ULTRA–LOW POWER STRAINTRONIC NANOMAGNETIC COMPUTING WITH SAW WAVES: AN EXPERIMENTAL STUDY OF SAW INDUCED MAGNETIZATION SWITCHING AND PROPERTIES OF MAGNETIC NANOSTRUCTURES

A recent International Technology Roadmap for Semiconductors (ITRS) report (2.0, 2015 edition) has shown that Moore’s law is unlikely to hold beyond 2028. There is a need for alternate devices to replace CMOS based devices, if further miniaturization and high energy efficiency is desired. The goal of this dissertation is to experimentally demonstrate the feasibility of nanomagnetic memory and logic devices that can be clocked with acoustic waves in an extremely energy efficient manner. While clocking nanomagnetic logic by stressing the magnetostrictive layer of a multiferroic logic element with with an electric field applied across the piezoelectric layer is known to be an extremely energy-efficient clocking scheme, stressing every nanomagnet separately requires individual contacts to each one of them that would necessitate cumbersome lithography. On the other hand, if all nanomagnets are stressed simultaneously with a global voltage, it will eliminate the need for individual contacts, but such a global clock makes the architecture non-pipelined (the next input bit cannot be written till the previous bit has completely propagated through the chain) and therefore, unacceptably slow and error prone. Use of global acoustic wave, that has in-built granularity, would offer the best of both worlds. As the crest and the trough propagate in space with a velocity, nanomagnets that find themselves at a crest are stressed in tension while those in the trough are compressed. All other magnets are relaxed (no stress). Thus, all magnets are not stressed simultaneously but are clocked in a sequentially manner, even though the clocking agent is global. Finally, the acoustic wave energy is distributed over billions of nanomagnets it clocks, which results in an extremely small energy cost per bit per nanomagnet. In summary, acoustic clocking of nanomagnets can lead to extremely energy efficient nanomagnetic computing devices while also eliminating the need for complex lithography. The dissertation work focuses on the following two topics: Acoustic Waves, generated by IDTs fabricated on a piezoelectric lithium niobate substrate, can be utilized to manipulate the magnetization states in elliptical Co nanomagnets. The magnetization switches from its initial single-domain state to a vortex state after SAW stress cycles propagate through the nanomagnets. The vortex states are stable and the magnetization remains in this state until it is ‘reset’ by an external magnetic field. 2. Acoustic Waves can also be utilized to induce 1800 magnetization switching in dipole coupled elliptical Co nanomagnets. The magnetization switches from its initial single-domain ‘up’ state to a single-domain ‘down’ state after SAW tensile/compressive stress cycles propagate through the nanomagnets. The switched state is stable and non-volatile. These results show the effective implementation of a Boolean NOT gate. Ultimately, the advantage of this technology is that it could also perform higher order information processing (not discussed here) while consuming extremely low power. Finally, while we have demonstrated acoustically clocked nanomagnetic memory and logic schemes with Co nanomagnets, materials with higher magnetostriction (such as FeGa) may ultimately improve the switching reliability of such devices. With this in mind we prepared and studied FeGa films using a ferromagnetic resonance (FMR) technique to extract properties of importance to magnetization dynamics in such materials that could have higher magneto elastic coupling than either Co or Ni

Electric field control of fixed magnetic Skyrmions for energy efficient nanomagnetic memory

To meet the ever-growing demand of faster and smaller computers, increasing number of transistors are needed in the same chip area. Unfortunately, Silicon based transistors have almost reached their miniaturization limits mainly due to excessive heat generation. Nanomagnetic devices are one of the most promising alternatives of CMOS. In nanomagnetic devices, electron spin, instead of charge, is the information carrier. Hence, these devices are non-volatile: information can be stored in these devices without needing any external power which could enable computing architectures beyond traditional von-Neumann computing. Additionally, these devices are also expected to be more energy efficient than CMOS devices as their operation does not involve translation of charge across the device. However, the energy dissipated in the clocking circuitry negates this perceived advantage and in practice CMOS devices still consume three orders of magnitudes less energy. Therefore, researchers have been looking for nanomagnetic devices that could be energy efficient in addition to being non-volatile which has led to the exploration of several switching strategies. Among those, electric field induced switching has proved to be a promising route towards scalable ultra-low power computing devices. Particularly Voltage Control of Magnetic Anisotropy (VCMA) based switching dissipates ~1 fJ energy. However, incoherence due to thermal noise and material inhomogeneity renders this switching error-prone. This dissertation is devoted towards studying VCMA induced switching of a spin spiral magnetic state, magnetic skyrmions, which can potentially alleviate this issue. Magnetic skyrmions has recently emerged as a viable candidate to be used in room temperature nanomagnetic devices. Most of the studies propose to utilize skyrmion motion in a magnetic track to implement memory devices. However, Magnetic Tunnel Junction (MTJ) devices based on skyrmions that are fixed in space might be advantageous in terms of footprint. To establish a new computing paradigm based on electrical manipulation of magnetization of fixed magnetic skyrmions we have studied: i) Purely VCMA induced reversal of magnetic skyrmions using extensive micromagnetic simulations. This shows sequential increase and decrease of Perpendicular Magnetic Anisotropy (PMA) can result into toggling between skyrmionic and ferromagnetic states. We also demonstrate VCMA assisted Spin Transfer Torque (STT) induced reversal of magnetic skyrmions. ii) Complete reversal of ferromagnets mediated by intermediated skyrmion state using rigorous micromagnetic simulation. We show that the switching can be robust by limiting the “phase space” of the magnetization dynamics through a controlled skyrmion state. Thus, the switching error can be lowered compared to conventional VCMA switching. iii) Finally, we perform preliminary experiments on VCMA induced manipulation of skyrmions. We demonstrate that skyrmions can be annihilated when Perpendicular Magnetic Anisotropy of the system is increased by applying a negative voltage pulse and can be recreated by decreasing PMA by applying a positive voltage pulse. The experimental observations are corroborated using micromagnetic simulation. Future research should focus on demonstrating reversal of skyrmions experimentally in MTJ like devices and study the downscaling of the proposed device. These can enable realization of energy efficient and robust nanomagnetic memory devices based on voltage control switching of fixed magnetic skyrmions as wells as other neuromorphic computing devices

Cellular Automata

Modelling and simulation are disciplines of major importance for science and engineering. There is no science without models, and simulation has nowadays become a very useful tool, sometimes unavoidable, for development of both science and engineering. The main attractive feature of cellular automata is that, in spite of their conceptual simplicity which allows an easiness of implementation for computer simulation, as a detailed and complete mathematical analysis in principle, they are able to exhibit a wide variety of amazingly complex behaviour. This feature of cellular automata has attracted the researchers' attention from a wide variety of divergent fields of the exact disciplines of science and engineering, but also of the social sciences, and sometimes beyond. The collective complex behaviour of numerous systems, which emerge from the interaction of a multitude of simple individuals, is being conveniently modelled and simulated with cellular automata for very different purposes. In this book, a number of innovative applications of cellular automata models in the fields of Quantum Computing, Materials Science, Cryptography and Coding, and Robotics and Image Processing are presented

STRAINTRONIC NANOMAGNETIC DEVICES FOR NON-BOOLEAN COMPUTING

Nanomagnetic devices have been projected as an alternative to transistor-based switching devices due to their non-volatility and potentially superior energy-efficiency. The energy efficiency is enhanced by the use of straintronics which involves the application of a voltage to a piezoelectric layer to generate a strain which is ultimately transferred to an elastically coupled magnetostrictive nanomaget, causing magnetization rotation. The low energy dissipation and non-volatility characteristics make straintronic nanomagnets very attractive for both Boolean and non-Boolean computing applications. There was relatively little research on straintronic switching in devices built with real nanomagnets that invariably have defects and imperfections, or their adaptation to non-Boolean computing, both of which have been studied in this work. Detailed studies of the effects of nanomagnet material fabrication defects and surface roughness variation (found in real nanomagnets) on the switching process and ultimately device performance of those switches have been performed theoretically. The results of these studies place the viability of straintronics logic (Boolean) and/or memory in question. With a view to analog computing and signal processing, analog spin wave based device operation has been evaluated in the presence of defects and it was found that defects impact their performance, which can be a major concern for the spin wave based device community. Additionally, the design challenge for low barrier nanomagnet which is the building block of binary stochastic neurons based probabilistic computing device in case of real nanomagnets has also been investigated. This study also cast some doubt on the efficacy of probabilistic computing devices. Fortunately, there are some non-Boolean applications based on the collective action of array of nanomagnets which are very forgiving of material defects. One example is image processing using dipole coupled nanomagnets which is studied here and it showed promising result for noise correction and edge enhancement of corrupted pixels in an image. Moreover, a single magneto tunnel junction based microwave oscillator was proposed for the first time and theoretical simulations showed that it is capable of better performance compared to traditional microwave oscillators. The experimental part of this work dealt with spin wave modes excited by surface acoustic waves, studied with time resolved magneto optic Kerr effect (TR-MOKE). New hybrid spin wave modes were observed for the first time. An experiment was carried out to emulate simulated annealing in a system of dipole coupled magnetostrictive nanomagnets where strain served as the simulated annealing agent. This was a promising outcome and it is the first demonstration of the hardware variant of simulated annealing of a many body system based on magnetostrictive nanomagnets. Finally, a giant spin Hall effect actuated surface acoustic wave antenna was demonstrated experimentally. This is the first observation of photon to phonon conversion using spin-orbit torque and although the observed conversion efficiency was poor (1%), it opened the pathway for a new acoustic radiator. These studies complement past work done in the area of straintronics