Spintronics Documentation

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spintronics SPINTRONICS

1.1 INTRODUCTIONAn emerging research field in physics focused on spin-dependent phenomena applied to electronic devices is called spintronics. The promise of spintronics is based on manipulation not only of the charge of electrons, but also their spin, which enables them to perform new functions. Currently, the ability to manipulate electron spin is expected to lead to the development of remarkable improvements in electronic systems and devises used in photonics, data processing and communications technologies only.

1.2 BRIEF HISTORYDuring the last half of the 20th century, the world witnessed a revolution based on a digital logic of electrons. The spintronics dream is a seamless integration justification, could be called the microelectronics era. From the earliest transistor to the remarkably powerful microprocessor in desktop computer, most electronic devices have employed circuits that express data as binary digits, or bitsones and zeroes represented by the existence or absence of electric charge. Furthermore, the communication between microelectronic devices occurs by the binary flow of electric charges. The technologies that emerged from this simple logic have created a multitrillion dollar per year global industry whose products are ubiquitous. Indeed, the relentless growth of microelectronics is often popularly summarized in Moores Law, which holds that microprocessors will double in power every 18 months as electronic devices shrink and more logic is packed into every chip. Yet even Moores Law will run out of momentum one day as the size of individual bits approaches the dimension of atomsthis has been called the end of the silicon road map.

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spintronicsFor this reason and also to enhance the multifunctionality of devices, another property of the electron has been exploiteda characteristic known as spin. Spin is a purely quantum phenomenon. The electrons have spin pointing either up or down in relation to a magnetic field. Spin therefore lends itself elegantly to a new kind of binary logic of ones and zeros. The movement of spin, like the flow of charge, can also carry information among devices.

1.3 SPINTRONICSSpintronics is a new, quickly developing field of science and technology, which deals with relationships responsible for specific features of the spin interactions in metals, semiconductors doped with transition or rare earth elements, and hetero structures that ensure unique properties of these materials. An electron is just like a spinning sphere of charge. It has a quantity of angular momentum (its "spin") and an associated magnetism, and in an ambient magnetic field its energy depends on how its spin vector is oriented. Every electron exists in one of two states, namely, spin-up and spin-down with its spin either +1/2 or 1/2. In other words, an electron can rotate either clockwise or counterclockwise around its own axis with constant frequency. . Two spins can be "entangled" with each other, so that neither distinctly up nor down, but a combination of the two possibilities. The main avenues of the development of spintronics are i) fabrication of magnetic nanostructures including novel materials, thin films, hetero structures, and multifunctional materials; ii) research on magnetism and spin control of magnetic nanostructures, theory of ferromagnetic exchange in dilute magnetic semiconductors (DMS),tunneling effects and spin injection, as spin transport and detection of magnetism; iii) magneto electronics and devices employing the giant magneto resistance (GMR) effect, tunneling devices, semiconductor hetero structures for spin injection, spin transport and detection, and pulsed ferromagnetism;

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spintronicsiv) magneto optical properties of dc magnetic semiconductor hetero structures and time resolved experiments, optical spin injection and detection, optically induced ferromagnetism, transmission; v) pattern recognition; imaging and metrology including magnetic pattern recognition and anomalous Hall effect; and vi) instrument engineering and applied studies. In order to make a spintronic device, the primary requirement is to have a system that can generate a current of spin polarized electrons, and a system that is sensitive to the spin polarization of the electrons. Most devices also have a unit in between that change the current of electrons depending on the spin states. The simplest method of generating a spin polarized current is to inject the current through a ferromagnetic material. The most common application of this effect is a giant magneto resistance (GMR) device. A typical GMR device consists of at least two layers of ferromagnetic materials separated by a spacer layer. When the two magnetization vectors of the ferromagnetic layers are aligned, then an electrical current will flow freely, whereas if the magnetization vectors are anti parallel then the resistance of the system is higher. Two variants of GMR have been applied in devices, current-in-plane where the electric current flows parallel to the layers and current-perpendicular-to-the-plane where the electric current flows in a direction perpendicular to the layers. ultrafast magneto optic switches; and quantum information

1.4 THE LOGIC OF SPINSpin relaxation (how spins are created and disappear) and spin transport (how spins move in metals and semi- conductors) are fundamentally important not only as basic physics questions but also because of their demonstrated value as phenomena in electronic technology. Spins can arrange themselves in a variety of ways that are important for spintronics devices. They can be completely random, with their spins pointing in every possible

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spintronicsdirection and located throughout a material in no particular order (upper left). Or these randomly located spins can all point in the same direction, called spin alignment (upper right). In solid state materials, the spins might be located in an orderly fashion on a crystal lattice (lower left) forming a nonmagnetic material. Or the spins may be on a lattice and be aligned as in a magnetic material (lower right),

Fig 1.1 Spin alignment in unmagnetized and magnetized field One device already in use is the giant magnetoresistive, or GMR, sandwich structure, which consists of alternating ferromagnetic (that is, permanently magnetized) and nonmagnetic metal layers. Depending on the relative orientation of the magnetizations in the magnetic layers, the electrical resistance through the layers changes from small (parallel magnetizations) to large (antiparallel magnetizations). Investigators discovered that they could use this change in resistance (called magnetoresistance, and giant because of the large magnitude of the effect in this case)

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spintronicsto construct exquisitely sensitive detectors of changing magnetic fields, such as those marking the data on the types of computations than is possible with electron-chargebased systems. Spintronic disk drive read/write heads have been wildly successful, permitting the storage of tens of gigabytes of data on notebook computer hard drives, and has created a billion-dollar per year industry. Groups have also been working to develop nonvolatile memory elements from these materials, possibly a route to instant-on computers.

1.5 ADVANTAGE OF SPIN OVER CHARGEOne advantage of spin over charge is that spin can be easily manipulated by externally applied magnetic fields, a property already in use in magnetic storage technology. Another more subtle (but potentially significant) property of spin is its long coherence, or relaxation, timeonce created it tends to stay that way for a long time, unlike charge states, which are easily destroyed by scattering or collision with defects, impurities or other charges. These characteristics open the possibility of developing devices that could be much smaller, consume less electricity and be more powerful for certain of electronic, optoelectronic and magnetoelectronic multifunctionality on a single device that can perform much more than is possible with todays microelectronic devices.

1.6 DEVELOPMENT OF SPINTRONIC DEVICESResearchers and developers of spintronic devices currently take two different approaches. In the first, they seek to perfect the existing GMR-based technology either by developing new materials with larger populations of oriented spins (called spin polarization) or by making improvements in existing devices to provide better spin filtering. The second effort, which is more radical, focuses on finding novel ways both to generate and to utilize spin-polarized currentsthat is, to actively control spin dynamics. The intent is to thoroughly investigate spin transport in semiconductors and search for ways in which semiconductors can function as spin polarizers and spin valves. This is crucial because, unlike semiconductor transistors, existing metal-based devices do not amplify signals (although they are successful switches or valves).

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1.7 SPINTRONIC DEVICEThe first scheme for a spintronic device based on the metal-oxide-semiconductor technology familiar to microelectronics designers was the field effect spin transistor proposed in 1989 by Supriyo Datta and Biswajit Das of Purdue University. In a conventional field effect transistor, electric charge is introduced via a source electrode and collected at a drain electrode. A third electrode, the gate, generates an electric field that changes the size of the channel through which the source-drain current can flow, akin to stepping on a garden hose. This results in a very small electric field being able to control large currents. In the Datta-Das device, a structure made from indium-aluminum-arsenide and indiumgallium-arsenide provides a channel for two-dimensional electron transport between two ferromagnetic