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MRAM using Spintronics

Dr. Ambedkar Institute of Technology Dept of E&C 2010-2011 Page 1

INTRODUCTION

All materials have an inherent magnetic character arising from the movements of their electrons.

Since dynamic electric fields induce a magnetic field, the orbit of electrons, which creates atomic

current loops, results in magnetic fields. An external magnetic field will cause these atomic

magnetic fields to align so that they oppose the external field. This is the only magnetic effect

that arises from electron pairs. If a material exhibits only this effect in an applied field it is

known as a diamagnetic material.

Magnetic properties other than diamagnetism, which is present in all substances, arise from the

interactions of unpaired electrons. These properties are traditionally found in transition metals,

lanthanides, and their compounds due to the unpaired dand felectrons on the metal. There are

three general types of magnetic behaviors: paramagnetism, in which the unpaired electrons are

randomly arranged, ferromagnetism, in which the unpaired electrons are all aligned, and

antiferromagnetism, in which the unpaired electrons line up opposite of one another.

Ferromagnetic materials have an overall magnetic moment, whereas antiferromagnetic materials

have a magnetic moment of zero. A compound is defined as being ferrimagnetic if the electron

spins are orientated antiparrallel to one another but, due to an inequality in the number of spins in

each orientation, there exists an overall magnetic moment. There are also enforced ferromagnetic

substances (called spin-glass-like) in which antiferromagnetic materials have pockets of aligned

spins


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Types of magnetism: (A) paramagnetism (B) ferromagnetism (C) antiferromagnetism (D)

ferrimagnetism (E) enforced ferromagnetism

Magnetic character of materials is typically analyzed relative to its magnetic susceptibility ().

Magnetic susceptibility is the ratio of magnetization (M) to magnetic field (H). The type of

magnetic behavior of a compound can be defined by its value of (see Table 1 for a

comparison of magnetic behavior versus and Table 2 for the susceptibilities of some common

paramagnetic materials).

Magnetic Behavior Value of

Diamagnetic small and negative

Paramagnetic small and positive

Ferromagnetic large and positive

Antiferromagnetic small and positive


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Table 1. Magnetic behavior versus values of magnetic susceptibility

Compound/

Element

Formula Mass

Susceptibility

(m) (m3/kg)

Mass Susceptibility

(m) (emu/Oeg) x

10-3

Cerium Ce 64.84 5.160

Chromium(III) oxide Cr2O3 24.63 1.960

Cobalt(II) oxide CoO 61.57 4.900

Dysprosium Dy 1301 103.500

Dysprosium oxide Dy2O3 1126 89.600

Erbium Er 556.7 44.300

Erbium oxide Er2O3 928.9 73.920

Europium Eu 427.3 34.000

Europium oxide Eu2O3 126.9 10.100

Gadolinium Gd 9488 755.000

Gadolinium oxide Gd2O3 668.5 53.200

Iron(II) oxide FeO 90.48 7.200

Iron(III) oxide Fe2O3 45.06 3.586

Iron(II) sulfide FeS 13.5 1.074

Neodymium Nd 70.72 5.628

Neodymium oxide Nd2O3 128.2 10.200

Potassium

superoxide

KO2 40.59 3.230

Praseodymium Pr 62.96 5.010

Samarium Sm 28.02 2.230

Samarium oxide Sm2O3 24.98 1.988

Terbium Tb 1822 146.000

Terbium oxide Tb2O3 984.4 78.340


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Thulium Tm 320.4 25.500

Thulium oxide Tm2O3 646.5 51.444

Vanadium oxide V2O3 24.83 1.976

Table 2. Mass susceptibilities of some common paramagnetic materials [emu = electromagnetic

unit (10-3ampm2), Oe = Oersted 1034 -1ampm-1)]

Antiferromagnetic materials can be distinguished from paramagnetic substances, in that the

value of increases with temperature, whereas shows no change or decreases in value as

temperature rises for paramagnetic compounds. Ferromagnetic and antiferromagnetic materials

will lose magnetic character and become paramagnetic if sufficiently heated. The temperature at

which this occurs is defined as the Curie temperature (Tc) for ferromagnetic compounds and the

Nel temperature (TN) for antiferromagnetic compounds. Some substances, particularly the later

lanthanides, will go from paramagnetic to antiferromagnetic to ferromagnetic as temperature

decreases (Table 3).

Curie

Temperature

Nel

Temperature

Curie

Temperature

Nel

Temperature

Metal TC (C) TN (C) TC (K) TN (K)

Ce -260.65 12.5

Pr -248 25

Nd -254 19

Sm -258.35 14.8

Eu -183 90

Gd 20 293

Tb -51 -44 222 229

Dy -188 -94 85 179


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Ho -253 -142 20 131

Er -253 -189 20 84

Tm -248 -217 25 56

Table 3. Curie and Nel temperatures of some lanthanides.1

There are several unique properties of magnetic materials which are exploited. Changing

magnetic fields induce an electrical voltage making magnetic materials a central component of

nearly all electrical generators. Magnetic materials are also essential components for

information storage in computers, sensors, actuators, and a variety of telecommunications

devices ranging from telephones to satellites.

Some materials, known as soft magnetic materials, exhibit magnetic properties only when they

are exposed to a magnetizing force such as a changing electric field. Soft ferromagnetic

materials are the most common of these as they are widely used in both AC and DC circuits to

amplify the electrical flux. Magnetic nanopowders have shown great promise in advanced soft

magnetic materials.2 Magnetocaloric materials heat up in the presence of a magnetic field and

subsequently cool down when removed from the magnetic field. Pure iron, for example will

change temperature by 0.5 2.0 C/Tesla. More recently alloys of the formula Gd5SixGe1-x

(where x = 0 5) will exhibit a 3 4 C/Tesla change.3,4 Some nanomagnetic materials have

shown significant magnetocaloric properties.

In general, molecule-based magnets have magnetic properties comparable to traditional

magnets. However, being molecular, they have many advantages over metal-based magnets in

terms of device fabrication. For example, they can be deposited as thin films by lowtemperature

(40 C) CVD, are low density, and can be transparent. This makes them ideal candidates for

such advanced devices utilizing magnetic imaging, data storage, magnetic shielding, or


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magnetic induction. In addition, molecule-based magnets can have more specialized properties

such as photomodulated magnetization.

Theories likening electron-transfer salts to molecular magnets date back to 1963. The

phenomenon was not observed until 1985,however, when Miller and coworkers identified

ferromagnetism in decamethylferrocenium tetracyanoethenide [Fe(Cp*)2][TCNE]. Since this

time many electron-transfer salts of decamethylmetallocenes with TCNE or TCNQ (7,7,8,8-

tetracyano-pquinodimethane) have been reported.

Structures of 7,7,8,8-tetracyano-p-quinodimethane (TCNQ) and tetracyanoethenide (TCNE).

Electron-Transfer Salt Curie Temperature(K)

Critical Temperature(K)

[CrCp*2]

+[TCNE]

-22.2 3.65

[CrCp*2]+[TCNQ]- 12.8 3.5

[FeCp*2]

+[TCNE]

-16.8 4.8

[FeCp*2]

+[TCNQ]

-12.3 2.55

[MnCp*2]

+[TCNE]

-22.6 8.8

[MnCp*2]

+[TCNQ]

-10.5 6.5

Table 5. Curie and critical temperatures of some Electron-Transfer salts.


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One aspect of molecule-based magnets that distinguish them from traditional magnetic materials

is dimensionality. Molecular magnets are often only magnetic in a single direction or along a

single dimension. For example, hexylammonium trichlorocuprate(II) (CuCl3(C6H11NH3) or

CHAC) consists of double-bridged chain of CuCl3 units with the hexylamine cations hydrogen

bonding parallel chains together . There is a ferromagnetic interaction along the chain within the

orthorhombic crystal structure.

The CuCl3 core of CHAC (chlorine atoms are not labeled).

The interaction parameter (J) along the c axis in figure 5 (JC) is 100 cm-1

and ferromagnetic.

This is about four orders of magnitude larger than the ferromagnetic interaction along the b axis

(Jb 10-1

cm-1

) and more than five orders of magnitude larger than the antiferromagnetic

interaction along the a axis (Ja = < -10-2 cm-1). Many ferromagnetic chain molecules have since

been reported among them [MnCu(dto)2(H2O)34.5 H2O] (dto = dithiooxalato) was the first

characterized.

Spintronics emerged from discoveries in the 1980s concerning spin-dependent electron transport

phenomena in solid-state devices. This includes the observation of spin-polarized electron


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injection from a ferromagnetic metal to a normal metal by Johnson and Silsbee (1985),and the

discovery of giant magnetoresistance independently by Albert Fert et al. and Peter Grnberg et

al. (1988). The origins of spintronics can be traced back even further to the

ferromagnet/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

Moore's Law - a dictum of the electronics industry that says the number of transistors that fit

on a computer chip will double every 18 months - may soon face some fundamental

roadblocks. Most researchers think there'll eventually be a limit to how many transistors they

can cram on a chip. But even if Moore's Law could continue to spawn ever-tinier chips, small

electronic devices are plagued by a big problem: energy loss, or dissipation, as signals pass

from one transistor to the next. Line up all the tiny wires that connect the transistors in a

Pentium chip, and the total length would stretch almost a mile. A lot of useful energy is lost as

heat as electrons travel that distance.

Theoretical physicists have found a way to solve the dissipation problem by manipulating a

neglected property of the electron - its ''spin,''or orientation, typically described by its quantum

state as ''up'' or down..

Electronics relies on Ohm's Law, which says application of a voltage to many materials results

in the creation of a current. That's because electrons transmit their charge through the

materials. But Ohm's Law also describes the inevitable conversion of electric energy into heat

when electrons encounter resistance as they pass through materials.

''Unlike the Ohm's Law for electronics, the new 'Ohm's Law' that we've discovered says that the

spin of the electron can be transported without any loss of energy, or dissipation. Furthermore,


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this effect occurs at room temperature in materials already widely used in the semiconductor

industry, such as gallium arsenide. That's important because it could enable a new generation of

computing devices.''

A celestial analogy to explain two important properties of electrons - their center

of mass and their spin: ''The Earth has two kinds of motion. One is that its center of mass

moves around the Sun. But the other is that it also spins by itself, or rotates. The way it

moves around the Sun gives us the year, but the way it rotates around by itself gives us the

day. The electron has similar properties.'' While electronics uses voltage to move an electron's

center of mass, spintronics uses voltage to manipulate its spin.

Ferromagnetic metallic alloy based devices are mainly used in memory and information storage.

They are also termed as magnetoelectronics devices . They rely on the giant magnetoresistance

(GMR) or tunnelling magnetoresistance effect. Magnetic interaction is well understood in this

category of devices

Semiconductor spintronics devices combine advantages of semiconductor with the concept of

magnetoelectronics. This category of devices includes spin diodes, spin filter, and spin FET. To

make semiconductor based spintronic devices, researchers need to address several following

different problems. A first problem is creation of inhomogeneous spin distribution. It is called

spin-polarisation or spin injection. Spin-polarised current is the primary requirement to make

semiconductor spintronics based devices. It is also very fragile state. Therefore, the second

problem is achieving transport of spin-polarised electrons maintaining their spin-orientation .

Final problem, related to application, is relaxation time. This problem is even more important for

the last category devices . Spin comes to equilibrium by the phenomenon called spin relaxation.

It is important to create long relaxation time for effective spin manipulation, which will allow

additional spin degree of freedom to spintronics devices with the electron charge . Utilizing spin


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degree of freedom alone or add it to mainstream electronics will significantly improve the

performance with higher capabilities.

The third category devices are being considered for building quantum computers. Quantum

information processing and quantum computation is the most ambitious goal of spintronics

research. The spins of electrons and nuclei are the perfect candidates for quantum bits or qubits.

Therefore, electron spin and nuclear based hardwares are some of the main candidates being

considered for quantum computers.

Spintronics based devices offers several advantages over conventional charge based devices.

Since magnetized materials maintain their spin even without power, spintronics based devices

could be the basis of non-volatile memory device. Energy efficiency is another virtue of these

devices as spin can be manipulated by low-power external magnetic field. Miniaturization is also

another advantage because spintronics can be coupled with conventional semiconductor and

optoelectronic devices.

However, temperature is still a major bottleneck. Practical application of spintronics needs

room-temperature ferromagnet in semiconductors. Making such materials represents a

substantial challenge for materials scientists.

Spin based Devices

The present status of spintronics devices at the commercial level is limited to giant

magnetoresistance (GMR) based devices. In GMR based memory devises electron spin play

passive role . It is limited to detect the change of magnitude of resistance depending on direction

of the spin . The change in resistance is controlled by a local or an external magnetic field . But,

it is predicted that spintronics can go beyond this passive spin device by integrating electron spin


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into conventional semiconductors. Thus, the technology based on spintronics may replace

conventional semi-conducting devices by introducing active control of electron spin.

Giant Magnetoresistance (GMR) devices

The read heads in modern hard drives and non-volatile, magnetic random access memory

(MRAM) are the two application of GMR effect.

In 1988, Albert Ferts group discovered GMR effect. They observed that when multi layers of

alternate magnetic/non-magnetic materials carrying electric current were placed in magnetic

field, they exhibit large change in electric resistance, which is also known as magneto

resistance.


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Figure 2

Figure 2 Giant magneto resistance effect; (a) electron transport takes place when magnetization

direction of both ferromagnetic regions aligned parallel to each other, (b) electrons are facing

high resistance and scattered away near interface when magnetization direction of both

ferromagnetic regions are opposite to each other (b).

The change in resistance depends on the relative orientation of the magnetization in magnetic

layers . The resistance to passage of current is low when the ferromagnetic layers align in the

same direction and transfer of current takes place dynamically (fig 1 (a)). If they align

themselves in opposite directions electrons scattering occurs near interface and a high resistance

path is produced (fig 1 (b)). The relative orientation of magnetic layers can be altered by the

applying external magnetic field . This effect is called spin-valve effect . These multi layers are

used to configure the GMR devices.

The read heads in hard disk drives utilize spin-valve effect to read data bits. The data bits are

stored as the minute magnetic areas on the surface of HDD . Zero is stored, when the magnetic


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layers align themselves in one direction and one when they align in opposite directions . The

read head reads the data by sensing a change in voltage corresponding to a change in resistance .

It reads 1 when resistance is higher and 0 when resistance is lower . Thus, the ability of read

head to sense minute changes in voltage corresponding to small changes in magnetic fields will

allow data storage at highest packing densities in small magnetic particles . The expected value

of storage densities may reach to 100 gigbites per square inch by using synthetic Ferromagnets.

There are three types of GMR.

Spin transistors

The spin-transistors exploit electron spin either by spin-valve effect or by active control of

electron spin . The design of transistor is similar to that of GMR devices. It consists of three

layers, out of which the non-magnetic layer is sandwiched between the two ferromagnetic layers

Johnson was the first to propose about spin-valve transistor. As per him, the first magnetic layer

acts as a spin injector or emitter while the second acts as a spin detector or collector . The non-

magnetic layer acts as a base . The magnetization direction of the collector can be changed by the

application of an external magnetic field . When the voltage is applied across the emitter-base, it

generate electrons with either spin-up or spin-down . When the magnetization direction of

emitter and collector is parallel, the current can flow throw the base to the collector . The

electrons face high resistance when the relative magnetization direction is opposite. Thus, device

acts as one-way switch . Electron spin plays passive role in Johnsons spin-valve transistor.


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Figure 3 Dutta-Das field effect transistor; at zero gate voltage, electron preserves spin state in

transport channel (a) it enables current flow from source to drain. With applied gate voltage,

electrons change their spin state from parallel to anti parallel to the direction of magnetization of

ferromagnetic layer (b) this offers high resistance to flow of current. Therefore, electron

scattering occurs at drain and no current flow from source to drain .

The first model of transistor using active control of electron spin was proposed by Datta and Das.

In the Datta-Das field effect transistor, the non-magnetic layer acts as a gate while two

ferromagnetic layers act as source and drain respectively (fig 2(a)) . The gate plays an important

role in Datta-Das field effect transistor. The gate modifies electron spin by generating effective

magnetic field and thereby in switching the transistor . When voltage is applied to the gate, it

generates effective magnetic field (fig 2(b)). Thus, by modifying gate voltage one can modify

electron spin [4]. The electrons ballistically transport in transport channel, if its spin is parallel to

the magnetization direction of drain (spin detector) . Otherwise, it will scattered away .


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The control of charge current in spin transistor is similar to the conventional transistors [2, 4].

But, the spin transistors possess advantage over conventional transistors. They are smaller in

size, and consume less power . Still, the spin-transistors are exist is in prototype models because

of theoretical limitation related to spin behavior in different materials.

Manipulation of Electron Spin

Spintronic devices are based on careful manipulation of the electron spin. The spin can be easily

manipulated by applying external magnetic field or by shining polarized light . In general, the

scheme of spin manipulation works fundamentally on: (1) generation of spin-polarized electron,

(2) injection and transportation of the spin-polarized electron, and (3) detection of the spin-

polarized carriers with information.

Generation of spin polarization

The generations of spin-polarized electron spins mean generation of spin polarized current. This

spin polarized current carries non-equilibrium spin population. The Spintronics devices detect

the distribution of spin-up and spin-down electrons in spin polarized current to control the

current . This phenomenon of controlling current in spintronic device makes it suitable to act as

electronic switch of transistor. Thus, the control of current is then either a control of phase of

electron spin or spin-population. It can be generated by transport, optical, and resonance methods

or by their combination . Figure 2 shows the schematic representation of generation of spin-

polarized current by transport method.


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Figure 4 current flow across interfaces;

current flow in the plane geometry (CIP)

(a), current flow perpendicular to the plane

geometry (CPP) (b)

Figure 5 Spin injection in non ferromagnetic region via

ferromagnetic region; equivalent circuit diagram for

ferromagnet/non-ferromagnet interface (a) accumulation of

nonequilibrium spin at non-ferromagnetic region (b) non-

e uilibrium s in state

Spin injection and spin-polarized transport

The spintronic device requires efficient transport of generated non-equilibrium spin (spin-

polarized current) across the electrode/sample interface. The transport of non-equilibrium spin

across interface is called spin injection. The non-equilibrium spin can be injected by driving

ordinary current through ferromagnetic electrode to sample. The current can be driven in plane

plan of interface called 'current in plane (CIP) geometry' (fig 4(a)) or perpendicular to the

interface called 'current perpendicular to plane (CIP) geometry' (fig 4(b)). The spin can be also

injected by optical method. The efficiency of spin injection is determined by rate of

accumulation of non-equilibrium spin in sample. There are several proposed ways to transport

spin-polarized current across interface. These are: (1) formation of Ohmic contact between

electrode-sample interface, (2) Ballistic electron injection, (3) electron tunneling from space

charge region and, (4) Hot spin injection.


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Ohmic injection

The most basic approach to spin injection is through the perfect Ohmic contact between

ferromagnetic/non-magnetic (F/N) interfaces (fig 4 (a)) . The interface can be produce by taking

metals or semiconductors or superconductors as non-magnetic region with ferromeget. The

degree of spin injection in non-magnetic region depends on the ratio of the conductivities of

ferromagnetic region (F) and non-magnetic region (N) . For typical conductivity mismatch, when

conductivity of F region N region, higher the spin injection efficiency (fig 4(b) and (c)). When

conductivity of F region non-magnetic region, smaller the spin injection efficiency. This

phenomenon is called conductivity mismatch . In the case of ferromagnet/semiconductor

interface, Ohmic contacts resulted from the doping of semiconductor surface. However, doping

leads to loss of spin polarization by spin-flip scattering . The electrochemical potential of N

region increases with spin injection. The difference of spin dependent electrochemical potentials

generates effective resistance R on either side of F/N interface. In superconductor/F interface,

increase in total resistance with spin injection results in switching superconducting state to

normal state of much higher resistance.

Ballistic electron injection

The ballistic spin injection works on principle of GMR effect and electrons are dynamically

transported. The ballistic transport is favorable in ferromagnet/non-magnet/ferromagnet (F/N/F)

interfaces. The F/N/F interface is formed by sandwiching a non- ferromagnetic layer of finite

thickness between two finite ferromagnetic layers . Fully ballistic transport takes place when

ferromagnetic layers aligned in the same direction . This condition provides low resistance path

to the spin polarize current.The probability of spin polarized electron back flow or reflection is

less in ballistic transport, once it enters in the non-magnetic region. The transmission probability


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of ballistic transport depends on difference of two spin conduction sub bands of the ferromagnet

and the conduction band of the semiconductor .

Tunneling spin injection via Schottky barrier (F/S interface)

The interface made up of GaAS (p-region) and ferromagnet (n-region) acts as a typical Schottky

barrier . Properties of Schottky barrier strongly depend on bias and doping of semiconductor.

There is no spin injection at small forward biases due to formation of depletion region near

interface . The depletion region offers resistance to flow of current . The spin injection is

possible at large forward bias because of electric drift lead by non-equilibrium spin already

present in the n-region . The results are more promising in the case of F (p-region)/S (n-region)

in reverse bias. The barrier acts as purely tunneling barrier in reverse bias due to extraction of

spin from non-magnetic region . Thus, tunneling junctions are considered to be most probable

candidate for enhanced spin injection.

Hot electron injection

An electron with energy higher than Fermi level is called Hot electron . These hot electrons are

injected in ferromagnetic region by tunneling. The transmission probability of electrons depends

on band structure of the F/N interface. As high as 90% efficiency will be possible this method,

when spin-flip scattering at the interface is very less. The disadvantage of this method is lower

overall efficiency


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Spin detection

Spin detection typically depends on the sensing the changes in the signal due to spin injection .

The injection of non-equilibrium spin either induces voltage or changes resistance corresponding

to buildup of the non-equilibrium spin . This voltage can be measured in terms of change in

resistance by potentiometric method; while change in resistance can be measured in terms of

voltage by balancing Wheastone Bridge . The transport and optical methods of spin detection are

most widely adopted to detect spin. The efficiency of spin detection in transport method is

depends on interface properties. Therefore, spin detection is low and also suffered from difficulty

discuss above . The optical spin detection technique is well established. The spin can be detected

by determining the helicity of emitted light from LEDs connected with interface.

Spin Relaxation

Non-equilibrium spin accumulates in non-magnetic region due to process of spin injection. It

comes to equilibrium by the phenomenon called spin relaxation . The rate of accumulation of

non-equilibrium spin depends on the spin relaxation . Electrons can remember their spin state for

finite period of time before relaxing. That finite time period is called Spin lifetime . Longer

lifetime is more desirable for data communication application while shorter for fast switching .

The distance traveled by the electron without loosing spin state is called Spin diffusion length .

It is most important variable in spintronic devices, which determines maximum allowable

thickness of the non-magnetic region in device. It is also depend on spin lifetime . There are four

proposed ways by which conduction electrons of metals and semiconductors relax: (A) The

Elliott-Yafet mechanism, (B) The Dyakonov-Perel mechanism, (C) The Bir-Aronov-Pikus

mechanism, and (D) hyperfine-interaction .


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Elliot-Yafet Mechanism

Elliot (1954) first suggested that electron spin relaxation occurs via momentum scattering.

Momentum scattering occurs when lattice ions or photons bring on spin-orbital coupling in the

electron wave function . This spin-orbital coupling introduces wave functions of opposite spin .

Now, electron wave functions with related spin have an admixture of the opposite spin state

These combinations of spin-up and spin-down momentum lead to relaxation of electron spin.

The mechanism is dominant in small-gap semiconductors with large spin-orbit splitting.

Dyakonove-Perel Mechanism

This mechanism comes into play, when the systems lack inversion symmetry . The electrons feel

an effective magnetic field, resulting from the lack of inversion symmetry, and from spin-orbit

interaction . These fluctuating magnetic fields randomly change the magnitude and direction of

electron spin precession . They also randomize the spin. This spin randomization is more

effective than momentum scattering . Therefore, spin dephasing occurs because of the

momentum dependent spin precession along with momentum scattering. This mechanism plays

important role with increase in temperature and increase in band gap.

Bir-Aronov-Pikus Mechanism

The holes also possess spin . The spin of hole can be exchange with conduction electrons. These

exchanges proceed through scattering and lead to spin relaxation of conduction electron in p-

doped semiconductors (Bir, 1975) . Holes have shorter spin coherence time and spin exchange

between electrons and holes is very effective. Ultimately, it will leads to spin decoherence. This

mechanism is of importance at low temperatures.


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Hyperfine-interaction Mechanism

Hyperfine-interaction comes from the magnetic interaction between the magnetic momentum of

nuclei and electrons. In semiconductor hetrostructures, this mechanism is responsible for spin

dephasing of localized or confined electron spins

Spin Relaxation

Non-equilibrium spin accumulates in non-magnetic region due to process of spin injection. It

comes to equilibrium by the phenomenon called spin relaxation . The rate of accumulation of

non-equilibrium spin depends on the spin relaxation . Electrons can remember their spin state for

finite period of time before relaxing. That finite time period is called Spin lifetime . Longer

lifetime is more desirable for data communication application while shorter for fast switching .

The distance traveled by the electron without losing spin state is called Spin diffusion length .

It is most important variable in spintronic devices, which determines maximum allowable

thickness of the non-magnetic region in device. It is also depend on spin lifetime . There are four

proposed ways by which conduction electrons of metals and semiconductors relax: (A) The

Elliott-Yafet mechanism, (B) The Dyakonov-Perel mechanism, (C) The Bir-Aronov-Pikus

mechanism, and (D) hyperfine-interaction.


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SPINTRONIC TECHNOLOGY is already in your computer, at least in a primordial incarnation.

Modern hard disk drives have a read head that relies on an effect known as giant magneto-

resistance, or GMR, which was discovered by French and German researchers in the late 1980s.

Basically, when the spins of electrons in the read head point in the same direction as those

creating the small magnetic domains on the disk, the heads electrical resistance decreases. When

the spins are in opposite directions, the resistance increases slightly. More recently. engineers

have developed even better read heads that rely on tunnel magnetoresistance, a kind of

enhanced GMR. It is this ability to sense very feeble magnetic fields that has allowed hard-disk

makers to keep doubling the capacities of hard- disk drives on a schedule thats even out- paced

Moores Law. Many advances in spintronics resulted from two big research programs that the

U.S. Defense Advanced Research Projects Agency, or DARPA, funded in the 1990s. The first

one produced the earliest MRAM prototypes. These devices used memory cells consisting of

magnetic tunnel junctions: two layers of a ferromagnetic material like iron separated by an

extremely thin, nonconductive barrier of magnesium oxide. When the spins of the electrons in

the two ferromagnetic layers point in the same direction ,in other words, when their

magnetizations are aligned,the electrical resistance across the junction decreases; when the spins

point in different directions, the junction becomes more resistant to current. The prototypes used

this change in resistance to sense whether a 1 or a 0 was stored. Some MRAM chips built at the

time contained millions of memory cells, each with dimensions of about 150 nanometers, an

impressive achievement back then. But the researchers soon discovered that going below too nm

was not going to be easy. The problem had to do with the method they used to change bits,

which was to drive currents through electrodes connected to each memory cell, creating a

magnetic field that oriented the spin state of the cell. This method required currents that proved

quite high, draining lots of power. Worse still, the magnetic fields affected not only the desired


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bit but also others nearby, resulting in errors. Researchers are now trying to improve on this

scheme. The most promising alternative is called spin-torque- transfer, or STT. The idea is to

send electrons through a magnetic layer of cobalt, which tends to orient their spins in the same

direction. The resulting spin- polarized current then flows into another layer of cobalt material.

There, by virtue of one of the many mysteries of quantum mechanics, the incoming spin-

polarized electrons transfer their spin orientation to the electrons on this second layer, thus

magnetizing it. So instead of writing a bit by applying a magnetic field, as early MRAM designs

do, STT uses a spin-polarized current of electrons. To be commercially viable, the magnetic

region where the bit is stored has to be quite small, of course. Researchers believe STT should

work down to at least 6 nm and possibly even smaller dimensions. Last year, engineers at Hitachi

and Tohoku University demonstrated a prototype capable of storing 32 megabits this way. But

thats not all that much. For comparison, a modern DRAM chip can hold ia8 times that amount.

And though in theory such memories should require very small currents to change a bit, in

practice the currents are still too high for most commercial applications. For such reasons, our

group and several others are betting on a different approach entirely. Forget about current-

induced magnetic fields and spin-polarized currents, Instead, find a storage medium with a

permanent magnetism that you can control by applying small voltages. These materials exist

They are called dilute magnetic semiconductors. As their name suggests, they are

semiconductors that are also somewhat magnetic. Their magnetism stems from certain metal

atoms added in a process similar to doping. Whats interesting about these materials is that the

presence of charge carriers electrons and holes (vacancies left when electrons are missing in

places where theyd normally be found)can alter their magnetic properties.

As part of DARPA. second MRAM research program. initiated in 1999, researchers investigated

several dilute magnetic semiconductors, in particular gallium manganese arsenide and indium


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manganese arsenide. Both proved to be good candidates, There was just one problem: A material

is magnetic only up to a given temperature in this case about 200 Kelvin, or 73oC. Thats

colder than nighttime in most parts of Mars! Go above that level known as the Curie temperature

and atomic vibrations cause the spins to lose the orderly arrangement that makes the material a

permanent magnet. If this was a memory chip, youd lose your data.

The resulting gallium manganese nitride turned out to be very promising. When a magnetic field

to this substance is applied, it becomes permanently magnetized. That is to say, when we remove

the field, the magnetization doesnt go away, so it can be used to store data.

The next major step, was the ability to manipulate the magnetic properties of this

semiconductor electrically. It ws started with ordinary gallium nitride, then applied a thin layer

of gallium nitride that contained a little added silicon, a dopant that donates electrons, thereby

creating an n-type semiconductor. (The n stands for negative. reflecting the addition of

negative charges, electrons ). Next there was an addition of another gallium nitride layer, this

time using magnesium as a dopant to remove electrons from the lattice of atoms, creating a p-

type layer (p stands for positive). Finally, a very thin veneer of gallium manganese nitride was

deposited on top of all this.

The junction between n- and p-type layers was key. Thats because we can control the

concentration of electrons and holes around a p-n junction by applying a voltage across it. And

thats exactly what should have been done. When 5 volts is applied across the p-n junction. the

magnetization of that upper layer approached 0. When voltage was removed , the magnetization

shot up.lt is a faint magnetization to be sure, but enough for storing bits.

To know why the voltage on a p-n junction change the magnetization nearby? To understand

that. you have to first think about what goes on at a p.n junction when no voltage is applied


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across It .First, recall that the n-type material has an abundance of negative charge

carriers,electrons,which are free to move around. In the p-type material, the charge carriers are

holes, spots in the atomic lattice that are lacking in electrons. When we put one of these materials

against the other, electrons move from the n-type material into the p-type material, filling what

were vacancies, or holes. So we end up depleting both types of charge carriers in the vicinity of

the p-n junction. which is called, naturally enough, the depletion zone. This process is self

limiting ,though. The loss of electrons from the n-type material leaves it with a positive charge,

while the gain of electrons in the p-type material makes it negatively charged. This sets up an

electric field that opposes the migration of any more electrons across the junction.

As with an ordinary diode. If the p-type material is made positive with respect to the n-type

material, the applied voltage can overcome this electric field, sending holes and electrons racing

toward the junction, reducing the thickness of the depletion zone. A voltage of the opposite sense

boosts the internal electric field and makes the depletion zone wider.

What makes this device different is that the p-type material is very thin and is positioned right

next to the magnetic layer of gallium manganese nitride. So by adjusting the voltage across the

p-n junction, we can control the concentration of holes in the p-type layer at the interface with

this magnetic material. Thats important because the pervasive quantum-mechanical weirdness

that arises at these scales allows these holes to interact with the manganese atoms sitting a few

hundred angstroms away. Though there is a debate in our community, we believe the quantum

phenomenon at work here is what is known as carrier-mediated ferromagnetism. Its as though

the holes told some of the electrons around these manganese atoms to align their spins and start

acting like a refrigerator magnet.

By the same token, a negative voltage is applied across the p-n junction, there is an increase in

the width of the depletion zone enough to diminish the number of holes at the interface with the


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magnetic material. That then allows the spins of the electrons in these manganese atoms to revert

to random directions. The devices magnetization vanishes.

This was the first demonstration that ferromagnetism can be controlled by applying voltages to a

p-njunction without relying on ultracold temperatures. Hope this discovery will help turn

spintronics into a hot topic again, so to speak.

The initial prototype built cant be readily used as a memory cell. First, we need a major

improvement on our design. The problem is that although we can control the magnetization of

our device using voltages, when we remove the voltages the magnetization returns to a baseline

level. For a device to work as a memory, we need to be able to switch back and forth between

two stable states.

One idea currently considered is making the devices layers even thinner and adding a barrier of

non magnetic material, also very thin, between the p-type and magnetic layers. There is a hope

that by applying a voltage across these two layers, we can change the concentration of holes in

the p-type region and also force some of the holes to cross the newly added barrier and migrate

into the magnetic section of the device. The barrier would then play a key role after the voltage is

removed, it would prevent the holes from migrating back to the p-type region. Thereby

maintaining the magnetization of the device even when its not powered on.

Now, if we take the device in this magnetized state and apply a voltage in the reverse direction,

the holes would cross the barrier back into the p-type region. The holes would remain trapped

there, and the magnetization would disappear. This approach would provide the two stable states

we need to use the device as a memory.

If this design is successful, the next step would be miniaturization fact, our initial prototype is

rather big each memory cell is about the size of a fingernail. To build smaller memory cells,


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investigation is done in two ways: One is using conventional photolithography, which we

believe could lead to cells about so nm in size. Another idea is to grow the cell structures as

nanowires, which we speculate might shrink them as small as 20 nm.

Such reduced dimensions would lead to another challenge: reading the bits in these tiny cells. As

we proceed to nanoscale dimensions, the strengths of the magnetic fields will become even

smaller, How to detect them remains an open question. We might have to equip each memory

cell with a tiny magnetic sensor, similar to a read head of a hard drive but etched as a series of

layers in the semiconductor. Its a possibility, but we dont know how it will perform and

whether the resulting device would be economically viable.

Finally, another issue crucial to the commercial success of MRAM proposal is its compatibility

with conventional semiconductor technology, In theory, because MRAM would be programmed

and interrogated electrically, It could be integrated with ordinary chip- making processes. Then

the MRAM devices could be made part of multifunctional integrated circuits, which would be

able to perform all the processing, storage, and communication tasks that today require separate

chips. Clearly, overcoming these hurdles will take a lot of work. But if all goes well, electrically

controlled magnetic material may help engineers to ensure their continued mastery over electrons

and their spins.


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Conclusion

The new technology based on spintronics utilizes electron spin and charge in conventional

electronics . The spin can be effectively utilized by careful manipulation of electron spin

dynamics . The effective manipulation adds additional spin degree of freedom to the devices .

The potential advantage is considerable increase in capacity of conventional electronic devices .

But, it suffers from fundamental limitations. The spin dynamics is not clearly understood in

transport across interface . This uncertainty imposes limits on design of devices . However, in

recent years, understanding of spin dynamics in metallic multilayer gives partial success in

utilizing electron spin as GMR read head and data storage devices . But, the projection of

spintronics will go beyond this and may end regime of charge based electronic .

Challenges

High-volume information-processing and communications devices are at present based on

semiconductor devices, whereas information-storage devices rely on multilayers of magnetic

metals and insulators. Switching within information-processing devices is performed by the

controlled motion of small pools of charge, whereas in the magnetic storage devices information

storage and retrieval is performed by reorienting magnetic domains (although charge motion is

often used for the final stage of readout). Semiconductor spintronics offers a possible direction

towards the development of hybrid devices that could perform all three of these operations, logic,

communications and storage, within the same materials technology. By taking advantage of spin

coherence it also may sidestep some limitations on information manipulation previously thought

to be fundamental.


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BIBLIOGRAPHY

y Special issue on spintronics IEEE, volume 91 , no. 5, May 2003y Spintronics info.comy A Discovery Company, How Stuff Worksy Wikipediay Whatis.techtarget.com(MRAM)y Scribd.com(MRAM)y Physics.umd.eduy IEEE Spectrum, Nov, 2010