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Spintronics
ABSTRACT
Control over spins in the solid state forms the basis for nascent spintronics and
quantum information technologies. There is a growing interest in the use of electronic and
nuclear spins in semiconductor nanostructures as a medium for the manipulation and storage
of both classical and quantum information.
Spin-based electronics offer remarkable opportunities for exploiting the robustness of
quantum spin states by combining standard electronics with spin-dependent effects that arise
from the interactions between Sections, nuclei, and magnetic fields. Here we provide an
overview of recent developments in coherent electronic spin dynamics in semiconductors ant
quantum structures, including a discussion of temporally- and spatially-resolved magnetooptical measurements that reveal an interesting interplay between electronic and nuclear
spins. In particular, we present an electrical scheme for local spin manipulation based on gtensor modulation resonance (g-TMR), functionally equivalent to electron spin resonance
(ESR) but without the use of time dependent magnetic fields.
The technique of g-TMR enables three-dimensional control of electron spins in
nanometer-scale geometries using a single voltage signal. These results provide a compelling
proof of concept that quantum spin Information can be locally manipulated using high-speed
electrical circuits. Furthermore, recent measurements of hybrid ferromagnet / semiconductor
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hetero structures under optical illumination reveal that nuclear spins become highly polarized
at low temperatures.
We explore the potential for exploiting this behavior to create complex nuclear
domains and arrays in lithographically patterned structures. A time-resolved polarization
microscope is used to directly image the nuclear landscape in hybrid nanostructures,
demonstrating the ability to design and control polarization patterns in the semiconductor.
These experiments investigate the electronic, photonic, and magnetic manipulation of
electron and nuclear spins in a variety of semiconductor structures and focus on investigating
the underlying physics for quantum information processing in the solid state.
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CONTENTS
INTRODUCTION
1
ELECTRONICS VS SPINTRONICS
5
SPIN DEVICES
7
GIANT MAGNETO RESISTANCE
8
MEMORYCHIPS
10
SENSORS
12
SPIN-VALVE TRANSISTOR
14
WORKING OF SPINTRONIC DEVICES
18
CONCLUSION
19
REFERENCES
20
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INTRODUCTION
In the weird world of quantum mechanics the fundamental, particle, electron
possesses a property Called ‘spin’. It is not the sort of spin used in common everyday speech
but, the angular momentum or the rotational momentum of a subatomic particle that creates
its own tiny magnetic field. By exploiting this spin property, in a field called spintronics,
computer scientists and physicists have the potential to revolutionise the basis of computer
processing and storage technologies.
‘Spintronics’ can be a fairly new term for you but the concept isn’t so very exotic this
technological discipline aims to exploit the subtle and mind-bending esoteric quantum
properties of the electron to develop a new generation of electronic devices. The word itself is
a blend of electronics with spin, the quantum property it exploits. Like so many words
applied to the subatomic realm, you can refer spin figuratively as a convenient label for a
property that has no equivalent in gross matter.
Every electron exists in one of two states, namely, spin-up and spin-down, with its
spin either +1/2 or - 1/2 (refer Figs 1 and 2). In other words, an electron can rotate either
clockwise or anticlockwise around its own axis with constant frequency. The two possible
spin states naturally represent ‘0’ and ‘1’ states in logical operations. And just because of this
it is possible to make a sandwich of gold atoms between two thin films of magnetic material
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that acts as, a filter or a valve permitting only the electrons in one of the two states to pass.
The filter can be changed from one state to the other using a brief and tiny burst of current.
There are total three categories of spintronics based devices: 1) ferromagnetic metallic alloy
based devices, 2) semiconductor based devices and 3) the devices that manipulate the
quantum spin states of individual electrons for information processing [8].
Ferromagnetic metallic alloy based devices are mainly used in memory and information
storage. They are also termed as magnetoelectronics devices [8]. They rely on the giant
magnetoresistance (GMR) or tunnelling magnetoresistance effect. Magnetic interaction is
well understood in this category of devices [5].
Semiconductor spintronics devices combine advantages of semiconductor with the concept of
magnetoelectronics. This category of devices includes spin diodes, spin filter, and spin FET.
To make semiconductor based spintronic devices, researchers need to address several
following different problems. A first problem is creation of inhomogeneous spin distribution.
It is called spin-polarisation or spin injection. Spin-polarised current is the primary
requirement to make semiconductor spintronics based devices. It is also very fragile state.
Therefore, the second problem is achieving transport of spin-polarised electrons maintaining
their spin-orientation. Final problem, related to application, is relaxation time. This problem
is even more important for the last category devices. Spin comes to equilibrium by the
phenomenon called spin relaxation. It is important to create long relaxation time for effective
spin manipulation, which will allow additional spin degree of freedom to spintronics devices
with the electron charge. Utilizing spin degree of freedom alone or add it to mainstream
electronics will significantly improve the performance with higher capabilities.
The third category devices are being considered for building quantum computers. Quantum
information processing and quantum computation is the most ambitious goal of spintronics
research. The spins of electrons and nuclei are the perfect candidates for quantum bits or
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qubits. Therefore, electron spin and nuclear based hardwares are some of the main candidates
being considered for quantum computers.
Spintronics based devices offers several advantages over conventional charge based devices.
Since magnetised materials maintain their spin even without power, spintronics based devices
could be the basis of non-volatile memory device. Energy efficiency is another virtue of these
devices as spin can be manipulated by low-power external magnetic field. Miniaturisation is
also another advantage because spintronics can be coupled with conventional semiconductor
and optoelectronic devices.
However, temperature is still a major bottleneck. Practical application of spintronics needs
room-temperature ferromagnet in semiconductors. Making such materials represents a
substantial challenge for materials scientists.
Spin of Electrons
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From this simple device it’s hoped to make incredibly tiny chips that will act as superfast memories whose contents will survive loss of power. The adjective is spintronic. The
ability to exploit spin in semiconductors promises new logic devices. With enhanced
fimctiona1ity higher speed, and reduced power consumption, and might spark a revolution in
the semiconductor industry. So far the problem of injecting electrons with a controlled spin
direction has held up the realization of such spintronic devices.
Spin up and down allows two electrons for each set of spatial quantum numbers.
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ELECTRONICS VS SPINTRONICS
One of the most inherent advantages of spintronics over electronics is that magnets
tend to stay magnetised, which is sparking in the industry an interest for replacing computers’
semiconductor-based components with magnetic ones, starting with the random access
memory (RAM), Let me tell you an example: You are in the mid of documenting a project
presentation that you need to present tomorrow morning and you face an electric power
failure. Your UPS was not recharged and, the worst part of all, you didn’t save your
presentation. I am sure that a condition like this is enough, to leave you back, pulling your
hair, for now you have to do the same task right from the scratch.
You need to do all this just because all the information that is stored via electron
charges is lost as soon as you face the power failure. And that is why before turning a
computer off, you are bound to save your new work to a disk.
Imagine a computer that retains all the information put into it: it’s really possible with
all-magnetic RAM. Most importantly, there would be no ‘boot-up’ waiting period when the
power is first turned on a great advantage, especially for laptop users.
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Another promising feature of spintronics is that it does not require the use of unique
and specialised semiconductors, thereby allowing it to work with common metals like copper,
alumimum, and silver. So the cost of such devices for you is unlikely to be high even in the
beginning.
Magnetic Domains in a Spin Valve
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SPIN DEVICES
A spin-valve device consists of two ferromagnetic layers separated by anon-magnetic
metallic spacer layer. The magnetisation direction (or direction of the net magnetic field of
the layer) of one of the two ferromagnetic layers is pinned by an adjacent antiferromagnetic
layer.
When the sensor is passed over the magnetic medium,-it sees a small applied field
that causes the magnetisation of the second ferromagnetic (free) layer to rotate in the plane of
the layer to align with this field and causes a reduction in the resistance of the spin-valve
device. The nature of the reversal of the magnetisation is complex due to magnetic
interactions between the antiferromagnetic layer and the pinned ferromagnetic layer.
Schematic Diagram of a spin valve
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GIANT MAGNETO RESISTANCE
Giant Magneto resistance (GMR) devices
The read heads in modern hard drives and non-volatile, magnetic random access memory
(MRAM) are the two application of GMR effect.
In 1988, Albert Fert’s group discovered GMR effect. They observed that when multi layers of
alternate magnetic/non-magnetic materials carrying electric current were placed in magnetic
field, they exhibit large change in electric resistance, which also known as magnetoresistance
.
Figure1: Giant magneto resistance effect; (a) electron transport takes place when
magnetization direction of both ferromagnetic regions aligned parallel to each other, (b)
electrons are facing high resistance and scattered away near interface when
magnetization direction of both ferromagnetic regions are opposite to each other (b).
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The change in resistance depends on the relative orientation of the magnetization in magnetic
layers. The resistance to passage of current is low when the ferromagnetic layers align in the
same direction and transfer of current takes place dynamically (fig 1 (a)). If they align
themselves in opposite directions electrons scattering occurs near interface and a high
resistance path is produced (fig 1 (b)). The relative orientation of magnetic layers can be
altered by the applying external magnetic field . This effect is called spin-valve effect . These
multi layers are used to configure the GMR devices.
The read heads in hard disk drives utilize spin-valve effect to read data bits. The data bits are
stored as the minute magnetic areas on the surface of HDD . ‘Zero’ is stored, when the
magnetic layers align themselves in one direction and ‘one’ when they align in opposite
directions. The read head reads the data by sensing a change in voltage corresponding to a
change in resistance. It reads 1 when resistance is higher and 0 when resistance is lower.
Thus, the ability of read head to sense minute changes in voltage corresponding to small
changes in magnetic fields will allow data storage at highest packing densities in small
magnetic particles. The expected value of storage densities may reach to 100 gigbites per
square inch by using synthetic Ferromagnets.
When electron spins are aligned (all spin-up or aft spin-down), these 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 is recorded and stored as tiny areas of magnetised 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 a
phenomenon called magneto resistance.
Spintronics burst on the scene in 1988 when French and German physicists
discovered much more powerful giant magneto resistance (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 magneto resistance. IBM soon realized that
read heads incorporating GMR materials can sense much smaller magnetic fields, allowing
the storage capacity of a hard disk to increase from 1 to 20 gigabytes. In 1997 it launched
GMR read heads into the market worth about $1 billion a year.
The basic GMR device is a three-layer sandwich of a magnetic metal (such as cobalt)
with a nonmagnetic metal filling (such as silver). A current passes through the layers
consisting of spin-up and spin-down electrons. The electrons 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.
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If the orientation of one 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 the spin orientations 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.
How magneto resistance works
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MEMORY CHIPS
Physicists have been quick to see further possibilities of spin valves. -The spin valves
are not only the highly sensitive magnetic sensors but these 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 (magnetisations of the layers parallel or anti parallel) as in a conventional
transistor memory device. An obvious application is the magnetic version of the RAM used
in your computer.
The advantage of magnetic random access memory (MRAM) is that it is nonvolatile,
i.e. information isn’t lost when the system is switched off The main advantages of MRAM
devices include lower cost, smaller size, faster speed, and less power consumption. These
devices would be much more robust in extreme conditions such as high temperature and 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 annually.
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For the past three years or so, researchers around the world have been working hard
on a range of MRAM devices. A particularly promising device is the magnetic tunnel
junction that has two magnetic layers separated by an insulating metal-oxide layer Electrons
can tunnel from one layer to the other only when magnetizations of the two layers in the same
direction. Other wise the resistance is high in fact, a thousand times higher than in the
standard spin valve.
Even-more interesting are the devices that combine the magnetic layers with
semiconductors like silicon. The advantage of using silicon is that it is still a favorite with the
electronics industry and is likely to remain so- Such hybrid devices could be made to behave
more A Ttke 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 computing.
Inductive write/GMR read head
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SENSORS
GMR sensors are already being developed in the UK. These have a wide range of
applications and their market is worth $8 billion a year.
Applications include:
•
Fast and accurate position and motion sensing of mechanical components in precision
engineering and robotics.
•
All kinds of automotive sensors for fuel handling systems, electronicengine control,
anti-skid systems, speed control, and navigation.
•
Missile guidance.
•
Position and motion sensing in computer video games.
•
Key-hole surgery and post-operative care.
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SPIN-VALVE TRANSISTOR
A new type of magnetic field sensor is the spin-valve transistor (Fig. 5). This
transistor is based on the magneto resistance found in. multilayers (for example, in
Co/Cu/Co). Usually, the resistance of a multiplayer 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 contributions of interface and bulk
spin-dependent scattering, are difficult to obtain using the CIP geometry items, mainly
because the electrons cross all magnetic layers. But a practical difficulty is encountered: the
perpendicular resistance of the ultra-thin multilayers is too small to be measured by ordinary
techniques.
Band structure of the spin
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Measuring with the current perpendicular to plane (CPP) solves most problems,
mainly because the electrons crass all magnetic layers. But a practical difficulty is
encountered; the perpendicular resistance of the ultra-thin multilayer is too small to be
measured by ordinary techniques.
Schematic cross-section of the spin-valve transistor
Fabrication
The spin-valve transistor consists of silicon emitter, a magnetic multi-layer as the base
and silicon collector (Fig. 6). 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. These electrons form
the collector-current.
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Figure Dutta-Das field effect transistor; at zero gate voltage, electron preserves spin state in
transport channel (a) it enables current flow from source to drain. With applied gate voltage, electrons
change their spin state from parallel to anti parallel to the direction of magnetization of ferromagnetic
layer (b) this offers high resistance to flow of current. Therefore, electron scattering occurs at drain and
no current flow from source to drain .
A Co/Cu multilayer is sputtered on one of the two silicon substrates and these are pressed
together at the last second of the sputter deposition. Because of the smoothness and freshness
of the metal surfaces, spontaneous adhesion occurs at room temperature. A metal layer
between two crystalline semiconductors is accomplished and the bond proves stronger than
silicon. Through lithography processes and wet chemical etching of the top substrate and the
metal base, spin-valve transistors are fabricated.
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Magnetic sensitivity
The number of electrons that reach the collector increases exponentially with the
mean free path of the electrons in the base. The mean free path varies with the applied
magnetic field; hence the collector current becomes strongly magnetic field-dependent.
Example of a LED structure device with magnetic semi conductor
The collector current variation at low temperatures is more than 400 per cent. Even
larger variations are expected with higher-quality bases; (The hysteresis is caused by the
hysteresis of the magnetic layers.) lie extreme magneto sensitivity makes the transistor an
interesting device for high technology read-heads for high-density hard disks and magnetic
RAMs.
Spin injection and spin-polarized transport
The spintronic device requires efficient transport of generated non-equilibrium spin (spinpolarized current) across the electrode/sample interface. The transport of non-equilibrium
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spin across interface is called spin injection. The non-equilibrium spin can be injected by
driving ordinary current through ferromagnetic electrode to sample. The current can be
driven in plane plan of interface called 'current in plane (CIP) geometry' (fig 3(a)) or
perpendicular to the interface called 'current perpendicular to plane (CIP) geometry' (fig
3(b)). The spin can be also injected by optical method. The efficiency of spin injection is
determined by rate of accumulation of non-equilibrium spin in sample. There are several
proposed ways to transport spin-polarized current across interface. These are: (1) formation
of Ohmic contact between electrode-sample interface, (2) Ballistic electron injection, (3)
electron tunneling from space charge region and, (4) Hot spin injection.
Figure 3 current flow across interfaces;
current flow in the plane geometry (CIP) (a),
current flow perpendicular to the plane
geometry (CPP) (b) [4]
Figure 4 Spin injection in non ferromagnetic
region via ferromagnetic region; equivalent
circuit
diagram
for
ferromagnet/nonferromagnet interface (a) accumulation of
nonequilibrium spin at non-ferromagnetic region
(b) non-equilibrium spin state in nonferromagnetic region (c) [4].
Ohmic injection
The most basic approach to spin injection is through the perfect Ohmic contact between
ferromagnetic/non-magnetic (F/N) interfaces (fig 4 (a)). The interface can be produce by
taking metals or semiconductors or superconductors as non-magnetic region with ferromeget.
The degree of spin injection in non-magnetic region depends on the ratio of the conductivities
of ferromagnetic region (F) and non-magnetic region (N). For typical conductivity mismatch,
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when conductivity of F region ≤ N region, higher the spin injection efficiency (fig 4(b) and
(c)). When conductivity of F region ≥ non-magnetic region, smaller the spin injection
efficiency. This phenomenon is called “conductivity mismatch”. In the case of
ferromagnet/semiconductor interface, Ohmic contacts resulted from the doping of
semiconductor surface. However, doping leads to loss of spin polarization by spin-flip
scattering. The electrochemical potential of N region increases with spin injection. The
difference of spin dependent electrochemical potentials generates effective resistance δR on
either side of F/N interface. In superconductor/F interface, increase in total resistance with
spin injection results in switching superconducting state to normal state of much higher
resistance.
WORKING OF SPINTRONIC DEVICES
The information is stored (written) into spins as a particular spin orientation (up or
down). The spins, being attached to mobile electrons, catty the information along a wire, and
the information is read at a terminal.
Spin orientation of conduction electrons survives for a relatively long time
(nanoseconds, compared to term of nanoseconds during which electron momentum decays).
This makes spintronic devices particularly attractive for memory storage and magnetic sensor
applications, and for quantum computing where electron spin would represent a bit (called
qubit) of the information.
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Spin Relaxation
Non-equilibrium spin accumulates in non-magnetic region due to process of spin injection. It
comes to equilibrium by the phenomenon called spin relaxation . The rate of accumulation of
non-equilibrium spin depends on the spin relaxation. Electrons can remember their spin state
for finite period of time before relaxing. That finite time period is called ‘Spin lifetime’.
Longer lifetime is more desirable for data communication application while shorter for fast
switching. The distance traveled by the electron without loosing spin state is called ‘Spin
diffusion length. It is most important variable in spintronic devices, which determines
maximum allowable thickness of the non-magnetic region in device. It is also depend on spin
lifetime. There are four proposed ways by which conduction electrons of metals and
semiconductors relax: (A) The Elliott-Yafet mechanism, (B) The D’yakonov-Perel’
mechanism, (C) The Bir-Aronov-Pikus mechanism, and (D) hyperfine-interaction.
Elliot-Yafet Mechanism
Elliot (1954) first suggested that electron spin relaxation occurs via momentum scattering.
Momentum scattering occurs when lattice ions or photons bring on spin-orbital coupling in
the electron wave function. This spin-orbital coupling introduces wave functions of opposite
spin. Now, electron wave functions with related spin have an admixture of the opposite spin
state.
These combinations of spin-up and spin-down momentum lead to relaxation of
electron spin. The mechanism is dominant in small-gap semiconductors with large spin-orbit
splitting.
D’yakonove-Perel’ Mechanism
This mechanism comes into play, when the systems lack inversion symmetry. The electrons
feel an effective magnetic field, resulting from the lack of inversion symmetry, and from
spin-orbit interaction. These fluctuating magnetic fields randomly change the magnitude and
direction of electron spin precession. They also randomize the spin. This spin randomization
is more effective than momentum scattering. Therefore, spin dephasing occurs because of the
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momentum dependent spin precession along with momentum scattering. This mechanism
plays important role with increase in temperature and increase in band gap.
Bir-Aronov-Pikus Mechanism
The holes also possess spin. The spin of hole can be exchange with conduction electrons.
These exchanges proceed through scattering and lead to spin relaxation of conduction
electron in p-doped semiconductors (Bir, 1975). Holes have shorter spin coherence time and
spin exchange between electrons and holes is very effective. Ultimately, it will leads to spin
decoherence. This mechanism is of importance at low temperatures.
Hyperfine-interaction Mechanism
Hyperfine-interaction comes from the magnetic interaction between the magnetic momentum
of nuclei and electrons. In semiconductor hetrostructures, this mechanism is responsible for
spin dephasing of localized or confined electron spins.
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CONCLUSION
Spintronics is still in its infancy and it’s difficult to predict how it will evolve. New
physics is being discovered and new materials are being developed, such as magnetic
semiconductors and exotic oxides that manifest an even more extreme effect called colossal
magneto resistance.
So what is the future of Spintronics? Two years ago, several experiments
demonstrated huge progress in transporting spins over long distances and in high electric
fields. This year, electrical spin injection, one of the main remaining obstacles of Spintronics,
is on the way to be solved.
Injecting spins with an efficiency of a few per cent using planar contacts or with the
tip of a scanning tunneling microscope is useless for commercial devices. But techniques for
highly efficient spin injection with planar contacts are being developed by various groups and
will probably prove successful in the near future.
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REFERENCES
1. Robert Matthews “Take a spin” New Scientist, Feb. 28, 1998, P. 24-28.
2. Peter Rodgers “Giants in their field” New Scientist, Feb. 10, 1996, P. 34-37.
3. Stuart A. Wolf and Daryl Treger “Spintronics: A new paradigm for electronics for the new
millennium” IEEE transactions on magnetics, Vol, 36, No.5 September 2000.
4. S. Das Sharma, Jaroslav Fabian, Igor zutic “Spintronics: Fundamental and applications”
Reviews of modern Physics, Vol. 76, No. 2, April 2004.
5. M Oestreich, M Bender, JH¨ubner, D H¨agele,WWR¨uhle, Th Hartmann, P J Klar, W
Heimbrodt, M Lampalzer, K Volz and WStolz “Spin injection, Spin transport and spin
coherence” Semiconductor science and Technology 17 (2002) P. 285-297
6.
S.A. Wolf, D. D. Awschalom, D. M. Treger “Spintronics: A spin-based electronics vision for
the future” Science Vol. 294, Nov. 2001.
7. Robert F. Service “Spintronics innovation bids to bolster bids” Science Vol. 297, July 5 2002.
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