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Spintronics
Dept of ECE, GVIC Page 1
A Seminar report On
“SPINTRONICS”
Submitted to
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY,
ANANTAPUR
In partial fulfilment of the requirements for the Award of the Degree of
BACHELOR OF TECHNOLOGY
IN
ELECTRONICS & COMMUNICATION ENGINEERING
By
B.DHANA LAKSHMI 119M1A0471
Under the esteemed guidance of
Mr. R. MALLIKARJUN REDDY (M.Tech)
(Head of the department)
DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING
BHARATH EDUCATIONAL SOCIETY’S GROUP OF INSTITUTIONS
GOLDENVALLEY INTEGRATED CAMPUS
(FACULTY OF ENGINEERING)
(Recognized by A.,I.,C.,T.,E., & Affiliated to J.N.T.U.A)
(An ISO 9001:2000 Certified institute)
Angallu, Madanapalli, Chittoor Dist. -517325 ,
(2014-2015

Spintronics
ABSTRACT
Spintronics also known as magneto electronics is an emerging technology that
exploits both the intrinsic spin of the electron and its associated magnetic moment, in addition to
its fundamental electronic charge, in solid-state devices. Spintronics emerged from discoveries in
the 1980s concerning spin-dependent electron transport phenomena in solid-state devices. The
origins of spintronics can be traced back even further to the ferromagnet/superconductor
tunneling experiments, and initial experiments on magnetic tunnel junctions. The use of
semiconductors for spintronics can be traced back at least as far as the theoretical proposal of a
spin field effect- transistor by Datta and Das in 1990. Recently integrated magnetic/spintronic
device micro arrays have demonstrated great potentials in both biomedical research and
practices. Also they have been widely used in creation of Magneto resistive Random Access
Memories. Motorola has developed a 1st generation 256 Kb MRAM based on a single magnetic
tunnel junction and a single transistor and which has a read/write cycle of under 50 nanoseconds.
The IBM-Infineon MRAM Development Alliance has recently developed a prototype 16Mb
MRAM. Thermal Assisted Switching (TAS) which is being developed by Crocus Technology,
and Spin Torque Transfer (STT) on which Crocus, Hynix, IBM, and several other companies are
working. Another design in development, called Racetrack memory, encodes information in the
direction of magnetization between domain walls of a ferromagnetic metal wire.
The Seminar Spintronics and Spintronic devices will give an introduction on
spintronics and will deal with the recent advances of spintronic devices like the MRAM, and will
make a comparison of the other memories available at present and the advantage of the MRAM
and its technical feasibility, the seminar will also cover spintronic logic devices and spintronic
Devices in Magnetic BioSensing.
Keywords:
spintronics, magneto electronics, spin electronics, spin‐based electronics, giant
magnetoresistance, spin valves, magnetic tunneling junctions, tunneling magnetoresistance,
colossal magnetoresistance, spin torque, momentum transfer, spin injection, magnetic
semiconductor, spin coherence, spin Hall effect, anomalous Hall effect.

Spintronics
TABEL OF CONTENTS
CHAPTERS PAGE NO
1. INTRODUCTION ……………………………………………………………….….....6
1.1 . LIMITATIONS OF ELECTRONICS……………………………………………..7
1.1.1. MOORE’S LAW ……………………………................................................7
1.1.2. GATE WIDTH…..……..……………............................................................8
1.2. ALTERNATIVES FOR ELECTRONICS …………………......................................8
2. LIST OF ABBREVIATIONS …………………………………....................................10
3. SPINTEONICS …………………………………………………...................................11
4. A BRIEF HISTORY……………………………………………....................................13
5. SPIN DEVICES………………………………………...………………………….......14
6. GAINT MAGNATORESISTANCE…………………….………………………..…..15
7. TYPES OF GMR……………………………………..………………………….….....18
7.1. MULTILAYER…………………………………………..…..................................18
7.2. GRANULAR ……………………………………...…………………………….........18
7.3. PSEUDO SPIN VALUE………………..……………………………………….…...18
8. MAGNATIC TUNNEL JUNCTIONS …………………….…..…................................20
9. SPIN VALUES…..............................................................................................................22
10. SPIN VALUETRANSISTOR………………….………………...…………..………25
10.1. APPLICATIONS……………………………………………….………….28
10.2. ADVANTAGES……………………………….……..………….………….28
11. SPIN TRANSFER TORQUE……………………..………………...……………….29

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12. SPIN INJECTION INTO SEMICONDUCTOR…………………………………..31
13. SPIN HALL EFFECT ………………………..……………………………………33
14. MAGNETIC RANDOM ACCESS MEMORY …..…………...................................34
14.1. HISTORY OF MRAM …………………………….…………..…………35
14 .2. HOW MRAM WORKS …..…...………………….…………...…………35
15. MAGNETIC (SPIN) TRANSISTORS …………………………………...…………39
16. ADVANTAGES OF SPINTRONICS ……....………………………………….........40
17. DISADVANTAGES OF SPINTRONICS …..…..………………………………......41
18. COMPUTATIONAL BENFITS .……..………………………………………….....42
19. ELECTRONICS V/S SPINTRONICS …..….………………………………………43
20. FUTURE DEMANDS ………………...…………………………………………...…44
21. CONCLUSION ……………………………………………..…………………….......45
22. REFERENCES ……………………………………………………………………….46

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LIST OF FIGURES:
1. MOORE’S LAW ……………………………………………………..7
2. GATE WIDTH ……………………………………………………….8
3. SPINTRONICS ……………………………………………………...12
4. GAINT MAGNETO RESISTANCE …………………………….....16
a. .CURRENT IN PLANE ………………………........................16
b. CURRENT PERPENDICULAR TO THE PLANE…….…...17
5. TUNNEL MAGNETORESISTANCE ……………………….....…..20
6. SPIN VALUE …………………………………………………...........22
7. SPIN VALUE TRANSISTOR ………………………………...……..25
8. ENERGY BAND ……………………………………………...……...26
9. SPIN TORQUE TRANSFER ……………………………………..…29
10. SPIN INJECTION INTO SEMICONDUCTOR ……….….....31
11. MRAM ………………………………………………………..…...…..36
12. SPIN TORQUE – CURRNET INDUCED MAGNETIC
SWITCHING ………………………………………………...………..36
13. MRAM IS A COMBINATION OF EXITING
TECHNOLOIES ……………………………………………………....38
14. MAGNETIC (SPIN) TRANSISTORS ………………………………..39
LIST OF TABLE:
1. COMPARISION OF MEMORY TYPES ……………...38

Spintronics
CHAPTER -1
INTRODUCTION
In our conventional electronic devices charge of electron used to
achieve functionalities and also semi conducting materials for logical operation and magnetic
materials for storage, but spintronics manipulates the electron spin and resulting magnetic
moment,to achieve improved functionalities and also magnetic materials are used for processing
and storage. These spintronic devices are more versatile and faster than the present one.
Spintronics (" SPIN Transport electronics "), also known as magneto electronics, is an emerging
technology that exploits the intrinsic spin of the electron and its associated magnetic moment, in
addition to its fundamental electronic charge, in solid-state devices.
From the above it is clear that understanding spintronics requires understanding of ferro-
magnetism and electron transport and combining them.
Main topics:
 magnetism
 electron transport
 spin transfer torques: interaction between magnetization and electron transport
 spin Hall effect: effects of spin-orbit coupling
Conventional electronic devices rely on the transport of electrical charge carriers – electrons in a
semiconductor such as silicon. Now, however, physicists are trying to exploit the 'spin' of the
electron rather than its charge to create a remarkable new generation of 'spintronic' devices
which will be smaller, more versatile and more robust than those currently making up silicon
chips and circuit elements. During that 50-year period, the world witnessed a revolution based on
a digital logic of electrons. From the earliest transistor to the remarkably powerful
microprocessor in your desktop computer, most electron IC devices have employed circuits that
express data as binary digits, or bits—ones and zeros represented by the existence or absence of
electric charge.

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1.1. Limitations of electronics:
Though the field of electronics is considered to be very vast, even his field is attaining its
limitations. The two main limitations which is propelling the scientists and researchers new
technology are
 Moore’s Law
 Gate Width
1.1.1. .MOORE’SLAW:
1 .Moore, one of the co- founders of Intel Corporation, visualized in the early 1970’s that
the number of transistors fabricated in a single chip will double for every 18 months.
Now, after almost three decades, the number of transistors fabricated in a single chip is so
large that it places severe demands on the material and fabrication technology used.
2 .Moore’s Law, which holds that microprocessors will double in power every 18 months
as electronic devices shrink and more logic is packed into every chip. Moore’s Law has
run out of momentum as the size of individual bits approaches the dimension of atoms—
this has been called the end of the silicon road map. For this reason and also to enhance
the multi functionality of devices investigators have been eager to exploit another property
of the electron—a characteristic known as spin. Spin is a purely quantum phenomenon .
Fig 1 : Moore’s Law

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1.1.2 GATE WIDTH:
Some scientist and experts have predicted that by the year of 2008,
the width of gate electrode in an FET will be around 45nm, which again places severe demands
on the material and fabrication technology used. The figure below shows the variation of the gate
electrode length over the years.
Fig 2 : Gate Width
1.2. ALTERNATIVES OF ELECTRONICS:
Due to the above mentioned limitations many alternatives for electronics have been considered
such as:
 Bottom down approach of fabrication
 Changing the characteristics of info carriers
 Bio-Electronics
 Polymer-Electronics

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 Molecular Electronics
 Spintronics
Of the above alternatives Spintronics has gained prominence because of the fact that spin devices
can be fabricated with small variations to present fabrication technology whereas other
alternatives require complete replacement of present fabrication units.

Spintronics
CHAPTER - 2
LIST OF ABBREVATIONS
MRAM - Magnetic Random Access Memory
TAS - Thermal Assisted Switching
STT - Spin Transfer Torque
AMR - Anisotropic Magneto Resistance
GMR - Giant Magneto Resistance
Fe –Iron
Cr – Chromium
Cu –Copper
RKKY – Ruderman-Kittel-Kasuya-Yosida Coupling mechanism
AFM - Anti Ferro Magnetic
FM – Ferro Magnetic
TMR -Tunnel Magneto Resistance
MTJ - Magnetic Tunnel Junctions
NVRAM - Non Volatile Random Access Memory

Spintronics
CHAPTER - 3
SPINTRONICS
The age of electrically‐based devices has been with us for more than six decades. With more
and more electrical devices being packed into smaller and smaller spaces, the limits of physical
space will prevent further expansion in the direction the microelectronics industry is currently
going. Also, volatile memory, which does not retain information upon being powered off, is
significantly hindering ultrafast computing speeds. However, a new breed of electronics, dubbed
“spintronics,” may change all of that. Instead of solely relying on the electron’s negative charge
to manipulate electron motion or to store information, spintronic devices would further rely on
the electron’s spin degree of freedom, the mathematics of which is similar to that of a spinning
top. Since an electron’s spin is directly coupled to its magnetic moment, its manipulation is
intimately related to applying external magnetic fields. The advantage of spin‐based electronics
is that they are very nonvolatile compared to charge‐based electronics, and quantum‐mechanical
computing based on spintronics could achieve speeds unheard of with conventional electrical
computing. Spintronics, also called magneto electronics, spin electronics, or spin‐based
electronics, is an emerging scientific field. The research on spintronics can be divided into the
following subfields.
The spin can be parallel or anti – parallel. This spin degree can be used to change the way data is
changed or carried. This field of tropics which we in addition to charge of electron, also use the
spin of electrons is called SPINTRONICS. Use of the spin provides additional functionalities
with increased speed.
The figure below explains the concept of Spintronics. It shows that the basic
properties of the electrons such as spin, charge, photon is used for data manipulation and
storage. The interaction between spin, charge, and photon opens a new field called spintronics
Now, let us see the main differences between electronics ad spintronics.

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Fig 3: Spintronics
In present electronics, each function required is designed and fabricated in separate chips
and these chips are interconnected to obtain desired functionalities. For example, in order to store
the data we will use memory unit, to process data we use processor and to transmit/receive data
we may use optical fiber. But, in the case of spintronics basic properties of electron itself is used
hence providing multi – functionalities the properties of electrons used for different functions
are:
Spin  Data
Charge  Processing
Photon  Transmission

Spintronics
CHAPTER - 4
A BRIEF HISTORY
Two experiments in 1920’s suggested spin as an additional property of the electron.
One was the closely spaced splitting of Hydrogen spectral lines, called fine structure. The other
was Stern - Gerlach experiment, which in 1922 that a beam of silver atoms directed through an
inhomogeneous magnetic field would be forced in to two beams. These pointed towards
magnetism associated with the electrons.
In 1965, Gordon Moore, Intel's co-founder, predicted that the number
of transistors on an integrated circuit would double every 18 month. That prediction, now known
as Moore’s Law, effectively described a trend that has continued ever since, but the end of that
trend the moment when transistors are as small as atoms, and cannot be shrunk any further—is
expected as early as 2015.
Magnetoresistance is the property of a material to change the value of
its electrical resistance when an external magnetic field is applied to it. The effect was first
discovered by William Thomson (more commonly known as Lord Kelvin) in 1856, but he was
unable to lower the electrical resistance of anything by more than 5%. This effect was later
termed Anisotropic Magnetoresistance (AMR) to distinguish it from GMR. Spintronics came
into light by the advent of Giant Magneto Resistance (GMR) in 1988. GMR is 200 times
stronger than ordinary Magneto Resistance. It results from subtle electron – spin effects in ultra
multi-layers of magnetic materials that cause a huge change in electrical resistance. Giant
magneto resistance is a quantum mechanical magneto resistance effect observed in thin film
structures composed of alternating ferromagnetic and non magnetic layers.
The 2007 Nobel Prize in physics was awarded to Albert Fert and Peter
Grunberg for the discovery of GMR. The effect is observed as a significant change in the
electrical resistance depending on whether the magnetization of adjacent ferromagnetic layers
are in a parallel or an anti-parallel alignment. The overall resistance is relatively low for parallel
alignment and relatively high for anti-parallel alignment. GMR is used by hard disk drive
manufactures.

Spintronics
CHAPTER - 5
SPIN DEVICES
Instead of solely relying on the electron’s negative
charge to manipulate electron motion or to store information, spintronic devices would further
rely on the electron’s spin degree of freedom, the mathematics of which is similar to that of a
spinning top. Since an electron’s spin is directly coupled to its magnetic moment, its
manipulation is intimately related to applying external magnetic fields. The advantage of
spin‐based electronics is that they are very nonvolatile compared to charge‐based electronics, and
quantum‐mechanical computing based on spintronics could achieve speeds unheard of with
conventional electrical computing. Spintronics, also called magnetoelectronics, spin electronics,
or spin‐based electronics, is an emerging scientific field. The research on spintronics can be
divided into the following subfields.
There are basically two spin devices which have been fabricated in industries and verified
it’s working. They are:
 GMR [Giant Magneto Resistance]
 MTJ [Magnetic Tunnel Junction]

Spintronics
CHAPTER - 6
GIANT MAGNETO RESISTANCE
One spintronic device that currently has wide commercial application is
the spin‐valve. Most modern hard disk drives employ spin‐valves to read each magnetic bit
contained on the spinning platters inside. A spin‐valve is essentially a spin “switch” that can be
turned on and off by external magnetic fields. Basically, it is composed of two ferromagnetic
layers separated by a very thin non‐ferromagnetic layer. When these two layers are parallel,
electrons can pass through both easily, and when they are antiparallel, few electrons will
penetrate both layers.
The principles governing spin‐valve operation are purely quantum mechanical.
Generally, an electron current contains both up and down spin electrons in equal abundance.
When these electrons approach a magnetized ferromagnetic layer, one where most or all
contained atoms point in the same direction, one of the spin polarizations will scatter more than
the other. If the ferromagnetic layers are parallel, the electrons not scattered by the first layer will
not be scattered by the second, and will pass through both. The result is a lower total resistance
(large current). However, if the layers are antiparallel, each spin polarization will scatter by the
same amount, since each encounters a parallel and antiparallel layer once. The total resistance is
then higher than in the parallel configuration (small current).
DISCOVERY:
GMR was independently discovered in 1988 in Fe/Cr/Fe trilayers by a
research team led by Peter Grunberg, who owns the patent, and in Fe/Cr multilayers by the group
of Albert Fert of the University of Paris-Sud, who first saw the large effect in multilayer’s (up to
50% change in resistance) that led to its naming, and first correctly explained the underlying
physics. The discovery of GMR is considered as the birth of Spintronics.
Grunberg and Fert have received a number of prestigious prizes and awards for their discovery
and contributions to the field of Spintronics, including the Nobel Prize in Physics in 2007.

Spintronics
THEORY:
Like other magneto resistive effects, giant magneto resistance (GMR) is
the change in electrical resistance of some materials in response to an applied magnetic field. It
was discovered that the application of a magnetic field to magnetic metallic multilayers such as
Fe/Cr and Co/Cu, in which ferromagnetic layers are separated by nonmagnetic spacer layers of a
few nm thick, results in a significant reduction of the electrical resistance of the multilayer.
Fig 4: Gaint Magneto Resistance
This effect was found to be much larger than other magneto resistive effects that
had ever been observed in metals and was, therefore, called “giant magneto resistance”. In Fe/Cr
and Co/Cu multilayers the magnitude of GMR can be higher than 100% at low temperature
Fig a : Current In Plane

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The change in the resistance of the multilayer arises when the applied
field aligns the magnetic moments of the successive ferromagnetic layers, as is illustrated
schematically in the figure below. In the absence of the magnetic field the magnetizations of the
ferromagnetic layers are antiparallel. Applying the magnetic field, which aligns the magnetic
moments and saturates the magnetization of the multilayer, leads to a drop in the electrical
resistance of the multilayer. Usually resistance of multilayer is measured with the Current in
Plane (CIP). For instance, Read back magnetic heads uses this property. But this suffers from
several drawbacks such as; shunting and channeling, particularly for uncoupled multilayers and
for thick spaced layers diminish the CIP magneto resistance. Diffusive surface scattering reduces
the magneto resistance for sandwiches and thin multilayers.
To erase these problems we measure with Current Perpendicular to the Plane
(CPP), mainly because electrons cross all magnetic layers, but a practical difficulty is
encountered the perpendicular resistance of ultra thin multilayers is too small to be measured by
ordinary technique.
Fig b:Current Perpendicular To The Plane
The use of Micro fabrication techniques for CPP measurements, from
4.2 to 300kwas first shown for Fe/Cr multi layers, where the multilayers were etched into micro
pillars to obtain a relatively large resistance (a few milli ohms). These types of measurements
have confirmed the larger MR for the CPP configuration, but they suffer from general
complexity of realization and measurement techniques. Experiments using electro deposited
nanowires showed CPP MR up to 15% at room temperature.

Spintronics
CHAPTER - 7
TYPES OF GMR
 MULTILAYER
 GRANULAR
 PSEUDO SPIN VALVE
7.1. MULTILAYER:
Two or more ferromagnetic layers are separated by a very thin (about 1
nm) nonferromagnetic spacer (e.g. Fe/Cr/Fe). At certain thicknesses the RKKY1 coupling
between adjacent ferromagnetic layers becomes anti ferromagnetic, making it energetically
preferable for the magnetizations of adjacent layers to align in antiparallel. The electrical
resistance of the device is normally higher in the anti-parallel case and the difference can reach
more than 10% at room temperature. The interlayer spacing in these devices typically
corresponds to the second anti ferromagnetic peak in the AFM-FM oscillation in the RKKY
coupling. The GMR effect was first observed in the multilayer configuration, with much early
research into GMR focusing on multilayer stacks of 10 or more layers.
7.2. GRANULAR:
Granular GMR is an effect that occurs in solid precipitates of a magnetic
material in a non-magnetic matrix. In practice, granular GMR is only observed in matrices of
copper containing cobalt granules. The reason for this is that copper and cobalt are immiscible,
and so it is possible to create the solid precipitate by rapidly cooling a molten mixture of copper
and cobalt. Granule sizes vary depending on the cooling rate and amount of subsequent
annealing. Granular GMR materials have not been able to produce the high GMR ratios found in
the multilayer counterparts.
1. It refers to a coupling mechanism of nuclear magnetic moments or localized inner d or f
shell electron spins in a metal by means of an interaction through the conduction
electrons.
7.3. PSEUDO SPIN VALVE:
Pseudo-spin valve devices are very similar to the spin valve
structures. The significant difference is the coercivities of the ferromagnetic layers. In a pseudo-

Spintronics
spin valve structure a soft magnet will be used for one layer; where as a hard ferromagnetic will
be used for the other. This allows an applied field to flip the magnetization of the hard
ferromagnet layer. For pseudo-spin valves, the non-magnetic layer thickness must be great
enough so that exchange coupling minimized. This reduces the chance that the alignment of the
magnetization of adjacent layers will spontaneously change at a later time.

Spintronics
CHAPTER -8
TUNNEL MAGNETORESISTANCE
The Tunnel magneto résistance (TMR) is a magneto resistive
effect that occurs in magnetic tunnel junctions (MTJs). This is a component consisting of two
ferromagnetisms separated by a thin insulator. If the insulating layer is thin enough (typically a
few nanometers), electrons can tunnel from one ferromagnetic into the other. Since this process
is forbidden in classical physics, the tunnel magneto resistance is a strictly quantum mechanical
phenomenon. Magnetic tunnel junctions are manufactured in thin film technology. On an
industrial scale the film deposition is done by magnetron sputter deposition; on a laboratory scale
molecular beam epitaxial, pulsed laser deposition and electron beam physical vapor deposition
are also utilized. The junctions are prepared by photolithography.
Fig 5 : Tunnel magneto résistance
The direction of the two magnetizations of the ferromagnetic films
can be switched individually by an external magnetic field. If the magnetizations are in a
parallel orientation it is more likely that electrons will tunnel through the insulating film than if
they are in the oppositional (anti parallel) orientation. Consequently, such a junction can be
switched between two states of electrical resistance, one with low and one with very high
resistance.

Spintronics
The effect was originally discovered in 1975 by M. Julliere (University of Rennes,
France) in Fe/Ge-O/Co-junctions at 4.2 K. The relative change of resistance was around 14%,
and did not attract much attention. In 1991 T. Miyazaki (University Tohoku, Japan) found an
effect of 2.7% at room temperature. Later, in 1994, Miyazaki found 18% in junctions of iron
separated by an amorphous aluminum oxide insulator and J. Moodera found 11.8% in junctions
with electrodes of CoFe and Co. The highest effects observed to date with aluminum oxide
insulators are around 70% at room temperature.
The read-heads of modern hard disk drives work on the basis of magnetic
tunnel junctions. TMR, or more specifically the magnetic tunnel junction, is also the basis of
MRAM, a new type of non-volatile memory. The 1st generation technologies relied on creating
cross-point magnetic fields on each bit to write the data on it, although this approach has a
scaling limit at around 90-130 nm. There are two 2nd generation techniques currently being
developed: Thermal Assisted Switching (TAS) and Spin Torque Transfer (STT) on which
several companies are working Further, magnetic tunnel junctions are also used for sensing
applications.

Spintronics
CHAPTER - 9
SPIN VALVE
Two ferromagnetic layers are separated by a thin (about 3 nm) non-
ferromagnetic spacer, but without RKKY coupling. If the coercive fields of the two
ferromagnetic electrodes are different it is possible to switch them independently. Therefore,
parallel and anti-parallel alignment can be achieved, and normally the resistance is again higher
in the anti-parallel case. This device is sometimes also called a spin valve. Spin valve GMR is
the configuration that is industrially most useful, and is used in hard drives. Stuart Parkin and
two groups of colleagues at IBM's Almaden Research Center, San Jose, Calif, quickly
recognized its potential, both as an important new scientific discovery in magnetic materials and
one that might be used in sensors even more sensitive than MR heads. Parkin first wanted to
reproduce the Europeans' results. But he did not want to wait to use the expensive machine that
could make multilayers in the same slow-and-perfect way that Grunberg and Fert had.
So Parkin and his colleague, Kevin P. Roche, tried a faster and less-
precise process common in disk drive manufacturing: sputtering. To their astonishment and
delight, it worked! Par kin’s team saw GMR in the first multilayers they made. This
demonstration meant that they could make enough variations of the multilayers to help discover
how GMR worked, and it gave Almaden's Bruce Gurney and co-workers hope that a room-
temperature, low-field version could work as a super-sensitive sensor for disk drives.
Fig 6: Spin value
The key structure in GMR materials is a spacer layer of a non
magnetic metal between two magnetic metals. Magnetic materials tend to align themselves in the
same direction. So if the spacer layer is thin enough, changing the orientation of one of the
magnetic layers can cause the next one to align itself in the same direction. Increase the spacer

Spintronics
layer thickness and you'd expect the strength of such "coupling" of the magnetic layers to
decrease. But as Parkin's team made and tested some 30,000 different multilayer combinations of
different elements and layer dimensions, they demonstrated the generality of GMR for all
transition metal elements and invented the structures that still hold the world records for GMR at
low temperature, room temperature and useful fields. In addition, they discovered oscillations in
the coupling strength: the magnetic alignment of the magnetic layers periodically swung back
and forth from being aligned in the same magnetic direction (parallel alignment) to being aligned
in opposite magnetic directions (anti-parallel alignment).
The overall resistance is relatively low when the layers were
in parallel alignment and relatively high when in anti-parallel alignment. For his pioneering work
in GMR, Parkin won the European Physical Society's prestigious 1997 Hewlett-Packard
Europhysics Prize along with Gruenberg and Fert. Searching for a useful disk-drive sensor
design that would operate at low magnetic fields, Bruce Gurney and colleagues began focusing
on the simplest possible arrangement: two magnetic layers separated by a spacer layer chosen to
ensure that the coupling between magnetic layers was weak, unlike previously made structures.
They also "pinned" in one direction the magnetic orientation of one layer by adding a fourth
layer: a strong anti ferromagnet. When a weak magnetic field, such as that from a bit on a hard
disk, passes beneath such a structure, the magnetic orientation of the unpinned magnetic layer
rotates relative to that of the pinned layer, generating a significant change in electrical resistance
due to the GMR effect. This structure was named the spin valve. Gurney and colleagues worked
for several years to perfect the sensor design that is used in the new disk drives. The materials
and their tiny dimensions had to be fine tuned so they
1) Could be manufactured reliably and economically,
2) Yielded the uniform resistance changes required to detect bits on a disk accurately, and
3) Werestable -- neither corroding nor degrading -- for the lifetime of the drive.
"That's why it's so important to understand the science," Parkin
says. "IBM's intensive studies of GMR enabled us to enhance considerably the performance of
some low-field sensors." The chief source of GMR is "spin dependent" scattering of electrons.
Electrical resistance is due to scattering of electrons within a material. By analogy, consider how
fast it takes you to drive from one town to another. Without obstacles on a freeway, you can
proceed quickly. But if you encounter heavy traffic, accidents, road construction and other

Spintronics
obstacles, you'll travel much slower. Depending on its magnetic direction, a single-domain
magnetic material will scatter electrons with "up" or "down" spin differently.
When the magnetic layers in GMR structures are aligned anti-
parallel, the resistance is high because "up" electrons that are not scattered in one layer can be
scattered in the other. When the layers are aligned in parallel, all of the "up" electrons will not
scatter much, regardless of which layer they pass through, yielding a lower resistance.

Spintronics
CHAPTER - 10
SPIN VALVE TRANSISTOR
A spin valve multilayer serves as a base region of an n silicon metal base
transistor structure. Metal base transistors have been proposed for ultrahigh frequency operations
because of
1. Negligible base transport time.
2. Low base resistance, but low gain prospects have limited their emergence.
The first evidence of a spin valve effect for hot electrons in Co/Cu multilayers is the spin
valve transistor. In this we see a very large change in collector current (215% at 77K) under
application of magnetic field of 500 Oe. In spin valve transistor (SVT) electrons are injected in
to metallic base across a Schottky barrier (Emitter side) pass through the spin valve and reach the
opposite side (Collector side) of transistor. When these injected electrons traverse the metallic
base electrons are above Fermi level, hence hot electron magneto transport should be considered
in Spin Valve Transistor (SVT).
The transport properties of hot electrons are different from Fermi electrons
.For example spin polarisation of Fermi electrons mainly depends on Density Of States (DOS)
at Fermi level, while the spin polarisation of hot electron is related to the density of unoccupied
states above the fermi level.
CONSTRUCTION:
Fig 7: Spin valve transistor

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The starting material for both emitter and collector is a 380um, 5-
10Ocm, n-si (100) wafer. After back side n++ implantation ,wafer is dry oxidised to anneal the
implant and to form a SIO2 layer .After depositing a Pt ohmic contact on to the back side, wafer
is sawn in to 10X10mm collector and 1.6X1.6mm emitters. Collector is subsequently dipped in
HNO3, 2% HF to remove the native oxide on siliconnfragments,5% Tetra methyl Ammonium
Hydroxide at 90˚, and buffered HF to remove thermal oxide .following each step the collector is
rinsed in demineralised water. After this procedure base multilayer (Cu 2nm/Co 1.5nm), is rf
sputtered through a laser cut metal shadow mask on to the collector substrate defining square
base regions slightly larger than the emitter surface. Directly after cleaning the emitter in a
similar manner its hydrophobic surface is contacted to the multilayer surface, forming a bond
through spontaneous adhesion.
Here metal parts were laid down directly on to the doped Silicon base
layer, which resulted in the information of metal silicides at the interface. These degrade device
performance due to the large depolarising effect they have on the flow of spin polarized charge
carriers through the interface which severely reduces the magnetic sensitivity of devices.
WORKING:
The energy band diagram of the bonded Co/Cu of spin valve transistor is shown
below.
Fig 8: Energy Band Diagram

Spintronics
The collector barrier height about 0.7eV while the emitter
barrier height is 0.6eV.The emitter and collector Schottky barrier are in forward
and reverse bias respectively as illustrated by the CB configuration in Fig. 1. The
emitter bias accelerates the electrons towards the emitter barrier, after which they
constitute the hot “Ballistic” electrons in the base. The probability of passing the
collector barrier is limited by the collisions in the base which effect their energy
and trajectory by optical phonon scattering in the semiconductor and by quantum
mechanical reflections at the base collector interface. For a base transistor with a
single metal base film, this can be expressed by the CB current transfer ratio or
current gain.
αo=(J c – J leak)/ J e=αcαe αqme−w / y
Where
αe = emitter efficiency
αc = collector efficiency
αqm = quantum mechanical transmission
W = base width
And λ is the hot electron mean free path (MFP), in the base. The factor
e−w/ λ represents the probability of transmission of the hot electrons through the base. J c is the
total collector current, J leak is the collector leakage current, determined by the reverse biased
collector Schottky barrier and J e is the injected emitter current. The αc and αe depend among
others, on the type and quality of the semi conductors.
In the SVT under consideration, the thickness of the individual layers
(Co/Cu) are much smaller than the spin-slip diffusion length ( a few nm as compared to several
tens of nm). Neglecting, therefore, spin flip scattering, we consider the spin up and spin down
electrons to carry the current in parallel (two current model). Furthermore it has been shown that
in this limit no spin relaxation occurs in the CPP-MR and that consequently the perpendicular
transport properties can be very simply described by considering a network of serial resistance
for each channel of electrons corresponding to the resistance’s of successive layers and
interfaces.

Spintronics
10.1. APPLICATIONS
 Spin transistors have huge potential for incorporation in stable, high sensitivity
magnetic field sensors for automotive, robotic, mechanical engg. & data storage
applications.
 This may also be used as Magnetically Controlled Parametric Amplifiers &
Mixers, as magnetic signal processors, for control of brush less DC motors & as
Magnetic Logic elements
 In log applications they have the advantage over conventional semiconductor chips
that they do not require power to maintain their memory slate
 It finds its application towards Quantum Computer, a new trend in computing.
 Here we use Qubits instead of bits.Qubit also represents only 1& 0 but here they
show superposition these classical states. But it is in pioneering stage.
There are major efforts ongoing at Honeywell, IBM,
Motorola in developing RAM based on spin valves and metal tunnel junctions such devices
called MRAM have demonstrated faster speed, high density low power consumption, non
volatility and radiation harness they are promising replacements for the Semi Conducting RAM
currently used.
10.2 ADVANTAGES
 Traditional transistors use on & off charge currents to create bits – the binary 0&1 of
Computer information. Quantum spin field effect transistor will use up & down spin
states to generate the same binary data.
 A currently logic is usually carried out using conventional electrons, while spin is used
for memory. Spintronics will combine both.
 In most Semi Conducting transistors the relative proportion of the up & down carries
types are equal. If Ferro Magnetic material is used as the carrier source then the ratio can
be deliberately skewed in one direction.
 Amplification and / or switching properties of the Device can be controlled by the
external magnetic field applied to the device.
 One of the problems of charge current electrons is that we pack more devices together,
the chip heats up. Spin current releases heat but it is rather less.

Spintronics
CHAPTER - 11
SPIN TRANSFER TORQUE
When a current of electrons passes through a magnetized
ferromagnetic layer, it becomes spin polarized in one direction, much like the polarization of
light through a filter. However, spin is the quantum mechanical analogue of angular momentum,
and when a current of electrons gets spin polarized by a ferromagnet, a small transfer of angular
momentum happens between the current and the magnet.
Spin-transfer torque is an effect in which the orientation of a magnetic
layer in a tunnel magnetoresistance or spin valve can be modified using a spin-polarized current.
Charge carriers (such as electrons) have a property known as spin which is a small quantity of
angular momentum intrinsic to the carrier. An electrical current is generally unpolarized
(consisting of 50% spin-up and 50% spin-down electrons); a spin polarized current is one with
more electrons of either spin. By passing a current through a thick magnetic layer, one can
produce a spin-polarized current. If a spinpolarized current is directed into a magnetic layer,
angular momentum can be transferred to the layer, changing its orientation. This can be used to
excite oscillations or even flip the orientation of the magnet. The effects are usually only seen in
nanometer scale devices.
Fig9: Spin Transfer Torque

Spintronics
Spin-transfer torque can be used to flip the active elements in magnetic random
access memory. Spin-transfer torque random access memory, or STT-RAM, has the advantages
of lower power consumption and better scalability over conventional MRAM which uses
magnetic fields to flip the active elements. The name STT-RAM was first coined by Grand is,
Inc. Spin-transfer torque technology has the potential to make possible MRAM devices
combining low current requirements and reduced cost; however, the amount of current needed to
reorient the magnetization is at present too high for most commercial applications, and the
reduction of this current density alone is the basis for current academic research in spin
electronics.
The spin‐torque effect can also be taken advantage of in the design and realization
of ultrahigh frequency (RF) microwave devices such as frequency standard devices, DC to AC
converters, microwave sources, antennas, and isolators.

Spintronics
CHAPTER - 12
SPIN INJECTION INTO SEMICONDUCTORS
The goal of spintronics research is to eventually relieve present
information technology from solely relying on the charge of electrons. The spin degree of
freedom of an electron has shown to be a very viable candidate to save the microelectronics
industry from the results of “Moore’s Law,” which describes a trend of electrical components
getting increasingly smaller, eventually reaching atomic scales. Though much progress has been
made, a final obstacle needs to be overcome for spintronics to emerge as dominant technology.
Spintronics is highly energy efficient, and spintronic devices generate less heat in operation than
semiconductor devices. This unique property may extend the life of “Moore’s Law” by having
higher integration levels without astronomical heat generation
Since nearly all electronic components currently rely on semiconductors,
namely Silicon, it would make sense to interface any new spintronics technology with
semiconductors as well. However, maintaining spin polarization in a semiconductor can prove to
be quite difficult. The major problem is that it is hard to form good atomic interface between a
ferromagnetic metal and a semiconductor. Poor interfaces can cause the electron spins to be
randomized in direction when electrons transit through the interface.
Fig 10 : spin Injection Into Semiconductors
One can eliminate the interface problem by making semiconductors
into ferromagnets. Unfortunately, a semiconductor does not make a good ferromagnet in general.

Spintronics
The magnetic semiconductors often work only at temperatures than room temperature , and
they are not strongly magnetic .
Just recently, researchers have successfully injected spin polarized current into Silicon
from a ferromagnet. Since Si has no nuclear spin, there are no hyperfine interactions, resulting in
very high spin preservation for electrons inside the semiconductor. This “final hurdle” may
finally allow spintronics to emerge as force in electronic technology. More research is needed in
achieving high quality spin injection techniques.

Spintronics
CHAPTER - 13
SPIN HALL EFFECT
In order to realize spintronics as a fully operational technology, the
ability to manipulate spin polarized electrons within a conductor is necessary .A phenomenon
called the spin Hall effect may be the solution.
In the regular Hall effect, if a magnetic field is placed perpendicular to
the direction of current flow in a conductor, a bias voltage will be created perpendicular to both
across the conductor. The reason for this is the electrons in the current interact with the magnetic
field and experience a Lorentz force at right angles to the field and direction of current flow.
They are pushed to one side of the conductor ,and an electric field is created across the
conductor.
In the spin Hall effect, a similar phenomenon occurs. Because the spin of an
electron is coupled to its magnetic moment, if an electric field is placed perpendicular to the
direction of current flow, the electrons’ spin degree of freedom interacts with the field and also
experiences a Lorentz force. However, since electron spin can point either up or down, the two
types of electrons will separate and move to opposite sides of the conductor .Although it was
predicted almost 40 years ago ,the spin hall effect has received significant interest within the past
decade.

Spintronics
CHAPTER - 14
MAGNETIC RANDOM ACCESS MEMORY
All modern hard disks are equipped with two different heads , one for writing
and the other for reading. The principle of the writing head is quite simple, i.e., generation of a
magnetic field as electricity passes through the head. The head focuses this magnetic field
generated to the area on the disk surface where the bit is to be written. Conceptually the
technique for reading is the reverse of that of writing, i.e., using electromagnetic induction and it
was the technique used in earlier hard disks. But as the storage density increased, it became very
difficult to read a bit from the disk surface as there was the interference of magnetic fields from
the neighboring bits.
Magneto resistive Random Access Memory (MRAM) is a non-volatile
computer memory (NVRAM) technology, which has been under development since the 1990s.
Continued increases in density of existing memory technologies notably Flash RAM and DRAM
kept MRAM in a niche role in the market, but its proponents believe that the advantages are so
overwhelming that MRAM will eventually become dominant. Unlike conventional RAM chip
technologies, in MRAM data is not stored as electric charge or current flows, but by magnetic
storage elements.
The elements are formed from two ferromagnetic plates, each of which
can hold a magnetic field, separated by a thin insulating layer. One of the two plates is a
Permanent magnet set to a particular polarity; the other's field will change to match that of an
external field. A memory device is built from a grid of such "cells". Reading is accomplished by
measuring the electrical resistance of the cell. A particular cell is (typically) selected by
powering an associated transistor which switches current from a supply line through the cell to
ground. Due to the magnetic tunnel effect, the electrical resistance of the cell changes due to the
orientation of the fields in the two plates. By measuring the resulting current, the resistance
inside any particular cell can be determined, and from this the polarity of the writable plate.
Typically if the two plates have the same polarity this is considered to mean "0", while if the two
plates are of opposite polarity the resistance will be higher and this means "1". On comparison
with existing memory technologies, MRAM is faster than SRAM, have a higher storage density
than DRAM, the power requirement is less than that of DRAM and it is faster than FLASH.

Spintronics
MRAM is the Memory of the future. If the researches turn up, it will replace both Volatile and
Non Volatile Primary memories.
14.1 HISTORY OF MRAM
2000 - IBM and Infineon established a joint MRAM development program.
2000 - Spintec laboratory's first Spin Torque Transfer patent.
2002 - NVE Announces Technology Exchange with Cypress Semiconductor.
2003 - A 128 kbit MRAM chip was introduced, manufactured with a 180 nm lithographic
process
2004 - Infineon unveiled a 16-Mbit prototype
2005 - Sony announced the first lab-produced spin-torque-transfer MRAM
2007 - Tohoku University and Hitachi developed a prototype 2 Mbit Non-Volatile RAM Chip
employing spin-transfer torque switching
2008 - Scientists in Germany have developed next-generation MRAM that is said to operate with
write cycles under 1 ns.
2009- Hitachi and Tohoku University demonstrated a 32-Mbit spin-transfer torque RAM.
14.2 HOW MRAM WORKS
MRAM (Magneto resistive Random Access Memory) uses
electron spin to store data. Memory cells are integrated on an integrated circuit chip, and the
function of the resulting device is like a semiconductor static RAM (SRAM) chip, with
potentially higher density and the added feature that the data are nonvolatile, that is data are
retained with power off. Typical “classic” or “conventional” MRAM uses spin-dependent tunnel
junction memory cells and magnetic row and column write lines as illustrated. The spin-
dependent tunnel junction produces a large change in resistance depending on the predominant
electron spin in a storage layer. The tunnel barrier is as thin as a few atomic layers--so thin that
electrons can “tunnel” through the normally insulating material, causing a resistance change.
Row and column magnetic write lines allow data to be written to a selected cell in a two-
dimensional array:

Spintronics
Fig 11: MRAM Diagram
Data are written by small electrical currents in the write lines that
create a magnetic fields, which flip electron spins in the spin-dependent tunnel junction storage
layer, thus changing the junction’s resistance. Data is read by the tunneling current or resistance
through the tunnel junction. Next generation MRAM could reduce cell size and power
consumption. Potential next-generation designs include Spin- Momentum Transfer, Magneto-
Thermal MRAM, and Vertical Transport MRAM. Spin-Momentum Transfer (also “Spin-
Transfer,” “Spin Injection,” or “Spin Torque Transfer”) MRAM is based on changing the spin of
storage electrons directly with an electrical current rather than an induced magnetic field. This
method has the potential to significantly reduce MRAM write currents, especially with
lithographic feature sizes less than 100 nanometers. M-T MRAM uses a combination of
magnetic fields and ultra-fast heating from electrical current pulses to reduce the energy required
to write data.
Fig 12: Spin Torque – Current Induced Magnetic Switching

Spintronics
The Table below shows the comparison of various memory type.
Comparesion Of Expected MRAM Features With Other Memory Technologies
SRAM DRAM FLASH MRAM
Real Time Fast Moderate Moderate Moderate-Fast
White Time Fast Moderate Slow Moderate-Fast
Non volatile No No Yes Yes
Refresh N/A Yes N/A N/A
Minimum Cell
Size
Large Small Small Small
Low voltage Yes Limited No Yes
Tabel 1:Memory Types
The elements are formed from two ferromagnetic plates, each of which can hold a
magnetic field, separated by a thin insulating layer. One of the two plates is a permanent magnet
set to a particular polarity; the other's field will change to match that of an external field. A
memory device is built from a grid of such "cells". Reading is accomplished by measuring the
electrical resistance of the cell. A particular cell is (typically) selected by powering an associated
transistor which switches current from a supply line through the cell to ground. Due to the
magnetic tunnel effect, the electrical resistance of the cell changes due to the orientation of the
fields in the two plates. By measuring the resulting current, the resistance inside any particular
cell can be determined, and from this the polarity of the writable plate. Typically if the two plates
have the same polarity this is considered to mean "0", while if the two plates are of opposite
polarity the resistance will be higher and this means "1".

Spintronics
On comparison with existing memory technologies, MRAM is faster
than SRAM, have a higher storage density than DRAM, the power requirement is less than that
of DRAM and it is faster than FLASH. MRAM is the Memory of the future. If the researches
turn up, it will replace both Volatile and Non Volatile Primary memories.
MRAM is a combination of the advantages of exiting technologies.
Fig 13: MRAM is a combination of exiting technologies

Spintronics
CHAPTER - 15
MAGNETIC (SPIN) TRANSISTORS
In an ordinary transistor, specifically an n‐p‐n type transistor, two n-type
semiconductors are separated by a p‐type semiconductor. Near the n‐p‐n type semiconductor.
When a voltage is placed across the p‐type semiconductor, free electrons either are attracted
towards the gate (base) or away from it, depending on the direction of the applied voltage. This
lack or presence of gate electrons controls the flow of current between the two n-type
semiconductor, allowing the transistor to occupy both on and off states.
Fig 14: Magnetic (spin) Transistors
The problem with electrically‐based transistors is their volatility. When power is
shut off, the electrons in the p‐type semiconductor are no longer confined to a single region and
diffuse throughout; destroying their previous on or off configuration .This is the reason why
computers cannot be instantly turned on and off. However, a new type of transistor may change
all of this.
In a magnetic transistor, magnetized ferromagnetic layers replace the role of n and
p‐type semiconductors. Much like in a spin‐valve, substantial current can flow through parallel
magnetized ferromagnetic layers. However, if, say, in a three layer structure, the middle layer is
Antiparallel to the two outside layers, the current flow would be quite restricted, resulting in a
high overall resistance. If the two outside layers are pinned and the middle layer allowed to be
Switched by an external magnetic field, a magnetic transistor could be made, with on and off
configurations depending on the orientation of the middle magnetized layer.

Spintronics
CHAPTER - 16
ADVANTAGES
The various advantages of spintronics is as follows:
1. Low power consumption.
2. Less heat dissipation.
3. Spintronic memory is non-volatile.
4. Take up lesser space on chip, so greater read & write speed.
5. Large storage capacity.
6. No electronic current required.
7. Spintronics RAM chips will:
 Increase storage densities
 Have faster operation
8.Compared to normal RAM chips.

Spintronics
CHAPTER - 17
DISADVANTAGES
The various types of disadvantages of spintronics:
1. High power consumption.
2. High heat dissipation.
3. Electronics memory is volatile
4. Take up higher space on chip, thus less compact.
5. Electron manipulation is lower, so poor read &write speed.
6. Common metals such as Fe, Al, Ag, etc., cannot used.
7. Controlling spin for long distances.
8. Difficult to INJECT and MEASURE spin.
9. Interference with nearest field.
10. Control of spin in Silicon is difficult.

Spintronics
CHAPTER - 18
COMPUTATIONAL BENEFITS
Spin interaction weaker than Coulomb interaction .Spin current flows almost
without dissipation.Spin can flip very fast; requires small energy.─More Miniaturizatio;Higher
speed.
Non-volatile memory,Manipulation by polarized photons ─ New features
 Simple device structure for degree of integration and high process yield.
 Large magneto current for high speed operation.
 High transconductance for high speed operation.
 High amplification capability (V, I, and / or power).
 Bit vs quits.

Spintronics
CHAPTER – 19
ELECTRONICS V/S SPINTRONICS
 One of the main advantage of spintronics over electronics is the magnets tend to stay
magnetize which is sparking in the industry an interest for replacing computer ‘s
semiconductor based components with magnetic ones , starting with the RAM.
 With an all-magnetic RAM, it is niw possible to have a computer that retains all the
information put into it. Most importantly, there will be no ‘boot – up’ waiting period
when power is turned on.

Spintronics
CHAPTER - 20
FUTURE DEMANDS
 Moore’s Law states that the number of transistors on a silicon chip will roughly double
every eighteen months.
 By 2008,it is projected that the width of the electrons in a microprocessor will be 45nm
across.
 As electronic devices become smaller, quantum properties of the wave like nature of
electrons are no longer negligible.
 Spintronic devices offer the possibility of enhanced functionality , higher speed, and
power consumption.
 Another promising feature of spintronics is that it doesn’t require the use of unique and
specialized semiconductor ,there by allowing it to work common metals like Cu, Al,
and Ag.
 Spintronics will use less power than convectional electronics, because the energy
needed to change spin is a minute fraction of what is needed to push charge around.

Spintronics
CHAPTER - 21
CONCLUSION
In this report we have seen the advantages of spintronic devices over the present electronic
devices. As said earlier this is the technology which will replace the present electronics era and
provides the advantages of speed, size, compactness so on. If the applications such as LED and
MRAM can be realized we can attain high efficiency of output in the case of LED and we can
attain high writing speed and reading efficiency in the case of MRAM. Though the area of
spintronics has some drawbacks which will be realized when the spin devices will be fabricated
we may still avoid these drawbacks to large extent.
One drawback of this emerging technology is that since the spintronics is mainly
based on the magnetic properties of the material, the magnetic field of the earth may affect the
magnetic field inside the spin devices and cause errors. One main disadvantages of this is that the
data stored in a MRAM may be altered and hence can lead to errors.
Hopefully, this and many unknown effects will be found out and efforts are made
to avoid such effects and lead to more reliable, more functional, with greater speed of operation
of spin devices will be achieved.

Spintronics
CHAPTER - 22
REFERENCES
[1] Stuart A. Wolf, Jiwei Lu, Mircea R.Stan, Eugene Chen and Daryl M. Tregger the Promise of
Nonmagnetic and Spintronics for future Logic and Universal memory IEEE, 2010.
[2] Michael E. Flatte Spintronics IEEE Transactions on Electronic Devices Vol 54, No.5, 2007.
[3] Shoji Ikeda, Jun Hayakawa Magnetic tunneling Junctions for spintronics Memories and
Beyond IEEE Transactions on Elects onic Devices Vol 54, No.5, 2007.
[4] Feynman, Leighton, Sands Feynman Lectures on Physics, Volume 3

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