Nickvash Kani

Non-volatile Clocked Spin Wave Interconnect for Beyond-CMOS Nanomagnet Pipelines

The possibility of using spin waves for information transmission and processing has been an area of active research due to the unique ability to manipulate the amplitude and phase of the spin waves for building complex logic circuits with less physical resources and low power consumption. Previous proposals on spin wave logic circuits have suggested the idea of utilizing the magneto-electric effect for spin wave amplification and amplitude- or phase-dependent switching of magneto-electric cells. Here, we propose a comprehensive scheme for building a clocked non-volatile spin wave device by introducing a charge-to-spin converter that translates information from electrical domain to spin domain, magneto-electric spin wave repeaters that operate in three different regimes - spin wave transmitter, non-volatile memory and spin wave detector, and a novel clocking scheme that ensures sequential transmission of information and non-reciprocity. The proposed device satisfies the five essential requirements for logic application: nonlinearity, amplification, concatenability, feedback prevention, and complete set of Boolean operations.

A Model Study of an Error-Free Magnetization Reversal Through Dipolar Coupling in a Two-Magnet System

A systematic study on the feasibility of an error-free switching of two nanomagnets coupled via their dipolar fields by applying a spin current to one of them is presented. It is demonstrated that the dynamic nature of dipolar interaction between the nanomagnets may cause the magnetization of the second magnet to precess back to its original state despite having crossed the free-axis equatorial plane during the transient phase. This non-reversal is very different than the purely successful or unsuccessful switchings noted in other reversal analyses presented in the literature and, in this paper, is referred to as a dipolar switching glitch. The dynamic dipolar coupling between nanomagnets creates temporary titled precessional trajectories pushing the magnetization of the nanomagnet into high-energy states. Relaxation from these high-energy positions leads to fast, but quasi-random switching behavior. The maximum separation between the magnets for perfect coupling has been quantified as a function of the magnet planar dimensions. It is also shown that the simpler models that do not consider the mutual coupling between the two magnets underestimate the maximum allowed separation between the two magnets and, as such, are too conservative.

Impact of dimensional scaling and size effects on beyond CMOS All-Spin Logic interconnects

The energy-per-bit and delay of All-Spin Logic (ASL) interconnects have been modeled. Both Al and Cu interconnect channels have been considered and the impact of size effects and dimensional scaling on their potential performance has been quantified. It is predicted that size effects will affect ASL interconnects more severely than electrical interconnects.

Strain-Mediated Magnetization Reversal Through Spin-Transfer Torque

Recent experiments have shown the ability to introduce an anisotropy energy to the energy landscape of a thin-film nanomagnet through the use of mechanical strain. Assuming this strain-induced anisotropy is large enough, the low-energy state of the nanomagnet is altered and can be used to initialize the magnetization along a given axis. Utilizing this effect, we propose a more energy efficient method of nanomagnet reversal through spin-transfer torque (STT). This is accomplished by first initializing the magnetization to a high-energy state and then applying a short current pulse to nudge the magnetization in the appropriate energy basin. Using extensive numerical simulations, we qualitatively analyze this type of reversal and find the optimal parameters for reliable functionality while in the presence of thermal noise. We demonstrate that despite negating the initial portion of nominal STT reversal, where the STT must fight against the damping torque of the initial energy-basin, the magnitude of spin current required for our proposed strain-mediated reversal is equivalent to the nominal case. However, the strain-meditated reversal is beneficial by minimizing the spin-current pulsewidth necessary for reliable operation allowing for large energy savings. Assuming the strain-anisotropy is significantly larger than the nanomagnet’s internal free-axis anisotropy, strain-mediated reversals offer a

Scaling Limits on All-Spin Logic

{10 \times }

Analysis of coupling strength in multi-domain magneto-systems

energy reduction over nominal STT reversals.

A Probability-Density Function Approach to Capture the Stochastic Dynamics of the Nanomagnet and Impact on Circuit Performance

Scaling limits on all-spin logic (ASL) are theoretically studied using the spin circuit theory and the stochastic Landau-Lifshitz-Gilbert equation under the macrospin approximation. It is found that as ASL circuits are scaled, the device delay significantly increases due to a stronger dipole coupling between the input and the output magnets. The effect of the dipole interaction can be mitigated by increasing the input current and by using smaller magnets with stronger material anisotropy and weaker saturation magnetization. Furthermore, the presence of the leakage current modifies the device delay. Finally, both delay and energy of ASL dramatically increase as the shunt path is shortened. Possible solutions to eliminate the leakage current and the shunt path are discussed.

Analytical models for coupling reliability in identical two-magnet systems during slow reversals

Summary form only given. In recent years, novel devices which use nanomagnets to store and transmit information have been a particular topic of interest. Many of these devices utilize dipolar coupling between nanomagnets to process or communicate information and the most important factor when designing these systems is knowing if two nanomagnets are perfectly coupled [1]. However, when modeling these devices, these nanomagnets are typically modeled as single-domain objects even though in many cases, the magnet dimensions far exceed the exchange length [2]. While it is expected that very small magnets can be treated as single-domain objects, it is not quite clear how small is small enough. Second, it is not clear whether using a single-domain analysis underestimates or overestimates the coupling. In this work, we quantify the coupling between nanomagnets while considering their multi-domain behavior. Our simulations conclusively prove that ignoring multi-domain effects beyond a certain size yields inaccurate results. While the single-domain assumption predicts similar coupling strength to the multi-domain models for thin-film magnets with areas smaller than 1 μm2, the two models disagree about the coupling strength between larger magnets.

Non-monotonic probability of thermal reversal in thin-film biaxial nanomagnets with small energy barriers

In this paper, we systematically evaluate the variation in the reversal delay of a nanomagnet driven by a longitudinal spin current under the influence of thermal noise. We then use the results to evaluate the performance of an all-spin-logic (ASL) circuit. First, we review and expand on the physics of previously published analytical models on stochastic nanomagnet switching. The limits of previously established models are defined, and it is shown that these models are valid for nanomagnet reversal times <;200 ps. Second, the insight obtained from previous models allows us to represent the probability density function (pdf) of the nanomagnet switching delay using the double exponential function of the Fréchet distribution. The pdf of a single nanomagnet is extended to more complex nanomagnet circuit configurations. It is shown that the delay-variation penalty incurred by nanomagnets arranged in parallel configuration is dwarfed by the average delay increase for nanomagnets arranged in a series configuration. Finally, we demonstrate the impact of device-level performance variation on the circuit behavior using ASL logic gates. While the analysis presented in this paper uses an ASL-AND gate as the prototype switching circuit in the spin domain, the physical concepts are generic and can be extended to any complex spin-based circuit.

Pipeline design in spintronic circuits

This paper follows previous works which investigated the strength of dipolar coupling in two-magnet systems. While those works focused on qualitative analyses, this manuscript elucidates reversal through dipolar coupling culminating in analytical expressions for reversal reliability in identical two-magnet systems. The dipolar field generated by a mono-domain magnetic body can be represented by a tensor containing both longitudinal and perpendicular field components; this field changes orientation and magnitude based on the magnetization of neighboring nanomagnets. While the dipolar field does reduce to its longitudinal component at short time-scales, for slow magnetization reversals, the simple longitudinal field representation greatly underestimates the scope of parameters that ensure reliable coupling. For the first time, analytical models that map the geometric and material parameters required for reliable coupling in two-magnet systems are developed. It is shown that in biaxial nanomagnets, the x an...