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SPINTRONICS

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What is an electron ?

Particle with negative

electric charge , q = -e

Spin =±(1/2) magnetic

moment , m = µB

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What is Spintronics?This refers to the study of :

Role played by electron spin

in solid state physics.

Possible devices that

specifically exploit spin

properties instead of or in

addition to charge.

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ELECTRONICS Vs SPINTRONICS

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Basic Principle:

In Spintronics , information is carried by orientation of

spin rather than charge.

Spin can assume one of the two states relative to the magnetic

field, called spin up or spin down.

These states, spin up or spin down, can be used to represent

‘1’ and ‘0’ in binary logic.

In certain Spintronics materials, spin orientation can be used

as Spintronics memory as these orientation do not change

when system is switched off.

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MOORE’S LAW: It states that the number of transistors on

a silicon chip will roughly double every eighteen months.

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Failure of Moore’s Law : Now a days, the transistors & other components have reached

nanoscale dimensions and further reducing the size would lead

to:

1. Scorching heat making the circuit inoperable.

2. Also Quantum effects come into play at nanoscale

dimensions.

So the size of transistors & other components cannot be

reduced further.

Why do we need Spintronics??

POWER DISSIPATION=GREATEST OBSTACLE FOR MOORE’S LAW !Modern processor chips consume ~100 W of power of which about 20% is wasted in leakage through the transistor gates.

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Spin injection

Spin manipulation

Spin detection

Phases in Spintronics

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Spin Injection

It is the transport of (or creating a) non-equilibrium spin population across interfaceUsing a ferromagnetic electrode

Effective fields caused by spin-orbit interaction.

Tunnel barrier could be used to effectively inject spins into a semiconductor

Tunneling spin injection via Schottky barrier

By “hot” electrons

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Spin Manipulation

To control electron spin to realize desired physical operation efficiently by means of external fields

Mechanism for spin transfer implies a spin filtering process.

Spin filtering means that incoming electrons with spin components perpendicular to the magnetic moment in the ferromagnet are being filtered out.

Spin-polarized current can transfer the angular momentum from carriers to a ferromagnet where it can change the direction of magnetization This effect is equivalent to a spin transfer torque.

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Spin Transfer Torque

The spin of the conduction electron is rotated by its interaction with the magnetization.

This implies the magnetization exerts a torque on the spin. By Conservation of angular momentum, the spin exerts an equal and Opposite torque on the magnetization.

2M1M

S

v v

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Spin Detection Technique

An ultrasensitive silicon

cantilever with a SmCo

magnetic tip positioned

125nm above a silica

specimen containing a low

density of unpaired

electron spins.

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Materials of Spintronics

• Currently used materials in conventional electronics are usually non-magnetic and only charges are controllable.

• Existing metal-based devices do not amplify signals. • Whereas semiconductor based spintronic devices could in

principle provide amplification and serve, in general, as multi-functional devices.

• All the available ferromagnetic semiconductor materials that can be used as spin injectors preserve their properties only far below room temperature, because their Curie temperatures (TC) are low.

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ADVANTAGES OF SPINTRONICS•Non-volatile memory

•Performance improves with smaller devices

•Low power consumption

•Spintronics does not require unique and specialized semiconductors

•Dissipation less transmission

•Switching time is very less

•Compared to normal RAM chips, spintronic RAM chips will:

– increase storage densities by a factor of three

– have faster switching and rewritability rates smaller

•Promises a greater integration between the logic and storage devices

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Giant magneto resistance (GMR) The basic GMR device consists of a layer of non -magnetic metal between two magnetic layers. A current consisting of spin-up and spin-down electrons is passed through the layers. 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.

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Concept of the Giant Magneto resistance (GMR)

1) Iron layers with opposite magnetizations : spin up and spin down are stopped → no current (actually small current only)

HIGH RESISTANCE

2) If a magnetic field aligns the magnetizations: spins go through LOW RESISTANCE

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Parallel and Perpendicular Currents Parallel Current GMR:• Current runs parallel between the

ferromagnetic layers• Most commonly used in magnetic read

heads• Has shown 200% resistance difference

between zero point and antiparallel states

Perpendicular Current GMR:• Easier to understand theoretically, think of

one FM layer as spin polarizer and other as detector

• Has shown 70% resistance difference between zero point and antiparallel states

• Basis for Tunneling MagnetoResistance

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Types of GMR:1) Multilayer GMR

Two or more ferromagnetic layers are

separated by a very thin non-ferromagnetic

spacer (e.g. Fe/Cr/Fe). The GMR effect was first

observed in the multilayer configuration.2) Granular GMR

Granular GMR is an effect that occurs

in solid precipitates of a magnetic material in a

non-magnetic matrix. 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.

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3) Spin Valves

If the orientation of one of the

magnetic layers be changed 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.

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Applications of GMR:

1)Magnetoresistive Random Access Memory:Magnetoresistive 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.

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2)HARD DISK:These devices are a hybrid of a hard disk and more up to date types of memory, such as flash memory, commonly used in digital cameras. Like flash, MRAM has no moving parts and retains all of its data even when the power is switched off. But, like a hard drive, it stores data as magnetic charges.

3)Quantum computer:With quantum computing you are able to attack some problems on the time scales of seconds, which might take an almost infinite amount of time with classical computers.

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4) RACE TRACK MEMORY:

A device that could increase

storage density by up to 100

times. It achieves this by building

"high-rise" chips". The racetrack is

a very tall column of magnetic

material. It is essentially a

magnetic nanowire standing on

end above the surface a silicon

wafer.

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Tunnel Magnetoresistance Magnetic tunnel junction has two

magnetic layers separated by an insulating

metal-oxide layer.

Is similar to a GMR spin valve except that

a very thin insulator layer is sandwitched

between magnetic layers instead of metal

layer .

The difference in resistance between the

spin-aligned and nonaligned cases is much

greater than for GMR device.

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Magnetic Tunnel Junction

• A magnetic tunnel junction (MTJ) consists of two layers of magnetic metal, such as cobalt-iron, separated by an ultrathin layer of insulator.

• Tunnel Magnetoresistive effect combines the two spin channels in the ferromagnetic materials and the quantum tunnel effect

Ferromagneticelectrodes

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Applications of TMR:Magnetoresistive Random Access

Memory

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MRAM combines the best characteristics of Flash, SRAM and DRAM

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Journey of MRAM Problems encountered:• 1. The density of bits was low.• 2. Cost of chips was high. Improved designs to overcome these problems would work

only at liquid nitrogen temperature. An important breakthrough was made in the year 2009. Scientists at the North Carolina State University discovered a

semiconductor material ‘ Galium manganese nitride’ that can store & retain spin orientation at room temperature.

And research is still going on…

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• GMR - Giant magnetoresistance - HDD read heads• MTJ - Magnetic Tunnel Junction - HDD read heads+MRAM• MRAM - Magnetic RAM - nonvolitile memory• STT - Spin Transfer Torque - MRAM+oscillator

Spintronic Research and Applications

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LimitationsProblems that all the engineers and scientists may have to overcome are:• To devise economic ways to combine ferromagnetic metals and

semiconductors in integrated circuits.• To find an efficient way to inject spin-polarized currents, or spin

currents, into a semiconductor.• To create long relaxation time for effective spin manipulation.• What happens to spin currents at boundaries between different

semiconductors?• How long can a spin current retain its polarization in a

semiconductor?

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Future Demands

Moore’s Law states that the number of transistors on a silicon

chip will roughly double every eighteen months

By 2012, it is projected that the width of the electrodes in a

microprocessor will be 40nm across

As electronic devices become smaller, quantum properties of

the wavelike nature of electrons are no longer negligible

Spintronics devices offer the possibility of enhanced

functionality, higher speed, and reduced power consumption.

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REFERENCES

Nanomagnetism and Spintronics, by Teruya shinjo.

The Physics and Chemistry of nanisolids, by Frank

J.Owens, Charles P.Poole.

Introduction to Spintronics, by S.Bandyopadhyay,

M.Cahay.

Nobel lecture: Origin, development and future of

Spintronics, Reveiws of Modern Physics,Vol.80,

October-December 2008.