Nanoelectronics and Nanophotonics_MNT-301 UNIT-5_(GGCT.pdf

8/14/2019 Nanoelectronics and Nanophotonics_MNT-301 UNIT-5_(GGCT.pdf

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MNT-301

UNIT-5

Introduction to Spintronics

Spin glasses,

Magnetismin metals,

Spindensity waves

Spinpolarizedtransport

Kondoeffect,

Kuboeffect,

Spinvalve effect

Spinrelaxationand injection

Spintronics memorydevicesand applications

Magnetic dipolediodes

Magnetic tunnelingdevices

Spindiodes,

spin filters and

spin transistors

SpinHall bars,.

spin Qbits


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Limitations of Conventional Electronic

The age of electricallybased 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, volati le 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.

Why Spintronic

Spin effects that show up in the resistance change.

the spin allows one to establish polarization-based electronic schemes in addition

to charge-based schemes, just like the polarization of l ight widens the field of

optics dramatically.

A second reason is the increased scattering

length:

under a wide range of circumstances, the spin interacts only weakly with its

environment.

spin is conserved over distances that are much larger than the elastic mean free

path.

This means that, in principle, spin is superior to charge in terms of coherent

effects and for quantum computation applications.


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The goal of spintronics research is to eventually rel ieve 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 Moores

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 Moores Law by having higher integration levels without astronomical heat generation.

Introduction to Spintronicmagnetoelectronics, spin electronics, or spinbased electronics

Spin Electronics is neatly summarized in J. M. D. Coeys observation that

Conventional Electronics has ignored the spin of the electron.

electron spin was known about for most of the 20th Century, no technical use is made

of this fact.

SPIN TRansport electrONICS or SPINTRONICS, in which the spin degree of

freedom of the electron will play an important role in addition to or in place of the

charge degree of freedom in mainstream electronics Spintronics is a multidisciplinary

field whose central theme is the active manipulation of spin degrees of freedom in

solid-state systems..

The goal of spintronics is to understand the interaction between the particle spin and

its solid-state environments and to make useful devices using the acquired

knowledge.

Fundamental studies of spintronics include investigations of spin transport in

electronic materials, as well as of spin dynamics and spin relaxation.


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What is meant by spin in spintronic?

Ans Ensemble spin (Magnetisation)

spin=N-N Spin Polarization ()= Spin/(N+N)

Magnetic Resonance:

Electronic states and magnetism in transition metals and alloys

B=0B

The Two Spin Channel Model

Mott postulated that certain electrical transport in the metallic ferromagnets come into

existence from the ability to consider the spin-up and spin-down conduction electrons as two

independent families of charge carriers, each with its own distinct transport properties.

Motts hypothesis essentially is that spin-flip scattering is sufficiently rare on the timescale of

all the other scattering processes canonical to the problem that defections from one spin

channel to the other may be ignored, hence the relative independence of the two channels.

Spin Asymmetry

The other necessary requirement of this model is that the two spin families contribute very

differently to the electrical transport processes.

This may be because the densities number of each carrier type are different, or it may

because they have different mobilitys in other words that the same momentum or energy

scattering mechanisms treat them very differently.

the ferromagnetic exchange field splits the spin-up and spin-down conduction bands, leaving

different bandstructures at the Fermi surface. If the densities of electron states differs at the

Fermi surface.


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The number of electrons participating in the conduction process is different for each

spin channel.

different densities of states for spin-up and spin-down implies that the susceptibility to

scattering of the two spin types is different, and this in turn leads to their having

different mobilitys.

Spin Accumulation:


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the spin-up electrons are performing the share of the electrical conducting, and,

moreover, that if a current is passed from such a spin-asymmetric material

For cobalt into a paramagnet like silver (where there is no asymmetry between

spin channels), there is a net influx into the silver of up-spins over down-spins.

Thus of up-spins appears in the silver and with it a small associated magnetic

moment per volume. This is known as a spin accumulation.

The field of spin electronics or spintronics has been developed, where the coupling of

electron spin and charge plays an important role.

In paramagnets, the number of up- and down-spin electrons is the same and no effect

of spin appears in the electrical transport.

However, the difference in the number of up- and down-spin electrons in ferromagnets

causes complex properties in which magnetism effects electrical transport and vice

versa.

For example, the control of spins by an electric field and the control of electrical

current by a magnetic field are fundamental issues in the field of spintronics.

Spin polarized current:


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Length scales (L): The fundamental properties of spintronics. There are several length

scales that characterize the properties of electrons in metals.

1. spin-flip mean free path ():

The z-component of spin sz takes one of two values 1/2 and is not necessarily conserved,

that is, it is time dependent due to such effects as the spinorbit interaction (SOI) and

interactions between electrons.

Therefore, the length for which the spin of an electron is conserved is finite, called the spin-

flip mean free path and typically takes values in the range 100 nm10 m.

Spin-flip time (): times the spin-flip time.

3. Fermi wave length F:

which characterizes the electronic states. In general, >> F.

This length scale becomes important when interference occurs between wave functions of

electrons.

The velocity of electrons on the Fermi surface is given by the Fermi velocity vF and hence

the time scale for an electron with vF travelling a distance is given by t= /vF, the

relaxation time.

3. spin-diffusion length (sd):

Due to scattering of electrons, the length an electron travels with a fixed spin direction

is much shorter than the spin-flip mean free path, called the spin-diffusion length.

the spin accumulation decays exponentially away from the interface on a lengthscale

called the spin diffusion length. (the average distance which the spin penetrates

into the nonmagnetic material (perpendicular to the interface)

To find the spin-polarized current in non-magnetic metals it is necessary that the

system length L be much shorter than spin.

3

=

F

sd

4. Spin Accumulation


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Spin Density wave

Spin-density wave (SDW) and charge-density wave (CDW) are names for two

similar low-energy ordered states of solids.

Both these states occur at low temperature in anisotropic, low-dimensional materials or

in metals that have high densities of states at the Fermi level N(EF).

Low-temperature ground states that occur in such materials are superconductivity,

ferromagnetism and antiferromagnetism.

Magnetism in Metals


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Spin Polarization

Spin polarization is the degree to which the spin, i.e., the intrinsic angular momentum

of elementary particles, is aligned with a given direction.

hence to the magnetic moment, of conduction electrons in ferromagnetic metals, such

as iron, giving rise to spin-polarized currents.

pin polarization of electrons or of nuclei, often called simply magnetization, is also

produced by the application of a magnetic field.

Spin Polarized transport

The spin of the electron is an angular momentum intrinsic to the electron that is separate

from the angular momentum due to its orbital motion.

The electrons spin is , implying that the electron acts as a Fermion (by the spin-

statistics theorem).

Like orbital angular momentum, the spin has an associated magnetic moment, the

magnitude of which is expressed as

In a solid the spins of many electrons can act together to affect the magnetic and electronic

properties of a material,

for example, a material with a permanent magnetic moment as in a ferromagnetQ.

In many materials, electron spins are equally present in both the up and the down state, and

no transport properties are dependent on spin.

requires generation or manipulation of a spin-polarized population of electrons, resulting in

an excess of spin up or spin down electrons.

The polarization of any spin dependent property can be written as


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A net spin polarization can be achieved either through creating an equilibrium energy

splitting between spin up and spin down such as putting a material in a large magnetic field

(Zeeman effect) or the exchange energy present in a ferromagnet; or forcing the system outof equilibrium.

The period of time that such a non-equilibrium population can be maintained is known as the

spin lifetime ()

In a diffusive conductor, a spin diffusion length () an also be defined as the distance over

which a non-equilibrium spin population can propagate.

In a pioneering work, Mott (1936a, 1936b) provided a basis for our understanding of spin-

polarized transport.

Mott sought an explanation for an unusual behavior of resistance in ferromagnetic metals.

He realized that at sufficiently low temperatures, where magnon scattering becomes

vanishingly small, electrons of majority and minority spin, with magnetic moment parallel and

antiparallel to the magnetization of a ferromagnet, respectively,

The conductivity can then be expressed as the sum of two independent and unequal parts

for two different spin projections the current in ferromagnets is spin polarized.

This is also known as the two-current model and has been extended by Campbellet al.

(1967) and Fert and Campbell (1968).

ferromagnet/insulator/ Ferromagnetic (F/I/F) junctions

has unambiguously proved that the tunneling current

remains spin polarized even outside of the ferromagnetic

region.

When unpolarized current is passed across a

ferromagnetic, the current becomes spin-polarized


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Kondo effect

The Kondo ef fect is an unusu al scatter ing mechanism of c onduct ion elect rons in a

metal due to m agnet ic impur i t ies, which contr ibutes a term to the elect r ica l resis t iv i tythat increases logarithm ical ly with temp erature as the temperature T is lowered (as

log(T)).

The mechanism of this anomalous resistance increase remained a mystery until Jun Kondo

explained in 1964 that it arises from scattering by the magnetic impurities that are

antiferromagnetically correlated with spins of the conduction electrons.

The temperature which marks the upturn of the resistance due to this spin interaction is

called the Kondo temperature,TK.

The resistance of some

metals containing a small

amount of magnetic

impurities was found to

increase at some low

temperature as depicted

in Fig.

It is sometimes used more generally to describe many-body scattering processes from

impurities.

Calculation: Consider a small amount of magnetic impurities in a metal.

So, to calculate the electrical resistivity arising from these impurities one first calculates the

scattering probability for an electron from a single impurity and then multiplies it by the

number of impurities.

Case: the electron with wave number (k), and spin down () collides with the impurity in a

state with its spin up () and is scattered into a state with wave number (k) with spin down

() while the impurity remains in a state with spin up ()

the matrix element for this process as : J (k , k , ) (eq-1)

where (J) is exchange between the spin of the conduction electron and the localized spin on

the impurity


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Case 2:

the electron is scattered into the state with wave-number(k) and spin () and leaving the

impurity is a spin down state () - a scattering process involving a spin flip of the impurity.

This is only an intermediate state.

and we have to take into account a further scattering process to arrive at the same final state as in

eq-1 in which the spin flip is reversed

means so that the scattered electron is in the state k, and the impurity is returned to the state

with spin up (see figure)

We sum k over all possible intermediate states and so, according to quantum mechanics, the

total matrix element for this process is given by

'' "

"1),',"().,",(

k kk

kfkkJkkJ

Eq (2)


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Kubo effect

In 1962 Ryogo Kubo of the university of Tokyo introduced his concept that the properties of

metal at nanoscale are determined by energy level statistics.


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Spin valve

Effect: When two or more conducting magnetic materials, whose electrical resistance can change

between two values (High or Low) depending on the relative alignment of the magnetisation in the

two layers.

The resistance change is a result of the Giant Magneto-resistive effect.

The magneticlayers align "up" or "down" depending on an external magneticfield.

Principle:

Theprinciplesgoverning spinvalve 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 f irst layer will not be

scatteredby the second, and will pass through both.

The resultis 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).


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The free ferromagnetic layer forms the sensing element and usually consists of Co or

Co90Fe10 or a Ni80Fe20/Co or Ni80Fe20/Co90Fe10 bilayer

The pinned ferromagnetic layer (Co or Co90Fe10) is coupled by exchange to an

antiferromagnet (for example Mn76Ir24, or Mn50Pt50) or a synthetic antiferromagnet

(NiO/Co/Ru/Co, Mn76Ir24/Co90Fe10/Ru/Co90Fe10).

Free and pinned layer easy axis can be set either parallel or antiparallel.

Typical MR values for these first generations of top-pinned or bottom-pinned spin

valves ranged from 6% to 10%.

Here the top-pinned or bottom-pinned designations relate to spin valves where the

pinned layers are above or below the Cu spacer respectively.

Construction:

The spin valve consists of two ferromagnetic

layers, separated by a Cu spacer.

One of these layers has it magnetization pinned

(Strong), while in the other it is free to rotate

(Soft).


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Required conditions: The blocking temperature (temperature where the exchange field

vanishes) should exceed 300C, to prevent accidental de-pinning of the pinned layer during

head fabrication or head life.

Another factor to take into account is the coupling fieldHfbetween the free and pinned layers,

which should not exceed 0.8 to 1.2 kA/m, to allow for proper biasing.

Spin injectionSpin Injection into semiconductors

Spin Injection into Semiconductors from

Ferromagnetic materials:

Injection of spin polarized current into Silicon

from a ferromagnet.

Transfer of spin angular momentum from a ferromagnet to a non-ferromagnetic material.

In above figure in Insulationlayer is added between FM andSC material.

Because, the depolarization processes occurring at the FM side that prevent a massive

injection of spins in the semiconductor part.

This is known as the impedance mismatch problem.

a spin-conserving tunnel barrier (I) at the FM/SC interface to form a FM/I/SC system, can

solve such impedance mismatch to restore a signif icant current spin-polarization in the

semiconductor part.

This results in a spin-splitting between spin-up and spin-down electro-chemical potentials as

the interfacial resistance played by the tunnel junction increases up to a certain threshold

value.


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Drawbacks

In conclusion, spin-injection from ferromagnetic metallic electrodes into semiconductors has

proven difficult. At present, effects due to spin-injection are small of the order of 1% at best.

In some geometries this might be due to an impedance mismatch. If a

ferromagnet/semiconductor contact is analyzed within the twocurrent model, the polarization

of the majority (I)and minority (I) currents is given by

HereR (R) denotes the majority (minority) resistance andRthe total resistance of the

ferromagnet,

Pthe spin-polarization of the ferromagnet,Pthe spin-polarization of the ferromagnet-

semiconductor structure,

Rs the resistance of the semiconductor that is assumed to be unpolarized.

In typical geometriesRs R, such that an impedance mismatch leads to a significant

reduction of the spin-polarization in the whole circuit as compared to the ferromagnetic

electrode.

Magnetic semiconductors:

In this approach spin-injection is facilitated in a band-gap-matched semiconductor

structure containing a strongly paramagnetic component, a so-called spin aligner.

At present three systems have been investigated, namely:

electrondoped Cd0.98Mn0.02Te/CdTe, electron-doped BexMnyZn1xySe/AlGaAs

and hole-doped Ga1xMnxAs/GaAs withx= 0.045.

The Mn doped semiconductors are strongly paramagnetic such that electrons or holes

passing through these can be easily aligned by an applied magnetic field.


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spin relaxation

Spin Glasses A spin glass is a magnet with frustrated interactions, augmented by stochastic (random

variable) disorder, where usually ferromagnetic and antiferromagnetic bonds are randomly

distributed. Its magnetic ordering resembles the positional ordering of a conventional,

chemical glass.

Geometric Frustration:

A simple 2D example is shown in Figure 1. Three

magnetic ions reside on the corners of a triangle with

antiferromagnetic interactions between themthe

energy is minimized when each spin is aligned

opposite to its neighbors.

Once the first two spins align anti-parallel, the third one is frustrated because its two

possible orientations, up and down, give the same energy. The third spin cannot

simultaneously minimize its interactions with both of the other two. Thus the ground state

is two-fold degenerate.

Figure 1: Antiferromagnetically interacting spins in a triangular arrangement


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Similarly in three dimensions, four spins arranged in a tetrahedron (Figure)

may experience geometric frustration. If there is an antiferromagnetic

interaction between spins, then it is not possible to arrange the spins so that

all interactions between spins are antiparallel.

There are six nearest-neighbor interactions, four of which are antiparallel and thus favourable,

but two of which (between 1 and 2, and between 3 and 4) are unfavourable. It is impossible to

have all interactions favourable, and the system is frustrated.

If we consider a tetrahedron with a spin on each vertex pointing

along theeasy axis (that is, directly towards or away from the centre

of the tetrahedron), then it is possible to arrange the four spins so

that there is no net spin (Figure 3).

This is exactly equivalent to having an antiferromagnetic interaction between each pair of

spins, so in this case there is no geometrical frustration.

Geometrical frustration is also possible if the spins are arranged in a non-collinear (not lying

on the same line) way.

Spin Glasses : Magnetic Behaviour:

It is the time dependence which distinguishes spin glasses from other magnetic

systems.

Beginning above the spin glass transition temperature,Tc, where the spin glass

exhibits more typical magnetic behavior (such as paramagnetism as discussed

here but other kinds of magnetism are possible),

if an external magnetic field is applied and the magnetization is plotted versus

temperature, it follows the typical Curie law, in which magnetization is inversely

proportional to temperature untilTc is reached, (at which point the magnetization

becomes virtually constant) (this value is called the field-cooled magnetization).

This is the onset of the spin glass phase.

When the external field is removed, the spin glass has a rapid decrease of

magnetization to a value called the remanent magnetization, and then a slow decay

as the magnetization approaches zero (or some small fraction of the original

valuethis remains unknown).


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This decay is non-exponential and no simple function can fit the curve of

magnetization versus time adequately. This slow decay is particular to spin

glasses.

If a similar test is run on a ferromagnetic substance, when the external field

is removed there is a rapid change to a remanent value that then stays

constant in time.

For a paramagnet, when the external field is removed the magnetization

rapidly goes to zero and stays there. In each case the decay is rapid andexponential.

Spin glasses have applications in neural networks.

Magnetic Tunnel Junction

A magnetic tunnel junction (MTJ) consists of two ferromagnetic electrodes separated by a

tunnel barrier. The resistance across this stack depends on the relative orientation of the

magnetization of the ferromagnetic layers.

The MTJ is composed of a fixed magnetic layer, a thin dielectric

tunnel barrier (typically a few nanometers), and a free magnetic

layer. When applying a current bias to the MTJ, electrons start

moving and become spin-polarized by the magnetic layer as

they traverse the dielectric. If the magnetic vectors are parallel

on both layers, a low resistance is detected and the result is a

zero; otherwise, a high resistance is detected and the result is

a one, (this effect is known as Tunnel magneto resistance)

This process is, strictly a quantum mechanical phenomenon,

because electrons can tunnel from one ferromagnet into the

other.


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Fabrication:

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 epitaxy, pulsed laser deposition and electron

beam physical vapor deposition are also utilized.

The junctions are prepared by photolithography.

Theory:

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 wil l

tunnel through the insulating film than if they are in the oppositional (antiparallel)

orientation.

Consequently, such a junction can be switched between two states of electrical

resistance, one with low and one with very high resistance.


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Spintronics memory devices

and applications

There is a strong interest in non-volatile memory devices based on magnetic materials,

Magnetic Random Access Memories (MRAMs), due to their nonvolatile characteristic,

radiation hardness, non-destructive read-out, low-voltage, and very large (>1015 ) read-write

cycle capability.

The MRAM architecture (Fig.) is an application of spintronics that combines magnetic-

tunnel-junction (MTJ) and CMOS technologies

MRAMs can be as fast as Dynamic Random Access Memories (DRAMs), and almost as

small as Static Random Access Memories (SRAM) in cell size.

To compete with CMOS embedded memories, they must be fabricated with < 125 nm

features, bringing several technological issues regarding micromagnetics and fabrication

issues for deep submicron magnetic elements. MRAMs compete also with Ferroelectric

Random Access Memories (FERAMs) for non-volatile memories.

Unlike conventional RAM chip technologies, in MRAM data is not stored as electric charge

or current flows, but by magnetic storage elements by aligning the spin of an electron.

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 others field can

be changed to match that of an external field to store memory.

This configuration is known as a MTJ and is the simplest structure for a MRAM bit. A

memory device is built from a grid of such "cells".

The simplest method of reading is accomplished by measuring the electrical resistance of

the cell.

Writing data: Data is written to the cells using a variety of means.

In the simplest, each cell l ies between a pair of write lines arranged at right angles to each

other, above and below the cell.

When current is passed through them, an induced magnetic f ield is created at the junction,

which the writable plate picks up.


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Figure shows schematically the MRAM matrix, where each cell consists of a tunnel junction.

In a memory array matrix, to

selectively read a bit, current must flow

through one single junction and

alternative current paths must be

blocked.

This can be achieved making use of

current directionality in diodes or on/off

transistor characteristics.

In the simplest case the basic

memory cell will be a diode

connected in series with a tunnel

junction. Figure shows

schematically the vertical

integration of a tunnel junction

with an amorphous Si diode


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Principle:

A particular cell is (typically) selected by powering an associated transistor that

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 "1",

while if the two plates are of opposite polarity the resistance will be higher and

this means "0".

Advantages of MRAM over DRAM

1. Power Consumstion:

Since the capacitors used in DRAM lose their charge over time, memory assemblies that

use DRAM mustrefresh all the cells in their chips approximately 20 times a second, reading

each one and re-writing its contents. As DRAM cells decrease in size, the refresh cycles

become shorter, and the power-draw more continuous.

In contrast, MRAM never requires a refresh. This means that not only does it retain its

memory with the power turned off but also there is no constant power-draw. While the read

process in theory requires more power than the same process in a DRAM, in practice the

difference appears to be very close to zero. However, the write process requires more

power in order to overcome the existing field stored in the junction, varying from three to

eight times the power required during reading. Although the exact amount of power savings

depends on the nature of the work more frequent writing will require more power in

general MRAM proponents expect much lower power consumption (up to 99% less)

compared to DRAM. STT-based MRAMs eliminate the difference between reading and

writing, further reducing power requirements.


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It is also worth comparing MRAM with another common memory system, flash RAM. Like

MRAM, flash does not lose its memory when power is removed, which makes it very common

as a "hard disk replacement" in small devices such as digital audio players or digital cameras.

When used for reading, flash and MRAM are very similar in power requirements. However,

flash is re-written using a large pulse of voltage (about 10 V) that is stored up over time in a

charge pump, which is both power-hungry and time-consuming. In addition, the current pulse

physically degrades the flash cells, which means flash can be written only to some finite number

of times before it must be replaced.

In contrast, MRAM requires only slightly more power to write than read, and no change in the

voltage, eliminating the need for a charge pump. This leads to much faster operation, lower

power consumption, and an indefinitely long "lifetime".

Performance

DRAM performance is limited by the rate at which the charge stored in the cells can be

drained (for reading) or stored (for writing). MRAM operation is based on measuring

voltages rather than charges or currents, so there is less "settling time" needed. IBM

researchers have demonstrated MRAM devices with access times on the order of 2 ns,

somewhat better than even the most advanced DRAMs built on much newer processes. A

team at the German Physikalisch-Technische Bundesanstalt have demonstrated MRAM

devices with 1 ns settling times, better than the currently accepted theoretical limits for

DRAM, although the demonstration was a single cell. The differences compared to flash are

far more significant, with write times as much as thousands of times faster.

The only current memory technology that easily competes with MRAM in terms of

performance is static RAM, or SRAM. SRAM consists of a series of transistors arranged in a

flip-flop, which will hold one of two states as long as power is applied. Since the transistors

have a very low power requirement, their switching time is very low. However, since an

SRAM cell consists of several transistors, typically four or six, its density is much lower than

DRAM. This makes it expensive, which is why it is used only for small amounts of high-

performance memory, a notable one being the CPU cache in almost all modern CPU

designs.


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Although MRAM is not quite as fast as SRAM, it is close enough to be interesting even in this

role. Given its much higher density, a CPU designer may be inclined to use MRAM to offer a

much larger but somewhat slower cache, rather than a smaller but faster one. It remains to be

seen how this trade-off will play out in the future.

Overall

MRAM has similar performance to SRAM, similar density of DRAM but much lower power

consumption than DRAM, and is much faster and suffers no degradation over time in

comparison to flash memory.

I t is this combination of features that some suggest make it the "universal memory", able to

replace SRAM, DRAM, EEPROM, and flash. This also explains the huge amount of research

being carried out into developing it.

However, to date, MRAM has not been as widely adopted in the market as other non-volatile

RAMs.

It may be that vendors are not prepared to take the risk of allocating a modern fab to MRAM

production when such fabs cost upwards of a few bil lion dol lars to build and can instead

generate revenue by serving developed markets producing flash and DRAM memories.

The very latest fabs seem to be used for flash, for example producing 16 Gbit parts

produced by Samsung on a 50 nm process. Slightly older fabs are being used to produce

most DDR2 DRAM, most of which is produced on a one-generation-old 90 nm process

rather than using up scarce leading-edge capacity.

In comparison, MRAM is still largely "in development", and being produced on older non-

critical fabs.

The only commercial product widely available at this point is Everspins 4 Mbit part,

produced on a several-generations-old 180 nm process.

As demand for flash continues to outstrip supply, it appears that it will be some time before

a company can afford to "give up" one of their latest fabs for MRAM production. Even then,

MRAM designs currently do not come close to flash in terms of cell size, even using the

same fab.

APPLICATIONS:

The read-heads of modern hard disk drives work on the basis of magnetic tunnel junctions.

A new type of non-volatile memory.


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spin Hall effect The Spin Hall Effect (SHE) is a transport phenomenon predicted

by the Russian physicists M.I. D yakonov and V.I. Perel in 1971.

It consists of an appearance of spin accumulation on the lateral

surfaces of a current-carrying sample, the signs of the spin

directions being opposite on the opposing boundaries.

Pas si ng en elec tr ic al c ur ren t t hr ough a c ondu ct or w il l

resu lt in a spin accumu lation at the edges of the

c on du ct or tr an sv er se to t he c ur ren t f lo w, d ue t o s pin -

dependen t scat ter i ng o f f impur it ies (Mot t scat ter i ng).

When the current direction is reversed, the directions of spin

orientation is also reversed.

In Spin Hall Effect, unlike the conventional hall effect, no

magnetic field is needed.

On the contrary, i f a strong enough magnetic field is applied in

the direction perpendicular to the orientation of the spins at the

surfaces,Spin Hall Effect will disappear because of the spin

precession around the direction of the magnetic field.

The bar exhibiting Spin Hall effect is called as the Spin Hall

Bar.

Fig: The Spin up and spin

down electrons are

deflected in two opposite

directions, when they

collide with the impurity

atoms.

Because of the spin-orbit coupling induced either by the impurities or by the host lattice,

electrons with a drift velocity along the sample scatter preferably left if, say, their spin is up, and

right, if their spin is down.

The difference in the scattering probabilities for the two spin orientations is typically small, say,

10 ppm, but even this small difference leads to spin currents transverse to the electron drift

motion.

In a finite sample, the spin currents at the edges need to be balanced by opposing diffusive

currents, which can be set up if there is spin accumulation at the edges, forming a gradient of

the spin density.

The origin of SHE is in the spin-orbit interaction, which leads to the coupling of spin and charge

currents: an electrical current induces a transverse spin current (a flow of spins) and vice versa.

This can be intuitively understood this by using the analogy between an electron and a spinning

tennis ball, which deviates from its straight path in air in a direction depending on the sense of

rotation (the Magnus effect).

The SHE might be used to manipulate electron spins electrically. For example, in combination

with the electric stirring effect, the SHE leads to spin polarization in a localized conducting

region.


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spin filters

A Spin Filter is a device that accepts or allows the electrons with the same spin, as the

device material itself is aligned, to pass through it and blocks the electrons of opposite

spin. Hence the name Spin Filter.

It is just like a tunnel junction, having three layers, arranged in the manner shown in Fig.1

below and is called an N-F-N Tunnel Junction.

These layers are as follows:

The first layer, start ing from left, is made upof a bulk normal metal and is denoted by N.

The second layer, middle one, is made up of ferromagnetic material Europium

chalcogenide and is denoted by F.

And the third layer, the right most, is again made up of a bulk normal metal and is denoted

by N.

Working:

To obtain the spin filtering, a magnetic f ield has to be appliedto the ferromagnetic f ilm F, so as to

induce the spin polarization into it. The orientation of all the spins in a material, in the same

particular direction, by application of a magnetic field or by shining a circularly polarized light, is

called as Spin Polarization.

In both the normal metal f ilms N, the electrons are unpolarized. This means that the spins of all

the electrons present in these films are randomly oriented (some may be up spin and some may

be down spin).

Now when a voltage is applied across the device, as shown in Fig.1, the electrons start f lowingfrom film N(left), through film F, to the film N(right).

But here, only the electrons of N layer, that are oriented in the same direction, as the spin

polarized electrons of film F, constitutes the current or are allowed.

to pass through. While the electrons that have opposite spin, recombine are become inactive.

Hence they do not flow down.

Hence, the tunneling current through F f ilm gets polarized because of difference in tunneling

barrier heights between N and F for two spin orientations.

Note: It has been proposed that spin filtering of Eu-chalcogenides with one electron

quantum dots may form an important ingredient in quantum computing.


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spin diodes Spin diodes are inhomogeneous two-terminal devices whose electronic or optical properties

depend on the spin polarization of the carriers.

a siliconp-njunction is shown whose current was modified by changing the spin polarization

of the recombination centers.

In a magnetic field both the mobile

carriers and the recombination centers

have an equil ibrium spin polarization

due to the Zeeman splitting.

The current in ap-njunction depends on

the recombination rate, which, in turn,

depends on the relative orientation of

the spin of the carriers and the centers

Several spin diodes have recently been proposed or demonstrated with the goal of either

maximizing the sensitivity of theI-Vcharacteristics to spin and magnetic field, or facilitating

spin injection and its detection through semiconductor interfaces comprising a magnetic

semiconductor as the injector.

The role of inhomogeneous doping in thep-n junction is played by the inhomogeneous spin

splitting of the carrier band, with the spin-up and spin-down carriers playing roles similar to

those of the electrons and holes in bipolar diodes.

Structure: Si-based p-i-n diode sandwiched between two ferromagnetic metals was

suggested to allow controlling the device performance by an externally applied magnetic field.

(it is discussed hear)

The magnetic bipolar diode or SPIN DIODE is ap-njunction diode with one or both regions

magnetic.

The MBD (Magnetic Bipolar Device) is the prototypical device of bipolar spintronics, a subfield

of spintronics in which both electrons and holes take part in carrier transport, while either

electrons or holes (or both) are spin polarized.

The most useful effects of the spin-charge coupling in MBDs are the giant magnetoresistive

effects.


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Scheme of a magnetic bipolar diode. Thep region (left) is

magnetic, indicated by the spin splitting of the conduction

band.

Then region (right) is nonmagnetic, but spin polarized by a

spin source: Filled circles, spin-polarized electrons; empty

circles, unpolarized holes.

If the nonequilibrium spin in then region is oriented parallel (top figure) to the equilibrium

spin in thep region, large forward current flows.

If the relative orientation is antiparallel (bottom), the current drops significantly.

Thep region is magnetic, by which we mean that it has a spin-split conduction band with the

spin splitting (Zeeman or exchange)

Zeeman splitting can be significantly enhanced magnetically doped or narrow-band-gapsemiconductors.

Using an MBD with a ferromagnetic semiconductor slightly above itsTC is also expected to

give largeg* factors.

Then region is nonmagnetic, but electrons can be spin polarized by a spin source (circularly

polarized light or magnetic electrode).

V-I Characteristics of Spin diode:

To illustrate the I-V characteristics of MBDs, consider the low-injection limit in the

configuration of Fig. The electron contribution to the total electric current is:


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spin transistors

There are two proposed schemes for spin transistors, one in analogy with the bipolar

transistor and other in analogy witn the field effect transistor.

Construction:

Just as in a conventional bipolar transistor, we have

an emitter, a base and a collector, in the bipolar

spin-transistor we have a semiconductor emitter, a

magnetic semiconductor base and a semiconductor

collector.

The emitter and collector are non-magnetic n-type

semiconductors.

The base is a p-type magnetic semiconductor, in which the conduction band is split due to

exchange interaction.

The junction between emitter and base is forward biased while the junction between base and

collector is reverse biased.

Working:

If a non equlibrium polarization is produced in the emitter, the spin polarized electrons will

flow towards the base.

A few of these electrons recombine with holes in the base to produce base current.

The rest of the spin polarized electrons will flow through the thin base to collector.

We may define the current amplification factor as the ratio of the collector current IC to

the base current IB.

But the current amplif ication factor will have different values for posit ive and negative

non-equilibrium polarizations.

Such a transistor has not yet been realized.


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Spin FET

The generic spintronic scheme on a prototypical device, the Datta-Das spin field-effect

transistor

The scheme shows the structure of the usual FET, with a drain, a source, a narrow

channel, and a gate for controlling the current.

The gate either allows the current to flow (ON)

or does not (OFF).

Construction:

In the Datta-Das SFET the source and the

drain are ferromagnets acting as the injector

and detector of the electron spin.

The gate either allows the current to flow (ON) or does not (OFF).

SourceDrain

WORKING:

In the Datta-Das SFET the source and the drain are ferromagnets acting as the injector

and detector of the electron spin.

The drain injects electrons with spins parallel to the transport direction.

The electrons are transported ballistically through the channel

When they arrive at the drain, their spin is detected. In a simpli fied picture, the electron

can enter the drain (ON) if its spin points in the same direction as the spin of the drain.

Otherwise it is scattered away (OFF).

The role of the gate is to generate an effective magnetic f ield.

This effective magnetic field causes the electron spins to precess.

By modifying the voltage, one can cause the precession to lead to either parallel or

antiparallel (or anything between) electron spin at the drain, effectively controlling the

current.


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Spin Q-bits

Semiconductor quantum dots are small devices in which charge carriers are confined in all

three dimensions: this can be achieved by electrical gating and/or etching techniques

applied e. g. to a two-dimensional electron gas.

In the quantum-dot scenario either the electron spin or the charge (orbital) degrees of

freedom can be chosen as the qubit. Here we will describe the spin dotwhich has two

immediate advantages:

the qubit represented by a real spin-1/2 is always well-defined as the two-dimensional

Hilbert space is the entire space available and therefore there are no extra dimensions into

which the qubit state could "leak";

real spins have quite long dephasing times (order of microseconds in GaAs).

In order to be able to perform quantum computation, in addition to a well-defined qubit, we also

need a controllablesource of entanglement, i. e. a mechanism by which two specific qubits at

a time can be entangled so as to produce the fundamental CNOT gate operation. This can be

achieved by temporarily coupling two spins via the Heisenberg Hamiltonian


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