COHERENT/INCOHERENT MAGNETIZATION DYNAMICS OF NANOMAGNETIC DEVICES FOR ULTRA-LOW ENERGY COMPUTING

Nanomagnetic computing devices are inherently nonvolatile and show unique transfer characteristics while their switching energy requirements are on par, if not better than state of the art CMOS based devices. These characteristics make them very attractive for both Boolean and non-Boolean computing applications. Among different strategies employed to switch nanomagnetic computing devices e.g. magnetic field, spin transfer torque, spin orbit torque etc., strain induced switching has been shown to be among the most energy efficient. Strain switched nanomagnetic devices are also amenable for non-Boolean computing applications. Such strain mediated magnetization switching, termed here as “Straintronics”, is implemented by switching the magnetization of the magnetic layer of a magnetostrictive-piezoelectric nanoscale heterostructure by applying an electric field in the underlying piezoelectric layer. The modes of “straintronic” switching: coherent vs. incoherent switching of spins can affect device performance such as speed, energy dissipation and switching error in such devices. There was relatively little research performed on understanding the switching mechanism (coherent vs. incoherent) in xiv straintronic devices and their adaptation for non-Boolean computing, both of which have been studied in this thesis. Detailed studies of the effects of nanomagnet geometry and size on the coherence of the switching process and ultimately device performance of such strain switched nanomagnetic devices have been performed. These studies also contributed in optimizing designs for low energy, low dynamic error operation of straintronic logic devices and identified avenues for further research. A Novel non-Boolean “straintronic” computing device (Ternary Content Addressable Memory, abbreviated as TCAM) has been proposed and evaluated through numerical simulations. This device showed significant improvement over existing CMOS device based TCAM implementation in terms of scaling, energy-delay product, operational simplicity etc. The experimental part of this thesis answered a very fundamental question in strain induced magnetization rotation. Specifically, this experiment studied the variation in magnetization orientation for strain induced magnetization rotation along the thickness of a magnetostrictive thin film using polarized neutron reflectometry and demonstrated non-uniform magnetization rotation along the thickness of the sample. Additional experimental work was performed to lay the groundwork for ultra-low voltage straintronic switching demonstration. Preliminary sample fabrication and characterization that can potentially lead to low voltage (~10-100 mV) operation and local clocking of such devices has been performed

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