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Introduction
Semiconductor spintronics
Conventional electronics rely upon and utilize the flow of an electron’s charge. The idea of Spintronics involves utilizing an electron’s spin as well. In addition to an electron’s orbital angular momentum, an electron has an intrinsic angular momentum called its spin angular momentum. This is simply known as spin, and it can be denoted by the vector S. Other elementary particles, such as protons, also have spin. This spin is associated with a particle’s intrinsic spin magnetic dipole moment, sμ. The vector Sand the moment sμ, are related in the following way:
Where e is the elementary charge and m is the mass of an electron. Now, S cannot be measured directly; however its component along any axis can be measured.In a narrow sense spintronics refers to spin electronics, the phenomena of spin-polarized transport in metals and semiconductors. The goal of this applied spintronics is to find effective ways of controlling electronic properties, such as the current or accumulated charge, by spin or magnetic field, as well as of controlling spin or magnetic properties by electric currents or gate voltages. The ultimate goal is to make practical device schemes that would enhance functionalities of the current charge-based electronics. An example is a spin field-effect transistor, which would change its logic state from ON to OFF by flipping the orientation of a magnetic field.
In a broad sense spintronics is a study of spin phenomena in solids, in particular metals and semiconductors and semiconductor heterostructures. Such studies characterize electrical, optical, and magnetic properties of solids due to the presence of equilibrium and nonequilibrium spin populations, as well as spin dynamics. These fundamental aspects of spintronics give us important insights about the nature of spin interactions—spin-orbit, hyperfine, or spin exchange couplings—in solids. We also learn about the microscopic processes leading to spin relaxation and spin dephasing, microscopic mechanisms of magnetic long-range order in semiconductor systems, topological aspects of mesoscopic spin-polarized current flow in low-dimensional semiconductor systems, or about the important role of the electronic band structure in spin-polarized tunneling, to name a few.
Spintronics refers commonly to phenomena in which the spin of electrons in a solid state environment plays the determining role. In a more narrow sense spintronics is an emerging research field of electronics: spintronics devices are based on a spin control of electronics, or on an electrical and optical control of spin or magnetism. While metal spintronics has already found its niche in the computer industry—giant magnetoresistance systems are used as hard disk read heads—semiconductor spintronics is yet to demonstrate its full potential. This review presents selected themes of semiconductor spintronics, introducing important concepts in spin transport, spin injection, Silsbee-Johnson spin-charge coupling, and spindependent tunneling, as well as spin relaxation and spin dynamics. The most fundamental spin-dependent interaction in nonmagnetic semiconductors is spin-orbit coupling. Depending on the crystal symmetries of the material, as well as on the structural properties of semiconductor based heterostructures, the spin-orbit coupling takes on different functional forms, giving a nice playground of effective spin-orbit Hamiltonians. The effective Hamiltonians for the most relevant classes of materials and heterostructures are derived here from realistic electronic band structure descriptions. Most semiconductor device systems are still theoretical concepts, waiting for experimental demonstrations. A review of selected proposed, and a few demonstrated devices is presented, with detailed description of two important classes: magnetic resonant tunnel structures and bipolar magnetic diodes and transistors. In view of the importance of ferromagnetic semiconductor materials, a brief discussion of diluted magnetic semiconductors is included. In most cases the presentation is of tutorial style, introducing the essential theoretical formalism at an accessible level, with case-study-like illustrations of actual experimental results, as well as with brief reviews of relevant recent achievements in the field.
Spintronics promises the possibility of integrating memory and logic into a single device. In certain cases, switching times approaching a picosecond are possible, which can greatly increase the efficiency of optical devices such as light-emitting diodes (LEDs) and lasers. The control of spin is central as well to efforts to create entirely new ways of computing, such as quantum computing, or analog computing that uses the phases of signals for computations. Spin is a fundamental quantum-mechanical property. It is the intrinsic angular momentum of an elementary particle, such as the electron. Of course, any charged object possessing spin also possesses an intrinsic magnetic moment. It has been known for decades that in ferromagnetism the spins of electrons are preferentially aligned in one direction. Then, in 1988, it was demonstrated that currents flowing from a ferromagnet into an ordinary metal retain their spin alignment for distances longer than interatomic spaces, so that spin and its associated magnetic moment can be transported just as charge. This means that magnetization as well can be transferred from one place to another.