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ABSTRACT
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. 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 ......................................................................6
4.2 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.INTRODUCTION
As 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 SPIN
An 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.
2
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 1
2
1
2
1
2
1
2 1
2
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 socalled 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 socalled Greenberger-Horne-Zeilinger (GHZ) states, which are entangled superpositions
of three qubits
"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
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
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,
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.
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Figure 3.2 Teleportation theory
In this scheme Alice wants t
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 whatsoe
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
instantly - which would violate special relativity. However, although Bob's photon
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
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
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materials like iron and cobalt. Magnetism is an intrinsic physical property associated
with the spins of electrons in a material.
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
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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.
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 semiconductors 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
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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.
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 DattaDas 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
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incorporate magnetism by doping the semiconductor crystals with atoms such as
manganese.
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 momentorienting 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’ semiconductorbased 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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