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Spintronics Feb, 2013.

A

SEMINOR

ON

SPINTRONICS

Submitted to SEER AKADEMI of JNTU HYDERABAD

Submitted By

G. AMARENDHAR(11011J6009)

DEPARTMENT OF

VLSI AND EMBEDDED SYSTEMS DESIGN

SEER AKADEMI, JNTU-HYDERABAD

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Spintronics Feb, 2013.

(FEB 2013)

INDEX

* ABSTRACT 3

* HISTORY 4

* SPINTRONICS 5

* WHY IS IT GOING TO BE ONE OF THE RAPIDLY

EMERGING FIELDS? 6

* ELECTRON SPIN: FUNDAMENTALS OF SPIN 7

* GIANT MAGNETO RESISTANCE 8

* CONSTRUCTION OF GMR 9

* SPIN VALVE TECHNOLOGY 10

* MRAM (MAGNETO RESISTIVE RANDOM ACCESS

MEMORY) 11

* METALS-BASED SPINTRONIC DEVICES 13

* SEMICONDUCTOR-BASED SPINTRONIC DEVICES 15

* SPIN TRANSISTOR CONCEPT 16

* LATEST DEVELOPMENTS 17

* CURRENT RESEARCH 18

* CONCLUSION 19

* REFERNCES 20

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Spintronics Feb, 2013.

ABSTRACT

In this paper we will discuss about a field of Nanotechnology,

which is believed to replace conventional electronics in the near future,

i.e. “spintronics”. Research and technology developments in the field of

spintronics have grown tremendously in the past 10-15 years and already

have had a major impact on the data storage industry. The future looks

even brighter, as many new spintronic discoveries have been recently

made a promise of even bigger impact in the future. This paper

summarizes the past accomplishments, describes some of the major

discoveries that will have a lasting impact on the field, and discusses

some of the technologies that may revolutionize data storage in the next

decade.

“Spintronics” is an emergent NANO technology, which uses the

spin of an electron instead of or in addition to the charge of an electron.

Electron spin has two states either “up” or “down”. Aligning spins in

material creates magnetism. Moreover, magnetic field affects the passage

of spin-up and spin-down electrons differently. The paper starts with the

detail description of the fundamentals and properties of the spin of the

electrons. It proceeds with a note on magneto resistance, the development

of Giant Magneto Resistance (GMR) and devices like Magneto Random

Access Memory, which are new versions of the traditional RAM. It

describes how this new version of RAM can revolutionize the memory

industry. There is also detailed explanation of the way, how this

revolution can increase the data density in our memory systems. It is

followed by an account of new Spin Field Effect Transistors. It also

specifies the difference between electronic devices and spintronic

devices.

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Spintronics Feb, 2013.

History

The research field of Spintronics emerged from experiments on

spin-dependent electron transport phenomena in solid-state devices done

in the 1980s, including the observation of spin-polarized electron

injection from a ferromagnetic metal to a normal metal by Johnson and

Silsbee (1985),and the discovery of giant magneto resistance

independently by Albert Fert et al. and Peter Grünberg et al. (1988). The

origins can be traced back further to the ferromagnetic/superconductor

tunneling experiments pioneered by Meservey and Tedrow, and initial

experiments on magnetic tunnel junctions by Julliere in the 1970s. 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.

Theory

Electrons are spin-1/2 fermions and therefore constitute a two-state

system with spin "up" and spin "down". To make a spintronic device, the

primary requirements are a system that can generate a current of spin-

polarized electrons comprising more of one spin species—up or down—

than the other (called a spin injector), and a separate system that is

sensitive to the spin polarization of the electrons (spin detector).

Manipulation of the electron spin during transport between injector and

detector (especially in semiconductors) via spin precession can be

Spin Up (0) Spin Down(1)

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Spintronics Feb, 2013.

accomplished using real external magnetic fields or effective fields

caused by spin-orbit interaction.

Spin polarization in non-magnetic materials can be achieved either

through the Zeeman effect in large magnetic fields and low temperatures,

or by non-equilibrium methods. In the latter case, the non-equilibrium

polarization will decay over a timescale called the "spin lifetime". Spin

lifetimes of conduction electrons in metals are relatively short (typically

less than 1 nanosecond) but in semiconductors the lifetimes can be very

long (microseconds at low temperatures), especially when the electrons

are isolated in local trapping potentials (for instance, at impurities, where

lifetimes can be milliseconds).

SPINTRONICS:

Imagine a data storage device of the size of an atom working at a

speed of light. Imagine a microprocessor whose circuits could be changed

on the fly. One minute is could be optimized for data base access. The

next for transaction processing and the next for scientific number

crunching. Finally, imagine a computer memory thousands of times

denser and faster than today’s memories. The above-mentioned things

can be made possible with the help of an exploding

science–“spintronics”.

Spintronics is a NANO technology which deals with spin

dependent properties of an electron instead of or in addition to its charge

dependent properties, Conventional electronics devices rely on the

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transport of electric charge carries electrons. But there is other

dimensions of an electron other than its charge and mass i.e. spin. This

dimension can be exploited to create a remarkable generation of

spintronic devices. It is believed that in the near future spintronics could

be more revolutionary than any other thing that nanotechnology has

stirred up so far.

WHY IS IT GOING TO BE ONE OF THE RAPIDLY EMERGING

FIELDS?

As there is rapid progress in the miniaturization of semiconductor

electronic devices leads to a chip features smaller than 100 nanometers in

size, device engineers and physists are inevitable faced with a looming

presence of a quantum property of an electron known as spin, which is

closely related to magnetism. Devices that rely on an electron spin to

perform their functions from the foundations of spintronics. Information-

processing technology has thus far relied on purely charge based devices

ranging from the now quantum, vacuum tube today’s million transistor

microchips. Those conventional electronic devices move electronic

charges around, ignoring the spin that tags along that side on each

electron.

1951mercury memory(UNIVAC)

Today0.85”HDD, 4 GBytes, 12.5 MB/sec

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ELECTRON SPIN:

FUNDAMENTALS OF SPIN

1. In addition to their mass and electric charge, electrons have an intrinsic

quantity of angular momentum called spin, almost of if they were tiny

spinning balls.

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

lined up with the spin axis.

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 electronic 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.

Electrons like all fundamental particles have a property called spin,

which can be oriented in one direction, or the other called spin-up or spin-

down. Magnetism is an intrinsic Physical property associated with the

spins. An intuitive notion of how an electron spins is suggested below.

Imagine a small electronically charged sphere spinning rapidly. The

circulating charges in the sphere amount to tiny loops of electric current

which creates a magnetic field.. a spinning sphere in an external magnetic

field changes its total energy according to how its spin vector is aligned

with the spin. In some ways, an electron is just like a spinning sphere of

charge, an electron has a quantity of angular momentum (spin) an

associated magnetism. In an ambient magnetic field and the spin

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Spintronics Feb, 2013.

changing this magnetic field can change orientation. Its energy is

dependent on how its spin vector is oriented. The bottom line is that the

spin along with mass and charge is defining characteristics of an

electron,. In an ordinary electric current, the spin points at random and

plays no role in determining the resistance of a wire or the amplification

of a transistor circuit.

GIANT MAGNETO RESISTANCE:

Magnetism is the integral part of the present day’s data storage

techniques. Right from the Gramophone disks to the hard disks of the

super computer magnetism plays an important role. Data is 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. In this way the read heads detect the data and

sent it to the various succeeding circuits. The magneto resistant devices

can sense the changes in the magnetic field only to a small extent, which

is appropriate to the existing memory devices. When we reduce the size

and increase data storage density, we reduce the bits, so our sensor also

has to be small and maintain very, very high sensitivity. The thought gave

rise to the powerful effect called “GIANT MAGNETO

RESISTANCE” OR (GMR):

Giant magnetoresistance (GMR) came into picture in 1988, which

lead the rise of spintronics. It results from subtle electron-spin effects in

ultra-thin ‘ multilayer’ of magnetic materials, which cause huge changes

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in their electrical resistance when a magnetic field is applied. GMR is 200

times stronger than ordinary magnetoresistance. It was soon realized that

read heads incorporating GMR materials would be able to sense much

smaller magnetic fields, allowing the storage capacity of a hard disk to

increase from 1 to 20 gigabits.

CONSTRUCTION OF GMR:

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. 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. In an ordinary electric Current, the spin points at random

and plays no role in determining the resistance of a wire or the

amplification of a transistor circuit. Spintronic devices, in contrast, rely

on differences in the transport of “spin up” and “spin down” electrons.

When a current passes through the Ferro magnet, electrons of one spin

direction tend to be obstructed.

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A ferromagnet can even affect the flow of a current in a nearby

nonmagnetic metal. For example, in the present-day read heads in

computer hard drives, 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 favoured direction flow through the conductor easily.

In the antiparallel case, all electrons are impeded. The resulting changes

in the current allow GMR read heads to detect weaker fields than their

predecessors; so that data can be stored using more tightly packaged

magnetized spots on a disk.

SPIN VALVE TECHNOLOGY

The first really significant technological discovery was the spin valve, 6 illustrated in Figure 1. This is a multilayer structure incorporating a “magnetically hard” or pinned, ferromagnetic layer on top (consisting of a bilayer of an anti ferromagnet strongly coupled to a ferromagnetic layer), a nonmagnetic conductor layer (typically copper) in the middle, and a “magnetically soft” or “free” layer on the bottom just

Basic heterostructure of a spin valve.

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above the substrate. The pinning of the top ferromagnetic layer significantly biases the switching field for this layer far away from zero fields, so it is not free to rotate at low fields. Thus, “pinning” means that this layer is always pointing in the same direction relative to the substrate. If the magnetic moments in the pinned and free layers are parallel, the current can flow easily throughout the structure, and the resistance is low. However, if the layers are magnetized oppositely, the current is impeded, and the resistance is high. Aspin valve can function as either a magnetic field sensor or a hysteretic memory device, depending on how easy it is to rotate the moment of the free layer from parallel to antiparallel with respect to the pinned layer. In the early 1990s, IBM started a project to develop such GMR devices as read-head sensors for magnetic disk drives and introduced them to the marketplace in 1998. This introduction had an almost immediate impact on disk drive capacity that has lasted to the present day.

MRAM (MAGNETORESISTIVE RANDOM ACCESS MEMORY):

An important spintronic device, which is supposed to be one of the

first spintronic devices that have been invented, is MRAM. Unlike

conventional random-access, MRAMs do not lose stored information

once the power is turned off...A MRAM computer uses power, the four

page e mail will be right there for you. Today’s pc use SRAM and

Figure 3. Schematic illustration of the operation of the toggling method of MRAM switching. Dark arrows represent the direction of magnetization of the film just above the tunnel barrier and determine the magnetoresistance of the tunnel junction. (a) Initial state of the structure (low resistance when the magnetization of the lower ferromagnet is aligned with the pinned layer on the other side of the tunnel barrier). (b) Current in the bit line is turned on, producing a field in the y-direction (Hy), as illustrated by the upper square wave in the lower part of the figure; the magnetizations of both layers rotate and “scissor,” producing a net moment in the y-direction. (c) Current in the word line is turned on (lower square wave in the figure), producing a field in the x-direction (Hx); the two layers scissor more and the direction of the net moment is rotated towards the x-direction. (d) When the bit line current is turned off, the net moment rotates to be aligned with the x-axis. (e) Finally, when the word line current is turned off, the layers have “toggled” a full 180°.

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DRAM both known as volatile memory. They can store information only

if we have power. DRAM is a series of Capacitors; a charged capacitor

represents 1 where as an uncharged capacitor represents 0. To retain 1

you must constantly feed the capacitor with power because the charge

you put into the capacitor is constantly leaking out.

MRAM is based on integration of magnetic tunnel junction (MJT).

Magnetic tunnel junction is a three-layered device having a thin

insulating layer between two metallic Ferro-magnets. Current flows

through the device by the process of quantum tunneling; a small number

of electrons manage to jump through the barrier even though they are

forbidden to be in the insulator. The tunneling current is obstructed when

the two ferromagnetic layers have opposite orientations and is allowed

when their orientations are the same. MRAM stores bits as magnetic

polarities rather than electric charges. When a big polarity points in one

direction it holds1, when its polarity points in other direction it holds 0.

These bits need electricity to change the direction but not to maintain

them. MRAM is non-volatile so, when you turn your computer off all the

bits retain their 1`s and 0`s.

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Metals-based spintronic devices

The simplest method of generating a spin-polarized current in a

metal is to pass the current through a ferromagnetic material. The most

common application of this effect is a giant magneto resistance (GMR)

device. A typical GMR device consists of at least two layers of

ferromagnetic materials separated by a spacer layer. When the two

magnetization vectors of the ferromagnetic layers are aligned, the

electrical resistance will be lower (so a higher current flows at constant

voltage) than if the ferromagnetic layers are anti-aligned. This constitutes

a magnetic field sensor.

Two variants of GMR have been applied in devices: (1) current-in-

plane (CIP), where the electric current flows parallel to the layers and (2)

current-perpendicular-to-plane (CPP), where the electric current flows in

a direction perpendicular to the layers.

Figure 2. Photomicrographs showing the increasing density of prototype magnetic random-access memory (MRAM) chips. (a) IBM 1 mm _ 1.5 mm, 1 kbit chip with a 5.4-m2 twin cell in 0.25-m technology with approximately 3–10-ns access time (from Reference 22, with permission). (b) Motorola 3.9 mm _ 3.2 mm, 256 kbit chip with 7.1-m2 cell in 0.6-m technology with 50-ns access time (from Reference 23,with permission). (c) Motorola 4.25 mm _ 5.89 mm, 1 Mbit chip with 7.1-m2 cell in 0.6-m technology with 50-ns access time (fromReference 24, with permission). (d) Motorola 4.5 mm _ 6.3 mm, 4 Mbit chip with 1.55-m2 cell in 180-nm technology with 25-ns access time(from Reference 17, with permission). (e) IBM 7.9 mm _ 10 mm, 16 Mbit chip with 1.42-m2 cell in 180-nm technology with 30-ns access time

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Other metals-based Spintronics devices:

Tunnel Magneto resistance (TMR), where CPP transport is

achieved by using quantum-mechanical tunneling of electrons

through a thin insulator separating ferromagnetic layers.

Spin Torque Transfer, where a current of spin-polarized electrons

is used to control the magnetization direction of ferromagnetic

electrodes in the device.

Applications

The storage density of hard drives is rapidly increasing along an

exponential growth curve, in part because Spintronics-enabled devices

like GMR and TMR sensors have increased the sensitivity of the read

head which measures the magnetic state of small magnetic domains (bits)

on the spinning platter. The doubling period for the areal density of

information storage is twelve months, much shorter than Moore's Law,

which observes that the number of transistors that can cheaply be

incorporated in an integrated circuit doubles every two years.

MRAM, or magnetic random access memory, uses a grid of

magnetic storage elements called magnetic tunnel junctions (MTJ's).

MRAM is nonvolatile (unlike charge-based DRAM in today's computers)

so information is stored even when power is turned off, potentially

providing instant-on computing. 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

(Everspin, Motorola's spin-off, has since developed a 4 Mbit version).

There are two 2nd generation MRAM techniques currently in

development: 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.

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Another design in development, called Racetrack memory, encodes

information in the direction of magnetization between domain walls of a

ferromagnetic metal wire.

Semiconductor-based spintronic devices:

In early efforts, spin-polarized electrons are generated via optical

orientation using circularly-polarized photons at the band gap energy

incident on semiconductors with appreciable spin-orbit interaction (like

GaAs and ZnSe). Although electrical spin injection can be achieved in

metallic systems by simply passing a current through a Ferro magnet, the

large impedance mismatch between ferromagnetic metals and

semiconductors prevented efficient injection across metal-semiconductor

interfaces. A solution to this problem is to use ferromagnetic

semiconductor sources (like manganese-doped gallium arsenide

GaMnAs), increasing the interface resistance with a tunnel barrier, or

using hot-electron injection.

Spin detection in semiconductors is another challenge, which has been

met with the following techniques:

Faraday/Kerr rotation of transmitted/reflected photons

Circular polarization analysis of electroluminescence

Nonlocal spin valve (adapted from Johnson and Silsbee's work

with metals)

Ballistic spin filtering

The latter technique was used to overcome the lack of spin-orbit

interaction and materials issues to achieve spin transport in silicon, the

most important semiconductor for electronics.

Applications

Advantages of semiconductor-based Spintronics applications are

potentially lower power use and a smaller footprint than electrical devices

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used for information processing. Also, applications such as

semiconductor lasers using spin-polarized electrical injection have shown

threshold current reduction and controllable circularly polarized coherent

light output. Future applications may include a spin-based transistor

having advantages over MOSFET devices such as steeper sub-threshold

slope.

SPIN TRANSISTOR CONCEPT:

Traditional transistors use on-and-off charge currents to create bits

—the binary zeroes and ones of computer information. “Quantum spin

field effect” transistor will use up-and-down spin states to generate the

same binary data. One can think of electron spin as an arrow; it can point

upward or downward; “spin-up and spin-down can be thought of as a

digital system, representing the binary 0 and 1. The quantum transistor

employs also called “spin-flip” mechanism to flip an up-spin to a

downspin, or change the binary state from 0 to 1. One proposed design of

a spin FET (spintronic field-effect transistor) has a source and a drain,

separated by a narrow semi conducting 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 in to the

channel, and this spin current flow easily if it reaches the drain unaltered

(top). A voltage applied to the gate electrode produces an electric field in

Parallel alignment → positive current Antiparallel alignment → negative current

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the channel, which causes the spins of fast-moving electrons to process,

or 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 filed.

Electronic Devices Spintronic devices

1. Based on properties of charge of

the electron

1. Based on intrinsic property spin

of electron

2. Classical property 2. Quantum property

3. Controlled by an external electric

field in modern electronics

3. Controlled by external magnetic

field

4. Materials: conductors and

semiconductors

4.Materials: ferromagnetic

materials

5. Based on the number of charges

and their energy

5. Two basic spin states; spin-up

and spin-down

6. Speed is limited and power

dissipation is high

6. Based on direction of spin and

spin coupling, high speed

LATEST DEVELOPMENTS:

Toshiba developed a spintronics-based MOSFET cell .[DEC 11,2009]

Researchers manipulated and detected spin at room temperature for

the first time .[NOV 27,2009]

Researchers developed a way to control electron spin using pure-

electric means .[OCT 29,2009]

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CURRENT RESEARCH:

Objective 1 : Magnetic FPGAs

The objective will be to design a magnetic FPGA which will incorporate finely distributed Magnetic Tunnel Junctions (MTJs) for non-volatile storage and configuration purposes above of a CMOS core circuit. In complement of existing high density FPGAs, it will provide better versatility with intrinsic re-configurability, instant on/off and energy saving. Such FPGAs can be used as general purpose standalone products. In the SPIN project, the FPGA will be targeted to provide intelligent processing of the magnetometers and sensors developed in objectives 2 and 3.

Objective 2: ultra-high sensitivity "spin valve" based magnetometers for biochips and medical applications, or "Biosensor"

The objective is to develop a new generation of ultra-high sensitivity integrated magnetometers. The highest demand now for this kind of sensors is for medical applications, mainly biochips, bio magnetism and MRI, but there is a large number of potential applications in magnetic imaging for non-destructive evaluation or field sensing for reliability testing in transport, electronics, etc.

Objective 3: highly integrated "spin valve" based current and voltage sensors, or "GMR sensor array"

The objective will be to fabricate a new generation of dense integrated sensors for current and voltage monitoring. One main output is the monitoring of fuel cells and batteries. Typical fuel cells for automotive applications will contain about 240 cells providing each one 1.3V. For the safety of the car and for efficient energy monitoring, it is necessary to follow in real time the voltage behavior of each cell with insulation between the control systems of at least 1.5kV.

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CONCLUSION

Interest in Spintronics arises, in part, from the looming problem of

exhausting the fundamental physical limits of conventional electronics.

However, complete reconstruction of industry is unlikely and Spintronics

is a “variation” of current technology. The spin of the electron has

attracted renewed interest because it promises a wide variety of new

devices that combine logic, storage and sensor applications.

So with this paper we have proved that the new generation of

computing and information technology is on its way to revolutionize the

21st century. We believe it makes sense instead to build on the extensive

foundations of conventional electronic semiconductor technology; we

exploit the spin of the electron and create new devices and circuits, which

could be more beneficial.

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REFERNCES

1. www.wikipidia.com

2. www.mrs.org/bulletin

3. www.dac.neu.edu

4. www.physics.udel.edu

5. www.nano.caltech.edu

6. www.spintronics-info.com

7. www.nanotech-now.com

8. www.technology24.com