memory.

Control of magnetic vortex polarity by the phase difference between voltage signals

The dynamic properties of radial vortex core (RVC) polarity reversal in PtCo20Fe60B 20Ti multilayers induced by a linearly attenuated perpendicular magnetic field are investigated. Micromagnetic simulation indicates that the polarity of the RVC can be quickly reversed through a continuous distortion of circular chirality around the core. The reversal time can be reduced to below one nanosecond when the magnitudes of the magnetic field and the attenuation time are selected properly. A quantitative analysis of the core exchange-energy density, Zeeman energy density, and magnetostatic energy density explains the RVC polarity reversal. At the moment of RVC polarity switching, the exchange energy density is sharply reduced, accompanied by the emission of strong spin waves. This scheme provides a new method to achieve ultra-fast RVC polarity reversal, which lays a foundation for radial vortex-based magnetoresistive memory devices.

Subnanosecond Radial Vortex Core Polarity Reversal Using Linearly Attenuated Perpendicular Magnetic Field

Spin-torque nano-oscillators based on radial vortex in the presence of interface Dzyaloshinskii-Moriya interaction

Under different nanomagnets’ size, switching behaviour of all spin logic (ASL) devices constructed with Co and permalloy (Py) nanomagnets are studied by using the coupled spin-transport/magneto-dynamics model. The results indicate that ASL devices’ switching delay and energy dissipation can be reduced by decreasing the thickness of nanomagnets. The switching delay and energy dissipation of PyASL are lower than those of CoASL in a smaller thickness of nanomagnet, but they increase much faster than those of CoASL when the nanomagnets (FM) thickness increases. With the dimensional scaling of nanomagnets, the ASL devices’ switching delay and energy dissipation decrease rapidly and the influence of thermal noise become weak. Moreover, under the same nanomagnet volume, ASL devices’ switching delay, energy dissipation, and energy barrier can be reduced by decreasing aspect ratio. These findings can provide guidelines for optimising the ASL devices’ materials and size.

https://ietresearch.onlinelibrary.wiley.com/doi/pdf/10.1049/mnl.2016.0163

Impact of nanomagnets size on switching behaviour of all spin logic devices

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