SPINTRONICS: AN INTRODUCTION
From the earliest transistor to the remarkably powerful microprocessers, most electronic devices have employed circuits that express data as binary digits, or bits: ones and zeroes represented by the existence or absence of electric charge. Furthermore, the communication between microelectronic devices occursby 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 Moore’s 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 Moore’s Law will run out of momentum one day as the size of individual bits approaches the dimension of atoms; this has been called the end of the silicon road map. For this reason and also to enhance the multifunctionality of devices (for example, carrying out processing and data storage on the same chip), investigators have been eager to exploit another property of the electron—a characteristic known as spin. Spin is a purely quantum phenomenon roughly akin to the spinning of a child’s top or the directional behavior of a compass needle. The top could spin in the clockwise or counterclockwise direction; electrons have spin of a sort in which their compass needles can point 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. One 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, time—once 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 types of computations than is possible with electron-charge-based systems. The word ‘spintronics” stands for spin electronics. By understanding the behavior of electron spin in materials we can learn something fundamentally new about solid state physics that will lead to a new generation of electronic devices based on the flow of spin in addition to the flow of charge.The spintronics is helpful in integration of electronic, optoelectronic and magnetoelectronic multifunctionality on a single device that can perform much more than is possible with today’s microelectronic devices.
THE LOGIC OF SPIN
Spin relaxation (how spins are created and disappear) and spin transport (how spins move in metals and semiconductors) are not only important in basic physics but also in electronic technology. 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 n (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) to construct exquisitely sensitive detectors of changing magnetic fields, such as those marking the data on a computer hard-disk platter. These disk drive read/write heads have been wildly successful, permitting the storage of tens of gigabytes of...
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