Monolithic and Hybrid Spintronics

Abstract “Spintronics” is the science and technology of using the spin degree of freedom of a charge carrier, either singly or in conjunction with the charge, to store, process and/or communicate information. Both, classical and quantum mechanical information can be encoded by spin. In this chapter, “hybrid” and “monolithic” spintronics are discussed; the distinction between the two is that, in the former case, spin is not used directly to store or process information; rather, that role is still delegated to charge. But spin influences how charge stores or manipulates information. The role of spin is secondary and may, at best, augment the role of charge. Examples of hybrid spintronic devices are spin field effect transistors (SPINFETs) and spin bipolar junction transistors (SBJTs). It is also noted that hybrid devices are not likely to yield major advances in device engineering in the short term. On the other hand, monolithic spintronics, which deals with devices in which spin single‐handedly stores and processes information, is much better poised to make major advances. An example of this is “single spin logic” (SSL), where a single electron acts as a binary element (two‐state switch), its two orthogonal spin polarizations being used to encode binary bits 0 and 1. Switching between bits is accomplished simply by flipping the spin of the electron, without physically moving charges in space and causing a current flow. In this way energy is saved, leading to a much‐reduced power dissipation. The switching time is also not limited by the speed of charge carriers as there is no charge motion. The maximum energy dissipated in SSL gate operation is the Landauer–Shannon limit of kT ln( p ) per operation (where 1/ p is the error probability). The chapter concludes with a brief discussion of spin‐based quantum computing, where the spin of an electron in a quantum dot is used to implement a qubit. Of note, this last section is necessarily brief and will most likely be outdated by the time this book is printed, as rapid strides are being made in that area.