chapter  4
The Exploitation of the Spin-Transfer Torque Effect for CMOS Compatible Beyond Von Neumann Computing
WithThomas Windbacher, Alexander Makarov, Siegfried Selberherr, Hiwa Mahmoudi, B. Gunnar Malm, Mattias Ekström, Mikael Östling
Pages 64

94The exponential growth in (affordable) computational power over the last decades was only sustainable due to continuous successful scaling of CMOS devices. The shrinking of the CMOS transistors allowed not only an increase in the speed and performance of circuits, but also ensured that the costs per transistor dropped for every technology generation. However, with each technology generation, new and ever harder to resolve obstacles appeared. Currently, out of the multitude of potential showstoppers in charge-based CMOS technology, the dissipated power and the energy associated with the transport of information are major concerns. The fast evolving field of spintronics offers a potential remedy for these problems by introducing “More than Moore” devices. The quest for the future universal memory candidate not only led to spin-based magnetoresistive random-access memory (MRAM), but also culminated in the first off-the-shelf MRAM products. Nevertheless, the core of the MRAM, the magnetic tunnel junction (MTJ), is not limited to memory applications. It can also be exploited for building logic-in-memory circuits with nonvolatile storage elements, as well as very compact on-chip oscillators with low power consumption. In general, the advent of nonvolatile elements, and especially spintronics in circuits, gives the unique opportunity to rethink how information is processed and moved. The concept of continuous information exchange between physically separated memory and processing units—also known as the Von Neumann architecture—has become a performance limiting bottleneck. The transition towards beyond Von Neumann architectures obviously also requires a redesign of all basic computational building blocks. In the this chapter, we will give an overview about the ideas and concepts for such beyond Von Neumann systems. First, we will present a short introduction into the physics necessary to understand the spintronic effects, like the magnetoresistance effect, spin-transfer torque (STT), spin Hall effect, and the magnetoelectric effect. Then we will move towards spintronic devices and circuits and their different concepts and architecture levels, where they introduce nonvolatility, such as thermally-assisted (TA)-MRAM, STT-MRAM, domain wall (DW)-MRAM, spin-orbit torque (SOT)-MRAM, spin-transfer torque and spin Hall oscillators, logic-in-memory, all-spin logic, buffered magnetic logic gate grid, ternary content 95addressable memory (TCAM), and random number generators. From our point of view, there will be no disruptive transition from pure CMOS to pure spintronic circuits. Instead, there will be a gradual introduction and substitution of existing CMOS devices by spintronic devices, where they outperform CMOS devices in one or more aspects. Therefore, we will concentrate on and emphasize concepts and devices that are CMOS compatible and present possibilities for different levels of integration into CMOS technology.