ABSTRACTSpintronics is a new branch of electronics in which electron spin, in addition to charge, is manipulated to yield a desired 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. Devices that rely on an electron's spin to perform their functions form the foundation of Spintronics, also known as Magneto electronics. Spin orientation of conduction electrons survives for a relatively long time (nanoseconds, compared to tens of femto seconds during which electron momentum decays), 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.

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TABLE OF CONTENTS

Abstract......................................................................................................................i

Table of contents........................................................................................................ii

List of figures.............................................................................................................iii

1.Introduction.............................................................................................................1

2.Electron spin...........................................................................................................1

2.1 Fundamentals of spin.....................................................................................2

3.Fundamentals of quantum information...................................................................3

3.1 Beyond the bit................................................................................................3

3.2 Quantum teleportation...................................................................................5

4.Related concepts.....................................................................................................6

4.1 Giant magneto static resistance......................................................................64.2 Memory chips..........................................................................................................................8

5. Spintronic devices..................................................................................................9

5.1 Spintronic transistor.......................................................................................9

5.2 Ballistic disks.................................................................................................11

5.3 Ultra fast drives..............................................................................................12

5.4 Voltage control of spin direction...................................................................13

6.Conclusion..............................................................................................................15

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LIST OF FIGURES

Figure 3.1 Entanglements..........................................................................................5

Figure 3.2 Teleportation theory.................................................................................6

Figure 4.1 Flow of Current through Ferromagnetic materials ..................................7

Figure 4.2 Resistance of Ferromagnetic materials.....................................................8

Figure 5.1 Spintronic Transistor...............................................................................10

Figure 5.2 Microscopic image of a disk....................................................................12

Figure 5.3 Magnetic Memory Device.......................................................................13

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1.INTRODUCTIONAs rapid progress in the miniaturization of semiconductor electronic devices leads toward chip features smaller than 100 nanometers in size, engineers and physicists are certainly faced with the alarming presence of quantum mechanics. One such peculiarity is a quantum property of the electron known as spin, which is closely related to magnetism. Devices that rely on an electron's spin to perform their functions form the foundation of Spintronics, also known as Magnetoelectronics. Information processing technology has thus far relied on purely charge-based devices -ranging from the now old-fashioned vacuum tube to today's million-transistor microchips. Those conventional electronic devices move electric charges around, ignoring the spin that tags along for the ride on each electron.

Spintronics is a new branch of electronics in which electron spin, in addition to charge, is manipulated to yield a desired 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 femto seconds during which electron momentum decays), 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.

2.ELECTRON SPINAn electron spin s = 1/2 is an intrinsic property of electrons. Electrons have intrinsic angular momentum characterized by quantum number 1/2. In the pattern of other quantized angular momenta, this gives total angular momentum

Spin "up" and "down" allows two electrons for each set of spatial quantum numbers.

The resulting fine structure which is observed corresponds to two possibilities for the z-component of the angular momentum.

This causes an energy splitting because of the magnetic moment of the electron 1

Two types of experimental evidence which arose in the 1920s suggested an additional property of the electron. One was the closely spaced splitting of the hydrogen spectral lines, called fine structure. The other was the Stern-Gerlach experiment which showed in 1922 that a beam of silver atoms directed through an inhomogeneous magnetic field would be forced into two beams. Both of these experimental situations were consistent with the possession of an intrinsic angular momentum and a magnetic moment by individual electrons. Classically this could occur if the electron were a spinning ball of charge, and this property was called electron spin.

Quantization of angular momentum had already arisen for orbital angular momentum, and if this electron spin behaved the same way, an angular momentum quantum number s = 1/2 was required to give just two states. This intrinsic electron property gives:

The electron spin magnetic moment is important in the spin-orbit interaction which splits atomic energy levels and gives rise to fine structure in the spectra of atoms. The electron spin magnetic moment is also a factor in the interaction of atoms with external magnetic fields (Zeeman effect).

2.1 FUNDAMENTALS OF SPIN

1   In addition to their mass and electric charge, electrons have an intrinsic quantity of angular momentum called spin, almost as if they were tiny spinning balls.

2 Associated with the spin is a magnetic field like that of a tiny bar magnet lined up with the spin axis.

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3 Scientists represent the spin with a vector. For a sphere spinning "west to east" the vector points "north" or "up." It points "down" for the opposite spin.

4 In a magnetic field, electrons with "spin up" and "spin down" have different energies.

5  In an ordinary electric circuit the spins are oriented at random and have no effect on current flow.

6 Spintronic devices create spin-polarized currents and use the spin to control current flow.

3.FUNDAMENTALS OF QUANTUM INFORMATION

The fact that information is physical means that the laws of quantum mechanics can be used to process and transmit it in ways that are not possible with existing systems.The important new observation is that information is not independent of the physical laws used to store and processes it. Although modern computers rely on quantum mechanics to operate, the information itself is still encoded classically. A new approach is to treat information as a quantum concept and to ask what new insights can be gained by encoding this information in individual quantum systems. In other words, what happens when both the transmission and processing of information are governed by quantum laws?

The elementary quantity of information is the bit, which can take on one of two values - usually "0" and "1". Therefore, any physical realization of a bit needs a system with two well defined states, for example a switch where off represents "0" and on represents "1". A bit can also be represented by, for example, a certain voltage level in a logical circuit, a pit in a compact disc, a pulse of light in a glass fibre or the magnetization on a magnetic tape. In classical systems it is desirable to have the two states separated by a large energy barrier so that the value of the bit cannot change spontaneously.

Two-state systems are also used to encode information in quantum systems and it is traditional to call the two quantum states 0 and 1. The really novel feature of quantum information technology is that a quantum system can be in a superposition of different states. In a sense, the quantum bit can be in both the 0 state and the 1 state at the same time. This new feature has no parallel in classical information theory and in 1995 Ben Schumacher of Kenyon College in the US coined the word "Qubit" to describe a quantum bit.

3.1 BEYOND THE BIT

Any quantum mechanical system can be used as a Qubit providing that it is possible to define one of its states as 0 and another as 1. From a practical point of view it is useful to have states that are clearly distinguishable. Furthermore, it is desirable to have states that have reasonably long lifetimes (on the scale of the experiment) so that the quantum information is not lost to the environment through decoherence. Photons,

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electrons, atoms, quantum dots and so on can all be used as qubits. It is also possible to use both internal states, such as the energy levels in an atom, and external states, such as the direction of propagation of a particle, as qubits.

The fact that quantum uncertainty comes into play in quantum information might seem to imply a loss of information. However, superposition is actually an asset, as can be seen when we consider systems of more than one qubit. What happens if we try to encode two bits of information onto two quantum particles? The straightforward approach would be to code one bit of information onto each qubit separately. This leads to four possibilities - 01 02 01 12 11 02 and 11 12 - where 01 12

describes the situation where the first qubit has the value "0" and second qubit has the value "1", and so on. This approach corresponds exactly to a classical coding scheme in which these four possibilities would represent "00", "01", "10" and "11".

However, quantum mechanics offers a completely different way of encoding information onto two qubits. In principle it is possible to construct any superposition of the four states described above. A widely used choice of superpositions is the so-called Bell states. A key feature of these states is that they are "entangled". Entanglement describes correlations between quantum systems that are much stronger than any classical correlations.

As in classical coding, four different possibilities can be represented by the four Bell states, so the total amount of information that can be encoded onto the two qubits is still two bits. But now the information is encoded in such a way that neither of the two qubits carries any well defined information on its own: all of the information is encoded in their joint properties. Such entanglement is one of the really counterintuitive features of quantum mechanics and leads to most of the paradoxes and other mysteries of quantum mechanics.

It is evident that if we wish to encode more bits onto quantum systems, we have to use more qubits. This results in entanglements in higher dimensions, for example the so-called Greenberger-Horne-Zeilinger (GHZ) states, which are entangled superpositions of three qubits. In the state 1/2( 000 + 111), for instance, all three qubits are either "0" or "1" but none of the qubits has a well defined value on its own. Measurement of any one qubit will immediately result in the other two qubits attaining the same value.

Although it was shown that GHZ states lead to violent contradictions between a local realistic view of the world and quantum mechanics, it recently turned out that such states are significant in many quantum-information and quantum-computation schemes. For example, if we consider 000 and 111 to be the binary representations of "0" and "7", respectively, the GHZ state simply represents the coherent superposition (1/2)( "0" + "7"). If a linear quantum computer has such a state as its input, it will process the superposition such that its output will be the superposition of the results for each input. This is what leads to the potentially massive parallelism of quantum computers.

It is evident that the basis chosen for encoding the quantum information, and the states chosen to represent 0 and 1, are both arbitrary. For example, let us assume that we have chosen polarization measured in a given direction as our basis, and that we have agreed to identify the horizontal polarization of a photon with "0" and its

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vertical polarization with "1". However, we could equally well rotate the plane in which we measure the polarization by 45º. The states in this new "conjugate" basis, 0´ and 1´, are related to the previous states by a 45º rotation in Hilbert space

0´ = (1/2)( 0 + 1)

1´ = (1/2)( 0 - 1)

This rotation is known in information science as a Hadamard transformation. When spin is used to encode information in an experiment we can change the basis by a simple polarization rotation; when the directions of propagation are used, a beam splitter will suffice. It is important to note that conjugate bases cannot be used at the same time in an experiment, although the possibility of switching between various bases during an experiment - most notably between conjugate bases - is the foundation of the single-photon method of quantum cryptography.

Figure 3.1 Entanglements

Imagine that Bob wants to send some information to Alice. (The characters in quantum information technology are always called Alice and Bob.) Entanglement means that, in theory, Bob can send two bits of information to Alice using just one photon, providing that Alice has access to both qubits and is able to determine which of the four Bell states they are in (see fig).

3.2 QUANTUM TELEPORTATION

Quantum dense coding was the first experimental demonstration of the basic concepts of quantum communication. An even more interesting example is quantum teleportation.

Suppose Alice has an object that she wants Bob to have. Besides sending the object itself, she could, at least in classical physics, scan all of the information contained in the object and transmit that information to Bob who, with suitable technology, could then reconstitute the object. Unfortunately, such a strategy is not possible because quantum mechanics prohibits complete knowledge of the state of any object.

There is, fortunately, another strategy that will work. What we have to do is to guarantee that Bob's object has the same properties as Alice's original. And most importantly, we do not need to know the properties of the original. In 1993 Bennett and co-workers in Canada, France, Israel and the US showed that quantum entanglement provides a natural solution for the problem.

Figure 3.2 Teleportation theory

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In this scheme Alice wants to teleport an unknown quantum state to Bob (see Fig). They both agree to share an entangled pair of qubits, known as the ancillary pair. Alice then performs a joint Bell-state measurement on the teleportee (the photon she wants to teleport) and one of the ancillary photons, and randomly obtains one of the four possible Bell results. This measurement projects the other ancillary photon into a quantum state uniquely related to the original. Alice then transmits the result of her measurement to Bob classically, and he performs one of the four unitary operations to obtain the original state and complete the teleportation.

It is essential to understand that the Bell-state measurement performed by Alice projects the teleportee qubit and her ancillary photon into a state that does not contain any information about the initial state of the teleportee. In fact, the measurement projects the two particles into a state where only relative information between the two qubits is defined and known. No information whatsoever is revealed about . Similarly, the initial preparation of the ancillary photons in an entangled state provides only a statement of their relative properties. However, there is a very clear relation between the ancillary photon sent to Bob and the teleportee photon. In fact, Bob's photon is in a state that is related to Alice's original photon by a simple unitary transformation.

. If Alice's Bell-state measurement results in exactly the same state as that used to prepare the ancillary photons (which will happen one time in four), Bob's ancillary photon immediately turns into the same state as . Since Bob has to do nothing to his photon to obtain , it might seem as if information has been transferred instantly - which would violate special relativity. However, although Bob's photon does collapse into the state when Alice makes her measurement, Bob does not know that he has to do nothing until Alice tells him. And since Alice's message can only arrive at the speed of light, relativity remains intact. In the other three possible cases, Bob has to perform a unitary operation on his particle to obtain the original state, . It is important to note, however, that this operation does not depend at all on any properties of .

4.RELATED CONCEPTS 4.1 GIANT MAGNETOSTATIC RESISTANCE

Electrons like all fundamental particles have a property called spin which can be orientated in one direction or the other - called 'spin-up' or 'spin-down' - like a top spinning anticlockwise or clockwise. When electron spins are aligned (i.e. all spin-up or all spin-down) they create a large-scale net magnetic moment as seen in magnetic materials like iron and cobalt. Magnetism is an intrinsic physical property associated with the spins of electrons in a material.

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Magnetism is already exploited in recording devices such as computer hard disks. Data are recorded and stored as tiny areas of magnetized iron or chromium oxide. To access the information, a read head detects the minute changes in magnetic field as the disk spins underneath it. This induces corresponding changes in the head's electrical resistance - an effect called magnetoresistance.

Figure 4.1 Flow of Current through Ferromagnetic materials

Spintronics burst on the scene in 1988 when French and German physicists discovered a much more powerful effect called 'giant magnetoresistance' (GMR). It results from subtle electron-spin effects in ultra-thin 'multilayers' of magnetic materials, which cause huge changes in their electrical resistance when a magnetic field is applied. GMR is 200 times stronger than ordinary magnetoresistance.

The basic GMR device consists of a three-layer sandwich of a magnetic metal such as cobalt with a nonmagnetic metal filling such as silver (see diagram, above). A current passes through the layers consisting of spin-up and spin-down electrons. Those oriented in the same direction as the electron spins in a magnetic layer pass through quite easily while those oriented in the opposite direction are scattered. If the orientation of one of the magnetic layers can easily be changed by the presence of a magnetic field then the device will act as a filter, or 'spin valve', letting through more electrons when the spin orientations in the two layers are the same and fewer when orientations are oppositely aligned. The electrical resistance of the device can therefore be changed dramatically.

A ferromagnet can even affect the flow of a current in a nearby nonmagnetic metal. For example, present-day read heads in computer hard drives use a device dubbed a spin valve, wherein a layer of a nonmagnetic metal is sandwiched between two ferromagnetic metallic layers. The magnetization of the first layer is fixed, or pinned, but the second ferromagnetic layer is not. As the read head travels along a track of data on a computer disk, the small magnetic fields of the recorded 1's and 0's change the second layer's magnetization back and forth, parallel or antiparallel to the magnetization of the pinned layer. In the parallel case, only electrons that are oriented in the favored direction flow through the conductor easily. In the antiparallel case, all electrons are impeded. The resulting changes in the current allow GMR read heads to detect weaker fields than their predecessors; so that data can be stored using more tightly packed magnetized spots on a disk, increasing storage densities by a factor of three.

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Figure 4.2 Resistance of Ferromagnetic materials

4.2 MEMORY CHIPS

Physicists have been quick to see the further possibilities of spin valves. Not only are they highly sensitive magnetic sensors, they can also be made to act as switches by flipping the magnetization in one of the layers. This allows information to be stored as 0s and 1s (magnetizations of the layers parallel or antiparallel) as in a conventional transistor memory device. An obvious application is a magnetic version of a random access memory (RAM) device of the kind used in the computer. The advantage of magnetic random access memory (MRAM) is that it is 'non-volatile' - information isn't lost when the system is switched off. MRAM devices would be smaller, faster, and cheaper, use less power and would be much more robust in extreme conditions such as high temperature, or high-level radiation or interference. The US electronics company Honeywell has already shown that arrays of linked MRAMS could be made to work. The potential market for MRAMS is worth 100 billion dollars annually.

Over the past three years or so, researchers around the world have been working hard on a whole range of MRAM devices. A particularly promising device is the magnetic tunnel junction, which has two magnetic layers separated by an insulating metal-oxide layer. Electrons can 'tunnel' through from one layer to the other only when magnetisations of the layers point in the same direction, otherwise the resistance is high - in fact, 1000 times higher than in the standard spin valve.

Even more interesting are devices that combine the magnetic layers with semi-conductors like silicon. The advantage is that silicon is still the favorite material of the electronics industry and likely to remain so. Such hybrid devices could be made to behave more like conventional transistors. They could be used as non-volatile logic elements which could be reprogrammed using software during actual processing to create an entirely new type of very fast computing.

The field of spintronics is extremely young and it's difficult to predict how it will evolve. New physics is still being discovered and new materials being developed, such as magnetic semiconductors, and exotic oxides that manifest an even more extreme effect called colossal magnetoresistance.

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Applications of GMR Fast accurate position and motion sensing of mechanical components in

precision engineering and in robotics All kinds of automotive sensors for fuel handling systems, electronic engine

control, antiskid systems, speed control and navigation Missile guidance Position and motion sensing in computer video games

Devices

The spin valve is the simplest magnetoresistive device. It consists of two ferromagnetic layers seperated by a metallic spacer, one of which is free to switch between parallel and antiparallel alignments corresponding to the low and high resistivity states, respectively.

A variant of the spin valve is the magnetic tunnel junction, where the ferromagnetic layers are separated by an insulator just a few atoms thick. Tunnel junctions are the basis of the new Magnetic Random Access Memory chips (MRAM).

 

Multiple electrical contacts are needed to measure the electrical properties of this patterned polycrystalline CrO2 film. The wire is 50 microns (m) long and 3 m wide.

 

5. SPINTRONIC DEVICES

5.1 SPINTRONIC TRANSISTOR

Spintronic transistors could play a major role in the quest for quantum computing, which exploits electron spin to process millions -- or even billions -- of bits of information, at once. Experiments have proved that "spin-polarized leads can be used to determine the spin state of the electron." The transistor, which is made from a tiny semiconductor called a "quantum dot," acts as a gateway that controls electrons by blocking them or letting them pass. This allows the storage of information that also can be read and erased by manipulating spin inside the dot.

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In 1990 Supriyo Datta and Biswajit A. Das, then at Purdue University, proposed a design for a spin-polarized field-effect transistor, or spin FET. In a conventional FET, a narrow semiconductor channel runs between two electrodes named the source and the drain. When voltage is applied to the gate electrode, which is above the channel, the resulting electric field drives electrons out of the channel (for instance), turning the channel into an insulator. The Datta-Das spin FET has a ferromagnetic source and drain so that the current flowing into the channel is spin-polarized. When a voltage is applied to the gate, the spins rotate as they pass through the channel and the drain rejects these antialigned electrons.

Figure 5.1 Spintronic Transistor

One proposed design of a spin FET (spintronic field-effect transistor) has a source and a drain, separated by a narrow semiconducting channel, the same as in a conventional FET.In the spin FET, both the source and the drain are ferromagnetic. The source sends spin-polarized electrons into the channel, and this spin current flows easily if it reaches the drain unaltered (top). A voltage applied to the gate electrode produces an electric field in the channel, which causes the spins of fast-moving electrons to rotate (bottom). The drain impedes the spin current according to how far the spins have been rotated. Flipping spins in this way takes much less energy and is much faster than the conventional FET process of pushing charges out of the channel with a larger electric field.

A spin FET would have several advantages over a conventional FET. Flipping an electron's spin takes much less energy and can be done much faster than pushing an electron out of the channel. One can also imagine changing the orientation of the source or drain with a magnetic field, introducing an additional type of control that is not possible with a conventional FET: logic gates whose functions can be changed on the fly. As yet, however, no one has succeeded in making a working prototype of the Datta-Das spin FET because of difficulties in efficiently injecting spin currents from a ferromagnetic metal into a semiconductor. Although this remains a controversial subject, recent optical experiments carried out at various laboratories around the world indicate that efficient spin injection into semiconductors can indeed be achieved by using unconventional materials, called magnetic semiconductors, that incorporate magnetism by doping the semiconductor crystals with atoms such as manganese.

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Some magnetic semiconductors have been engineered to show ferromagnetism, providing a spintronic component called a gateable ferromagnet, which may one day play an important role in spin transistors. In this device, a small voltage would switch the semiconductor between nonmagnetic and ferromagnetic states. A gateable ferromagnet could in turn be used as a spin filter--a device that, when switched on, passes one spin state but impedes the other.

The transistor is among a number of nanoscale devices that may revolutionize telecommunications, computing and daily life. Restricting the movement of information based on electron spin, rather than charge, can increase computing power exponentially and, by including memory with processing, create "a sort of computer on a chip. Quantum computing is still at least 25 years away, but related quantum information applications are likely to come sooner.

5.2 BALLISTIC DISKS

One of the challenges to cramming more information onto computer hard drives is making a sensor sensitive enough to measure the presence or absence of a magnetic field in a microscopic bit of material. Reading a bit means sensing if its magnetic field affects the flow of electrons through an electric circuit. If the magnetic field is strong enough to change a sensor's electron flow, the bit represents a 1, if not, it is a 0. The smaller the bit, the smaller its magnetic field, and the harder it is to sense the difference between a 1 and a 0.

The key to making sensors that can read smaller bits is increasing the magnetoresistance of the sensor, or read head, used to distinguish the magnetic states of the bits. The higher a material's magnetoresistance, the greater the difference in the number of electrons flowing through it when it is surrounded by a magnetic field versus when it is not. If the difference is significant, it can be used to distinguish weak magnetic fields like those of very small bits that represent 1's from bits that have no magnetic field and represent 0's.

Ballistic magnetoresistance produces a greater difference between 1 and 0 signals and other types of magnetoresistance, but the challenges to using it in disk drives is that it works best with either very strong magnetic fields or extremely low temperatures. Strong magnetic fields can't be used with small bits, and a low-temperature requirement makes for impractical devices.

The device's strong effect at room temperature and in small magnetic fields makes it "potentially very interesting" for data storage technology .The tiny contact point between the wires forces the boundary of the magnetic field to be very narrow, effectively blocking electrons.

The ballistic magnetoresistance the researchers produced could be used in practical applications in 4 to 6 years.

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5.3 ULTRA FAST DRIVES

The slowest part of a typical computer is the hard drive, which is no surprise to anyone who has waited for a PC to start up. There are several steps involved in storing and retrieving data from a disk, but the process of encoding information into magnetically aligned atoms is reaching its practical limits of speed. Researchers have tried reversing the alignment of groups of atoms in as little as 100 picoseconds--at least ten times faster than today's disk drives--by using ultra short laser pulses to influence the material's magnetic properties. The technique allows to investigate the fundamental interactions involved in such fast magnetic switching, and it may lead to extremely fast data storage devices in the future.

Figure 5.2 Microscopic image of a disk

This magnetic force microscope image of a disk shows the individual data bits in tracks of different densities. The far right tracks can store 10 billion bits per square inch. Techniques using ultra fast lasers may allow recording on these tracks at extremely high speeds.

A disk drive "writes" a one or zero by applying a magnetic field to a small region on the disk, which forces the internal "bar magnets" (magnetic moments) of those atoms to align parallel to the field. There is a limit to the speed with which the moment-orienting field can be turned on and off by conventional electronics, but the intrinsic speed limit on flipping magnetic moments may be much higher. To address that question, researchers have taken advantage of ultrafast laser pulse technology. The researchers used a subpicosecond laser pulse to disrupt the coupling between the two materials, freeing the ferromagnet to respond to an oppositely-directed field they had applied from the outside. The team recorded the quick magnetic reversal with a weak laser pulse whose polarization was affected by the direction of atomic moments in the sample.

The whole reversal process occurred in roughly 100 picoseconds (10-10 s), whereas conventional disk drives take more than a nanosecond to flip magnetic moments. "Magneto-optical" disk drives also make use of laser pulses in writing data, but in that technology the light heats the atoms to erase their "memory" of any previous orientation before a magnetic field re-aligns them. The heating process makes those drives even slower than conventional hard drives, although they have other advantages for storing large amounts of data.

While the concept could some day be used in fast data storage, the team is using it to study the process of moment-flipping. Many physicists have studied the reversal of a single atom's magnetic moment, but the collective process of flipping the moments of many thousands of atoms at once is not well understood at a fundamental level. Progress in the basic physics, will certainly advance the technology.

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Although applications of the method are far away, it's certainly a new avenue of thinking, especially the "great idea" of creating a "built-in" magnetic field as part of the material.

5.4 VOLTAGE CONTROL OF SPIN DIRECTION

All over the globe, and particularly in the United States, the pace of progress in computing speed, power, and performance has made the computer industry the fastest-growing, most vibrant in existence. But if its expansion is to continue, eventually the industry must go beyond incremental improvements to embrace radically new technologies. The particles we call electrons have both charge and spin. Conventional electronic devices use only the charge, while spintronic devices take advantage of both properties. When the spins of a material’s electrons are aligned along a common direction, rather than pointing randomly, it is said to be magnetized. Today, most of the information we deal with is processed and stored magnetically. The magnetic recording industry, which includes everything from audio and video products to information storage on computer hard disks, accounts for $150 billion annually. 

A computer’s key components consist of a hard disk, for storage; random access memory, or RAM, for programming; and a central processing unit, or CPU, the "logic device" that performs the computing operations. In present-day machines, the CPU and RAM are semiconductor-based, while the hard disk stores information magnetically.

Figure 5.3 Magnetic Memory Device

An inherent advantage of spintronics over electronics the fact that magnets tend to stay magnetized is sparking industry interest in replacing computers’ semiconductor-based components with magnetic ones, starting with the RAM. If we cut off an electronic device’s power the information stored via electron charges is lost. That is why, before turning a computer off, the user has to save new work to a disk. A computer with all-magnetic RAM would always retain the information put into it. But most important, there would be no "boot-up" waiting period when the power is first turned on a great advantage, especially for the laptop user.

One challenge in realizing magnetic RAM involves addressing individual memory elements, flipping their spins up or down to yield the zeros and ones of binary computer logic. The most commonly envisioned strategy running current pulses through wires to induce magnetic fields that will rotate the elements is flawed, because the fringe fields generated could interfere with neighboring elements.

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Using a change in voltage (not current) to flip the memory elements’ spins produces no magnetic fringe fields. This approach to control offers an inherently better match to spintronic technology. Still in the conceptual stage, voltage-controlled spin rotation is a potentially valuable strategy for the design of magnetic RAM devices. 

Each generation of computer processors has achieved greater efficiency, but CPUs are still hardware rigidly configured and not amenable to change. Computing could be further speeded up and customized for users with different needs if logic devices were reprogrammable. In principle, a magnetic CPU’s architecture could be reconfigured, in real time, for the task at hand.

Reprogrammable magnetic processors could be combined and essentially unlimited magnetic RAM (thus, no need for information storage on disks) with the high density and superior heat-dissipating ability of magnetic materials ferromagnetic layers sandwiched between spacers and insulators and the result could be pocket-size machines surpassing today’s most advanced computers!

Major companies are pursuing the development of magnetic RAM technology, and though it remains farther away, they are also thinking about the possibilities for magnetic CPUs. 

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6.CONCLUSION

The most exciting developments in semiconductor spintronics will probably be devices we have not imagined yet. A key research question for this second category of spintronics is how well electrons can maintain a specific spin state when traveling through a semiconductor or crossing from one material to another. For instance, a spin FET will not work unless the electrons remain polarized on entering the channel and after traveling to its far end.

Recent experiments have successfully driven coherent spins across complex interfaces between semiconductor crystals of different composition (for instance, from GaAs into ZnSe). A wealth of semiconductor applications, from lasers to transistors, are based on heterostructures, which combine disparate materials. The same design techniques can be brought to bear on spintronics.

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