spintronics

Spintronics
THE FUTURE OF THE DIGITAL WORLD
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.


Above plot spin relaxation time T1 of aluminum in nanoseconds (on a logarithmic scale) versus temperature in Kelvin’s. The solid curve is our first-principles calculation. Symbols come from two measurements: spin injection (Johnson and Silsbee) and CESR (Lubzens and Schultz). The agreement between theory and experiment is very good, showing, for the first time, that the Elliott-Yafet mechanism and the spin-hot-spot model works.




Above: The function measures how effective phonons with a given frequency (Omega--horizontal scale) are in scattering electrons in such a way that the electrons' spins flip. Here plot it for aluminum (solid line). The long-dashed curve is the phonon density of states (F) and the short-dashed curve is the ordinary (non-spin-flip) Eliashberg function which is important for superconductivity.

FINDING THE SPIN OF AN ELECTRON
Decoherence is the process in which objects of the quantum world -- like electrons -- lose their wavelike characteristics by interacting with the surrounding environment. Electron spin control could be crucial for the creation of nanoscale electronics, the magnetic resonance imaging of single molecules and the development of quantum computers. The spin orientation of the electron was converted to an electrical charge, which was then measured using a device called a Field effect transistor, or FET. An FET can sense current changes in electrostatic charge. The discovery sets the stage for the practical study of single electron spin physics using test transistors in conventional, commercial silicon integrated circuits. Electron spins in semiconductors have proven particularly attractive for such studies because of their long decoherence times. In addition, single electron spin resonance opens new opportunities in surface science by allowing researchers to individually study single defects and their environments at the semiconductor-insulator interfaces. This may lead to applications in semiconductor technology where design of reliable devices with ever decreasing feature sizes requires detailed understanding of the interfaces at the nanoscale

CONTROLING THE SPIN OF AN ELECTRON
An approximate understanding of the nature of spin can be gleaned by analogy with the orbit of planets in the solar system. In this analogy, electrons orbit a nucleus in a fashion similar to the Earth's orbit around the Sun. Just as the Earth rotates about it's axis during the orbit, electrons have a quality of rotation called 'spin.' The spin of electrons is characterized by the direction of rotation, so that spin 'up' or 'down' electrons rotate in opposite directions (i.e., clockwise or counter-clockwise).While magnetic fields are conventionally used to manipulate spins in familiar magnetic devices like hard-disk drives, electrical control of aligned spins represents a significant step towards making new spin-based technologies. One future technology is quantum computing, where many schemes make use of electron spin states as bits of information analogous to the 0's and 1's of binary computing. Unlike ordinary bits, 'quantum bits' can be any combination of both 0 and 1 simultaneously, corresponding to continuous range of possible directions. Magnetic fields can change the direction of spins by inducing "precession" which is an additional rotation of the spin orientation about the magnetic field, similar to the periodic movement of the axis of a top after it is spun. While the speed of electron spin precession in a magnetic field is generally fixed by the particular materials used, the research reported in Nature has shown that both the speed and direction of precession can be continuously adjusted by applying electric fields in specially engineered quantum structures.

The problem was an old one. So-called "spin resonance" techniques, used extensively for magnetic resonance imaging (MRI) and chemical identification, manipulate electron and nuclear spins in three dimensions using rapidly alternating magnetic fields. However, such magnetic fields are difficult to generate and control on a local scale. By contrast, local control over electric fields forms the basis of all of electronics, from CPUs to cell phones. The challenge was how to figure out a way to control electron spins using electric fields. if a host for electrons could be designed for which the axis of spin rotation changed with an applied electric field, the spin direction could itself be controlled. That is, they could turn electric fields into effective magnetic fields, That is, they could turn electric fields into effective magnetic fields.
Semiconductor sandwiches made of aluminum gallium arsenide and gallium arsenide could provide just this sort of control. The material was built atomic layer by atomic layer in the Molecular Beam Epitaxy (MBE) the deposition of the material such that the parabolic quantum wells (PQWs) "are grown by varying the aluminum concentration x, ranging from 7% at the center to 40% at the barrier, to shape the conduction band into a parabolic potential," The parabolic quantum wells are grown with varying concentrations of aluminum gallium arsenide, sandwiched between gallium arsenide, flanked by metal plates, grown by deposition on the gallium arsenide. Think of a sandwich with Swiss cheese in the middle (aluminum gallium arsenide quantum wells) flanked by meat (gallium arsenide) in turn flanked by bread (the metal plates). The plates are the gates with one lead for the application of electrical current that in turn creates the electrical fields, which enable manipulation of the electrons through the material whose varying concentrations of aluminum govern the rotational speed of the electrons and the direction of their axes.
The result is "electron spin resonance (ESR) on a chip." This engineered nanostructure allows use of very small voltages in traditional gates to operate on electron spin in all three directions in which the axis can point without requiring fast alternating magnetic fields manipulation Using Voltage Controlled g-Tensor Modulation." "This describes and demonstrates how to replace magnetic with electrical fields for the control of spin information in semiconductors," and the findings as an "enabling technology" for spintronics and a "feasibility demonstration" for quantum information processing. For the latter, the size of the gates must be reduced so that the spin of a single electron can be precisely controlled. Such a feat would enable electron spins to be used as quantum bits or "qubits" for a quantum computer. "Ultimately," "these electrical gates may be scaled down for operation on single spins and in quantum dots to form qubits."
In contrast to bits, the "1"s and "0"s of present-day computers, qubits can be in both the "1 state" and "0 state" at the same time, enabling a much richer and more powerful paradigm for computation. The orientation of an electron spin can be used to store one qubit of information. Quantum gates are then needed to reorient the electron spins and perform "quantum information processing".
And then there is the issue of spin-spin interactions, "Control over single spin operations is sufficient for universal quantum gating, provided there is a 'backbone' of spin-spin interactions."
TRANSFER SPIN WITHOUT CHANGE
Hold a pencil upright and rotate it in the same direction by turning it alternately between the thumb and index finger of one hand and the other. While rotating it, turn it upside down. Note that when inverted, the direction of rotation changes from, say, clockwise to counterclockwise. That pencil is analogous to the axis of rotation of an electron. The two orientations of the axis of rotation--up or down--are the conventional or classical ways physicists describe spin. That description is sufficient for understanding, though spin as a quantum mechanical property is understood not merely as up or down, but the superposition of all orientations of the axis of rotation. The question was whether a cloud or bundle of electrons all spinning the same way would retain that same spinning when the cloud is moved to an adjacent semi conducting material. The spins in fact stayed aligned. For example, if a voltage pushes an electron out of the gallium arsenide into zinc selenide, the electron’s precession characteristics change .How ever,if the a higher voltage pushes the electron sharply enough into zinc selenide , the precession characteristics do not change but remain same of that of gallium arsenide Certain semiconductors were found to work as spin reservoirs because spins survive there for long times. In analogy with conventional charge-based electronics, the electrons can be withdrawn from such reservoirs with their spin intact, using electric fields. Spin reservoirs are thereby "sourcing" a spin current
"Unexpectedly, if you keep pulling spin from one material to another, the spins in the adjacent layer acquire the original spin frequency and lifetime of the reservoir. Therefore the total transferred spin can have the properties of either the reservoir or the adjacent layer, and an external electric field 'gates' the transition between the two very different regimes. That is the 'persistent sourcing' The fact that this behavior can be tuned with either electric or magnetic fields results in a new multi-functional type of 'spintronics.'" this "newly-identified, persistent mode of spin transfer º makes the reservoir act, in effect, as a spin 'battery.'"
SPINTRONIC DEVICES
Here, we try and explain main devices which are already existing today like GMR which can also used as spin valve or filter, devices which increases memory handling capability like MRAM and possibilities of preparing spin valve transistors
The basic GMR device is a three layer sandwich of a magnetic metal such as cobalt with a non magnetic metal filling like silver .A current passes through the layers consisting of spin up and spin down electrons .The electrons oriented in the same direction as electron spins in the magnetic layer pass through quite easily , while those oriented in the opposite direction are scattered. if the orientation of the magnetic layers is changed by the presence of a magnetic field, The device will act as a filter or a spin valve , letting through more electrons when in the two layers are the same and fewer electrons when the spin orientations are oppositely aligned .The electrical resistance of the device can therefore be changed dramatically.
MRAM devices such as magnetic tunnel junctions that has two layers of ferromagnetic material separated by a non magnetic barrier when spin orientations of electronics in the two ferro magnetic layers are the same, a voltage is quite likely to push the electrons to tunnel through the barrier ,resulting in high current flow .but flipping the spins in one of the layers have oppositely aligned spins ,restricts the flows of the current due to the standard spin valve. Any memory device can also be used to build logic circuits and spin devices like tunnel junctions are no exception
To achieve a spintronic device in which spin-transport dominates, a magnetic material must be brought in close contact with the semiconductor. A common way to combine these materials is to epitaxially grow the magnetic materials on the semiconductor substrate. Epitaxy helps control the crystalline orientation of the metallic layer and as such, the magnetic anisotropy of the film. However, two important drawbacks limit epitaxy's applicability: (1) the interface is in many cases not thermodynamically stable and (2) the fabrication of a buried structure turns out to be nearly impossible, because the re-growth of a semiconductor layer on a metal has so far not led to acceptable semiconductor properties. More complex device concepts embed a magnetic material in the device structure. For example, the spin-valve transistor (SVT) incorporates a spin-valve in the metal base. In a spin-valve, which basically consists of two magnetic layers separated by a precious metal, the resistance depends on the relative orientation of the magnetization in the layers. SVT fabrication requires an approach other than epitaxy.

APPLICATIONS

1) GMR SENSORS
GMR sensors find a wide range of applications .Fast and accurate position and motion sensing of mechanical components in precision engineering and robotics
• All kinds of automotive sensors for fuel handling systems, anti skid systems, speed control and navigation.
• Missile guidance
• Position and motion sensing in computer video games
• Key hole surgery and post operative care


2) SPIN VALVES
Spin valves are not only highly sensitive magnetic sensors but these can also be made to act as switches by flipping magnetization in one of the layers parallel or anti parallel as in a convectional transistor memory device

3) MRAM
The magnetic version of the ram used in computer is non volatile ,i.e, the information isn’t lost when the system is switch off and other advantages are lower cost, smaller size ,faster speed and less power consumption. These can survive even in high temperature and high level radiations or interferenence

4 )CREATING FASTER COMPUTERS
Silicon is still a favorite with the electronics industry and is likely to remain so, hybrid devices that combine magnetic layers with semiconductors like silicon could be made to behave more like conventional transistors. These could be used as non-volatile logic elements that could be reprogrammed using software during actual processing to create an entirely new type of very fast computers.

5) QUANTUM COMPUTERS
one of most ambitions devices is the spin based quantum based computer in solid state structures using electron spin for this purposes is a obvious idea since fermions with ½ spin is a natural and intrinsic qubit.
The applications of spintronics in quantum computation has focused on using quantum-dot-trapped electron spins in GaAs .Because of the three dimensional confinement and the fact that GaAs conduction band is mainly band in mainly formed from ‘s’ atomic orbital , the trapped electrons have a small spin –orbit coupling and therefore small decoherence rate. In the Qd-Qc model, one electron spin per quantum dot works as a quantum qubit. Two coupled spins on two neighboring dots provide two qubit operations through the inter dot electronics exchange coupling. The external magnetic fields provide means to manipulate single qubits.

6) MAGNETIC FIELD SENSORS
A new type of magnetic field sensor is the spin valve transistor. this transistor is based on the magneto resistance found in multilayers.
Usually, the resistance of a multilayer is measured with the current in plane (CIP). The CIP configuration suffers from several drawbacks; for example, the CIP magneto resistance is diminished by shunting and diffusive surface scattering. Hence the fundamental parameters of the spin valve effect, such as the relative contribution of the interface and bulk spin dependent scattering, are difficult to obtain using the CIP geometry.
Measuring with the current perpendicular to plane solves most problems, mainly because the electrons cross all magnetic layers. The spin valve transistor consists of a silicon emitter, a magnetic multilayer as the base and a silicon collector.
Electrons are injected from the emitter, passing the first Schottky barrier (semiconductor metal interface) into the base. Because of the thin base multilayer (10 nm), most electrons are not directed to the base contact and travel perpendicular through the multilayer across the second Schottky barrier to form the collector current.
A Co-Cu multilayer is sputtered on one of the silicon substrates. The last layer is sputtered on both substrates and these are pressed together at the last section of the sputter deposition. A metal layer between the two crystalline semiconductors is accomplished and the bond proves stronger than silicon.
The mean free path varies with the applied magnetic field ,hence the collector current becomes strongly
Field dependent the extreme magentosensitivity makes the transistors device for high technology read heads for high density
Hard disks and magnetic RAM’S


CONCLUSION:
SPINTRONICS, which depend on the spin of the electron, has a great potential of spinning this global village into a unexpected digital atomic world which has a capability of manipulating at atomic level and this can even made further smaller
With the integration with new emerging technology called “NANOTECHNOLOGY”
This would make things smaller and cheaper and more affordable by a common man
That what is the aim of an engineer or a scientist! What ever may be the discovery or
Invention made will have its worth forever only if it finds its use in common man’s
Life. We wish and hope spintroincs will have its into common man’s life finally we conclude with a Telugu saying
“SUKSHMUM LO NAE MOKSHUM VUNNADI”
That’s what we were discussing about from the beginning.

REFERENCES:
1)mechanisms of spin relaxation in electronic systems (metals and semiconductors) in J. Fabian and S. Das Sarma, J. Vac. Sc. Technol. B 17, 1708 (1999).
2) Spintronics, a new nanoelectronics adventure by Jo De Boeck and Mieke Van Bavel, IMEC, Leuven, Belgium
3) Euro physics news (2003) vol.34 no.6

4) Spintronics :fundamentals and applications igor ,jarosiav Fabian, s.das sarma

5) New spin transistors nature july 05, 2002

6) Introduction to spintronics –Stuart Wolf

7) Spintronics by Mark Johnson, Naval research lab, Washington, DC