Advancing Hardware Security Using Polymorphic and Stochastic Spin-Hall Effect Devices

Protecting intellectual property (IP) in electronic circuits has become a serious challenge in recent years. Logic locking/encryption and layout camouflaging are two prominent techniques for IP protection. Most existing approaches, however, particularly those focused on CMOS integration, incur excessive design overheads resulting from their need for additional circuit structures or device-level modifications. This work leverages the innate polymorphism of an emerging spin-based device, called the giant spin-Hall effect (GSHE) switch, to simultaneously enable locking and camouflaging within a single instance. Using the GSHE switch, we propose a powerful primitive that enables cloaking all the 16 Boolean functions possible for two inputs. We conduct a comprehensive study using state-of-the-art Boolean satisfiability (SAT) attacks to demonstrate the superior resilience of the proposed primitive in comparison to several others in the literature. While we tailor the primitive for deterministic computation, it can readily support stochastic computation; we argue that stochastic behavior can break most, if not all, existing SAT attacks. Finally, we discuss the resilience of the primitive against various side-channel attacks as well as invasive monitoring at runtime, which are arguably even more concerning threats than SAT attacks.Comment: Published in Proc. Design, Automation and Test in Europe (DATE) 201

Spintronic device modeling and evaluation using modular approach to spintronics

Spintronics technology finds itself in an exciting stage today. Riding on the backs of rapid growth and impressive advances in materials and phenomena, it has started to make headway in the memory industry as solid state magnetic memories (STT-MRAM) and is considered a possible candidate to replace the CMOS when its scaling reaches physical limits. It is necessary to bring all these advances together in a coherent fashion to explore and evaluate the potential of spintronic devices. This work creates a framework for this exploration and evaluation based on Modular Approach to Spintronics, which encapsulate the physics of transport of charge and spin through materials and the phenomenology of magnetic dynamics and interaction in benchmarked elemental modules. These modules can then be combined together to form spin-circuit models of complex spintronic devices and structures which can be simulated using SPICE like circuit simulators. In this work we demonstrate how Modular Approach to Spintronics can be used to build spin-circuit models of functional spintronic devices of all types: memory, logic, and oscillators. We then show how Modular Approach to Spintronics can help identify critical factors behind static and dynamic dissipation in spintronic devices and provide remedies by exploring the use of various alternative materials and phenomena. Lastly, we show the use of Modular Approach to Spintronics in exploring new paradigms of computing enabled by the inherent physics of spintronic devices. We hope that this work will encourage more research and experiments that will establish spintronics as a viable technology for continued advancement of electronics

Towards building a prototype spin-logic device

Since the late 1980s, several key discoveries, such as Giant and Tunneling Magne- toresistance, and advances in magnetic materials have paved the way for exponentially higher bit-densities in magnetic storage. In particular, the discovery of Spin-Transfer Torque (STT) has allowed information to be written to individual magnets using spin-currents. This has replaced the more traditional Oersted-field control used in field-MRAMs and allowed further scaling of magnetic-memories. A less obvious con- sequence of STT is that it has made possible a logic-technology based on magnets controlled by spin-polarized currents. Charge-coupled Spin Logic (CSL) is one such device proposal that couples a giant spin Hall effect(GSHE) write-unit with a Mag- netic Tunnel Junction read-unit. Several theoretical reports have demonstrated that a CSL-style device can function as a fundamental building block for neuromorphic computing by harnessing the intrinsic properties of magnets. This thesis describes the working of a CSL device. Experimental progress towards building the individual components of CSL and also our efforts to integrate these components into a CSL prototype will be presented. In addition to the integration effort, this work also explores spin-injection from a GSHE metal to a nanoscale magnet through an intermediate non-magnetic metal. Our results indicate that with the right choice of intermediate layers, the spin-angular mo- mentum absorbed by the magnet can be increased without engineering the intrinsic spin Hall angle of the GSHE metal. Finally, this work also proposes a Schottky-barrier model to describe the current flow through low-dimensional semiconductors and uses it to extract the band gap of black-phosphorus thin-films in an attempt to characterize novel 2D-materials

Transport Theory for Materials with Spin-Orbit Coupling: Physics to Devices

Materials with spin-orbit coupling (SOC) exhibiting spin-momentum locking (SML) are of great current interest in spintronics because of their ability to efficiently convert charge signals into spin signals and vice versa. This dissertation develops a generalized diffusion equation with four electrochemical potentials starting from the standard Boltzmann transport equation and maps it to a transmission line model. This model applies to diverse materials with SOC including topological insulators, transition metals, narrow bandgap semiconductors, perovskite oxides, etc. and presents a new viewpoint suggesting that materials with low Fermi wave vector lead to larger spin voltages. The model has been used to make a number of predictions some of which have later received experimental confirmation up to room temperature. We also use it to propose new devices for writing and reading information to and from magnets. Specifically, we show using experimentally established phenomena that magnetic state can be read without conventional magnetoresistive devices. We analyze the proposals with SPICE compatible multi-physics framework along with a new model developed in this dissertation for pure spin conduction by magnon diffusion in ferromagnetic insulators