06-07-2012, 01:54 PM
SPINTRONICS
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ABSTRACT
In our electronics field ,all these years from the discovery of the electron so many scientists have made themselves as bricks to build this large structure called ‘digital world’ which is what we are enjoying today .To still satisfy common man needs they thought of rebuilding these structure and started doing it from the scratch the basement of this world “a ELECTRON” all these what we are using today depend on the charge of the electron rather then its other basic property ‘spin’ .so, they started exploiting this property giving rise to new field of electronics called “spintronics”
We try to present this ambitious side of electronics in layman terms, By the end of the paper we make u understand what is spintronics ? .we start with a brief history of it and continue with explaining some important terms like spin hotspots, spin relaxation and moving further we explain how to control the spin and basic devices that formed basing spintronics and sailing through we make u know the fruits of this field (applications) and leaving u with the leads that has further scope of research and chances of exploitations
INTRODUCTION
Spintronics burst on the scene in 1988 when French and German physicists discovered a much more powerful effect called 'giant magneto resistance' (GMR). It results from subtle electron-spin effects in ultra-thin 'multilayer' of magnetic materials, which cause huge changes in their electrical resistance when a magnetic field is applied. GMR is 200 times stronger than ordinary magneto resistance
Spintronics is a new branch of electronics in which electron spin, in addition to charge, is manipulated to yield a desired electronic outcome. All spintronic devices act according to the simple scheme: (1) information is stored (written) into spins as a particular spin orientation (up or down), (2) the spins, being attached to mobile electrons, carry the information along a wire, and (3) the information is read at a terminal. Spin orientation of conduction electrons survives for a relatively long time (nanoseconds, compared to tens of femtoseconds during which electron momentum and energy decay), which makes spintronic devices particularly attractive for memory storage and magnetic sensors applications, and, potentially for quantum computing where electron spin would represent a bit (called qubit) of information.
PRINCPLE
Spintronics is based on the spin of the electron exists in one of the two states, namely spin up and spin down, with spins either positive half or negative half. In other words ,an electron can rotate Either in clockwise or anticlockwise around its own axis with constant frequency
Spin is the root cause of magnetism and is a kind of intrinsic angular momentum that a particle cannot gain or lose, The two possible spin states naturally represent ‘0’and ‘1’in logical operations spin is the characteristics that makes the electron A tiny magnet complete with north and south poles .The orientation of the tiny magnet ‘s north-south poles depends on the particle’s axis of spin .In the atoms of an ordinary material,
Some of these spin axes point ‘up’ and equal number points ‘down’. The particle’s spin is associated with a magnetic moment, which may be thought of as the handle that lets a magnetic field torque the electron’s axis of spin .thus in a ordinary material, the up moments cancel the down ones, so no surplus moment piles up.
For that, a ferromagnetic material like iron, nickel or cobalt is needed.
These have tiny regions called ‘domains’ in which an excess of electrons have spins with axes pointing either up or down –at least, until heat destroys the magnetism, above the metal’s curie temperatures. The many domains are ordinarily randomly scattered and evenly divided between majority up and majority down. But an externally applied magnetic field will move the walls between the domains and line up all the domains in the direction of the field , so, they point is a permanent magnet
There are two ways for spins to decay, and both include spin-orbit coupling of some kind. First, impurities can induce a spin-orbit interaction that can flip an electron spin. Second, a spin-orbit interaction can be induced by host-lattice ions. The second mechanism is important at high temperatures where electrons scatter off phonons, but also at low temperatures, if the impurities are light—meaning they induce small spin-orbit coupling. The second mechanism is somewhat tricky. One has to realize that in the presence of spin-orbit coupling, spin up and spin down states are no longer good quantum numbers even scalar (spin independent) interactions due to impurities or phonons can cause spins to flip
SPIN HOT SPOTS
The two groups of metals (one that follows the scaling and one with spin relaxation rates off by orders of magnitude) have different valence: monovalent metals (Na, Cu, ...) follow the Gruneisen behavior, while polyvalent metals (Al, Pd, Be, and Mg) do not. What is so peculiar about polyvalent metals? Band structure. Because of the complicated character of Bloch bands in polyvalent metals one cannot use b^2 from atomic physics. Instead, b^2 is band-renormalized by the presence of band-structure anomalies--spin hot spots. Spin hot spots are points on the Fermi surface where the surface cuts through a Brillouin zone boundary, special symmetry point, or a line of accidental degeneracy. If an electron jumps from (or, into) a spin hot spot, the electron’s spin flips with much larger probability than usual. Since the resulting spin-flip probability is an average over the whole Fermi surface, one has to know how large spin hot spots are. They are large enough to completely monopolize spin relaxation: to calculate the average it suffices to count contributions from spin hot spots only.
For alkali and noble metals, which are monovalent, an electron performing a random walk on the Fermi surface has a small chance of flipping its spin everywhere on the surface. All Fermi states are equivalent. By contrast, polyvalent metals have so called spin hot spots where a chance of a spin flip is much greater, by several orders of magnitude.
The spin-hot-spot model is a general concept which explains the details of spin relaxation in metals within the framework of the Elliott-Yafet mechanism. A real calculation that would clearly show, without any fitting or adjusting, that the mechanism works, was still lacking and we chose aluminum, for it is both simple to calculate and complicated enough (polyvalent) to exhibit the spin-hot-spot model attributes.