A

SEMINAR REPORT

ON

“SPINTRONICS – TECHNOLOGY FOR THE FUTURE”

SUBMITTED

BY

Nitish Kumar Chauhan (EI)

[0805232014]

Seminar Under guidance of Seminar Incharge

Er. Ripu Daman Dr. V. K. Singh

(Associate Professor) (Associate Professor )

Department of Electronics Engineering

INSTITUE OF ENGINEERING & TECHNOLOGY, SITAPUR ROAD,LUCKNOW.

Acknowledgement

It gives me a great sense of pleasure to present the B.Tech Seminar report undertaken

during B. Tech. Third Year. I owe special debt of gratitude to respected Er. Ripu Daman for his

constant support and guidance throughout the course of our work. His sincerity, thoroughness and

perseverance have been a constant source of inspiration for me. It is only his cognizant efforts that

mine endeavor have seen light of the day.

I also do not like to miss the opportunity to acknowledge the contribution of all dignitary

Staff-members of I.E.T. Lucknow for their kind assistance and cooperation during the

development of our Seminar report. Last but not the least, I acknowledge my friends for their

contribution in the completion of the seminar report.

Apart from the efforts of me, the success of this project depends largely on the

encouragement and guidelines of many others. I take this opportunity to express my gratitude to

the people who have been instrumental in the successful completion of this report.

INDEX

Topic Page no.

1. Introduction ……………………………………………………………………………………………………………....4

1.1 Semiconductor spintronics…………………………………………………………………………………5

1.2 Electronics versus spintronics…………………………………………………………………………….6

2. Advantages of spin………………………………………………………………………………………………………8

3. Future demands………………………………………………………………………………………………………….8

4. How spintronics works?...................................................................................................9

4.1 Long distance transport……………………….……………………………………………………………..9

4.2 Spin injection…………………………………………………………………………………………………….10

5. Spintronics and semiconductors………………………………………………………………………………..12

6. Spintronic technologies……………………………………………………………………………………………..13

6.1 GMR effect……………………………………………………………………………………………………….13

6.2 Spin valve transistor………………………………………………………………………………………….15

6.3 Spintronic FET…………………………………………………………………………………………………..17

6.4 SMT-MRAM……………………………………………………………………………………………………...21

6.5 Quantum computing…………………………………………………………………………………………24

7. Spintronics: The future……………………………………………………………………………………………..26

8. Hindrances in the path of development of spintronics as an industry……………………….27

9. Conclusion………………………………………………………………………………………………………………..28

10. References……………………………………………………………………………………………………………….29

I. Introduction

Semiconductor spintronics

Conventional electronics rely upon and utilize the flow of an electron’s charge. The idea of Spintronics involves utilizing an electron’s spin as well. In addition to an electron’s orbital angular momentum, an electron has an intrinsic angular momentum called its spin angular momentum. This is simply known as spin, and it can be denoted by the vector S. Other elementary particles, such as protons, also have spin. This spin is associated with a particle’s intrinsic spin magnetic dipole moment, sμ. The vector Sand the moment sμ, are related in the following way:

Where e is the elementary charge and m is the mass of an electron. Now, Scannot be measured directly; however its component along any axis can be measured.In a narrow sense spintronics refers to spin electronics, the phenomena of spin-polarized transport in metals and semiconductors. The goal of this applied spintronics is to find effective ways of controlling electronic properties, such as the current or accumulated charge, by spin or magnetic field, as well as of controlling spin or magnetic properties by electric currents or gate voltages. The ultimate goal is to make practical device schemes that would enhance functionalities of the current charge-based electronics. An example is a spin field-effect transistor, which would change its logic state from ON to OFF by flipping the orientation of a magnetic field. In a broad sense spintronics is a study of spin phenomena in solids, in particular metals and semiconductors and semiconductor heterostructures. Such studies characterize electrical, optical, and magnetic properties of solids due to the presence of equilibrium and nonequilibrium spin populations, as well as spin dynamics. These fundamental aspects of spintronics give us important insights about the nature of spin interactions—spin-orbit, hyperfine, or spin exchange couplings—in solids. We also learn about the microscopic processes leading to spin relaxation and spin dephasing, microscopic mechanisms of magnetic long-range order in semiconductor systems, topological aspects of mesoscopic spin-polarized current flow in low-dimensional semiconductor systems, or about the important role of the electronic band structure in spin-polarized tunneling, to name a few.

Spintronics refers commonly to phenomena in which the spin of electrons in a solid state environment plays the determining role. In a more narrow sense spintronics is an emerging research field of electronics: spintronics devices are based on a spin control of electronics, or on an electrical and optical control of spin or magnetism. While metal spintronics has already found its niche in the computer industry—giant magnetoresistance systems are used as hard disk read heads—semiconductor spintronics is yet to demonstrate its full potential. This review presents selected themes of semiconductor spintronics, introducing important concepts in spin transport, spin injection, Silsbee-Johnson spin-charge coupling, and spindependent tunneling, as well as spin relaxation and spin dynamics. The most fundamental spin-dependent interaction in nonmagnetic semiconductors is spin-orbit coupling. Depending on the crystal symmetries of the material, as well as on the structural properties of semiconductor based heterostructures, the spin-orbit coupling takes on different functional forms, giving a nice playground of effective spin-orbit Hamiltonians. The effective Hamiltonians for the most relevant classes of materials and heterostructures are derived here from realistic electronic band structure descriptions. Most semiconductor device systems are still theoretical concepts, waiting for experimental demonstrations. A review of selected proposed, and a few demonstrated devices is presented, with detailed description of two important classes: magnetic resonant tunnel structures and bipolar magnetic diodes and transistors. In view of the importance of ferromagnetic semiconductor materials, a brief discussion of diluted magnetic semiconductors is included. In most cases the presentation is of tutorial style, introducing the essential theoretical formalism at an accessible level, with case-study-like illustrations of actual experimental results, as well as with brief reviews of relevant recent achievements in the field.

Spintronics promises the possibility of integrating memory and logic into a single device. In certain cases, switching times approaching a picosecond are possible, which can greatly increase the efficiency of optical devices such as light-emitting diodes (LEDs) and lasers. The control of spin is central as well to efforts to create entirely new ways of computing, such as quantum computing, or analog computing that uses the phases of signals for computations. Spin is a fundamental quantum-mechanical property. It is the intrinsic angular momentum of an elementary particle, such as the electron. Of course, any charged object possessing spin also possesses an intrinsic magnetic moment. It has been known for decades that in ferromagnetism the spins of electrons are preferentially aligned in one direction. Then, in 1988, it was demonstrated that currents flowing from a ferromagnet into an ordinary metal retain their spin alignment for distances longer than interatomic spaces, so that spin and its associated magnetic moment can be transported just as charge. This means that magnetization as well can be transferred from one place to another.

Successful spintronics applications need to satisfy three basic requirements: efficient spin injection or spin generation, whereby spin is injected from a ferromagnetic into a nonmagnetic conductor, reasonably long spin (magnetization, M) diffusion, at least tens of nanometers, and possibility of efficient spin manipulation, and, finally, spin detection. Spin detection, if performed by spin-to-resistance conversion, is at the heart of spintronics devices. All three processes are equally important, though the hierarchy starts naturally with spin injection, as a way to introduce nonequilibrium spin into a conductor. If you take a piece of iron and aluminum,

connect the two in series and make electrical current flow through them, you have likely achieved electrical spin injection. If electrons flow from the iron, where most electrons are spin polarized (there are more spin up, say, than spin down electrons), to the aluminum, the spin is accumulated in aluminum, the result of spin injection. If the current is reversed and electrons flow from the aluminum into the iron, the spin is taken from the aluminum and we speak of spin extraction.3 We understand these processes reasonably well, at least for the most studied cases of highly degenerate charge-neutral electronic systems. In non-degenerate semiconductors, for example, spin injection may be absent due to space charges and electron population statistics. What we call the standard theory of spin injection, as well as of spin transport and spin-dependent tunneling is presented in detail in this text.

Once the spin is injected, we need to manipulate it or control it. This is usually achieved by applying an external magnetic field to rotate the spin, although the presence of spin-orbit coupling allows one to control spin electronically. Indeed, the spin-orbit coupling in semiconductor heterostructures can be tailored by voltage gates on the top of the heterostructures, allowing to control the spin by voltage. We still need to find practical ways to do that; understanding the spin-orbit interactions is crucial. This article present detailed derivations of the effective Hamiltonians describing the spin-orbit interactions in the most studied classes of semiconductors and their heterostructures—the so-called Dresselhaus and Bychkov-Rashba Hamiltonians.

The injected spin has to survive sufficiently long, and travel sufficiently far, to transfer information between the injected point and the point of detection. The transfer is inhibited by irreversible processes of spin relaxation and spin dephasing. These processes arise due to the combined actions of the spin-orbit interaction and momentum relaxation. The former provides

spin flips or spin rotations, the latter gives irreversible time evolution. The interaction of spin with a solid-state environment is a complex process whose description relies on effective perturbativeapproximations. Such a formalism is introduced here, along with the most relevant spin relaxation mechanisms in semiconductors and in important classes of tailored semiconductor superstructures–lateral quantum dots which are potentially important for spin-based quantuminformation processing.

Finally, the spin has to be detected. Even if you pass current from the aluminum to the iron, you have to prove that spin-polarized electrons indeed accumulate in the aluminum. This is a highly nontrivial task. In Fig. I.1 the detection scheme is based on the Silsbee-Johnson spincharge coupling. This coupling is the inverse of the spin injection. In a spin injection electrical current drives spin-polarized electrons from a ferromagnetic metal to a nonmagnetic conductor. In a spin-charge coupling an electrical contact between a ferromagnet and a nonmagnetic conductor containing a nonequilibrium spin population results in electrical current (or electromotive force in an open circuit). The presence of the electron spin can then be detected electrically. Other frequently encountered ways of detecting spin include a spin-valve effect, in which the injected spin-polarized electrons enter a detecting ferromagnetic electrode with an efficiency given by the relative orientation of the injecting and detecting electrodes, or optical detection in which spin-polarized electrons recombine with unpolarized holes and emit circularly polarized light.

II. Advantages of Spin

Information is stored into spin as one of two possible orientations

Spin lifetime is relatively long, on the order of nanoseconds

Spin currents can be manipulated

Spin devices may combine logic and storage functionality eliminating the need for separate components

Magnetic storage is nonvolatile

Binary spin polarization offers the possibility of applications as qubits in quantum computers

III. 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

Spintronic devices offer the possibility of enhanced functionality, higher speed, and reduced power consumption.

IV. How Spintronics Works ?

Shortly after the discovery of GMR, Supriyo Datta and Biswajit Das of Purdue University in the US proposed a new type of field effect transistor (FET) that exploits the spin of the electrons travelling through a semiconductor without being scattered. When a voltage is applied to the gate electrode of a FET, the resulting electric field creates a conducting channel between the source and the drain electrodes.Datta and Das suggested that the field could also be used to control the orientation of the spin so that it modulates the current.The beauty of their idea is that the “spin-FET” can be fabricated using the standard equipment in microelectronics to produce new logic and sensor applications. Little wonder that their concept has become a paradigm of semiconductor spintronics and has stimulateda worldwide research effort. To understand how an electric field can control spin, we have to look at the effect of relativity on the spin of the electron as formulated in the Dirac equation. In simple terms, an electron has an intrinsic magnetic dipole moment and behaves like a miniature bar magnet that is aligned along its axis of angular momentum.The electron can have spin of either +h_/2 or –h_/2, where h_ is the Planck constant divided by 2π. As it orbits around the nucleus, the electron produces a magnetic field that modifies its own magnetic moment – an interaction know as “spin–orbit coupling”. In the rest frame of the electron, however, the electric field of the nucleus appears to be a magnetic field – this is a purely relativistic effect. As a result, the spin of the electron actually precesses as it orbits the nucleus. Similar effects are felt by electrons moving through the electric field inside certain types of semiconductor crystals.

Long-distance transport

For semiconductor spintronics to work, the electrons must first be polarized so that all their spins point in the same direction. It is also important that the spin polarization is largelypreserved as the electrons propagate through the semiconductor. Scientists have recently made great advances in this particular direction.Their results show that electron spins can be transported for over 100 micrometres in gallium arsenide, much further than the length of the semiconductor channel envisaged for future spintronics devices. In addition, a group of researchers reported that a “packet” of electrons remains coherent over the same distance.The successful spin transport was detected using sophisticated optical techniques – for example the amount of circularly polarized light produced by the recombination of spin-polarized electrons with holes gives a measure of the spin orientation.Curiously, intense research suggests that the number of defects in bulk semiconductors, such as gallium arsenide and gallium nitride, has little effect on spin orientation. Spin can also be transported successfully across the interface between two different semiconductors. Scientists have observed that a spin-polarized current can flow uninterrupted from a layer of gallium arsenide to a layer of zinc selenide. Both the amplitude and the phase of the spin current can be controlled, even on femtosecond timescales (10–15 s). The ability to control the phase of the electron spin with a stack of semiconductor interfaces offers intriguing possibilities for future applications in quantum computation. This is technologically feasible because molecular beam epitaxy can routinely produce semiconductor layers just one atom thick.

Spin injection

Unlike multilayer devices made from metals or metal-oxides, semiconductors can transport electron-spin information over macroscopic distances, and from one device to another.Engineers envisage a wealth of spinbased optoelectronic devices, including light-emitting diodes (LEDs) that generate polarized light intrinsically. Such LEDs would eliminate the need for the polarizing filters that are currently inserted into conventional devices and reducetheir brilliance. The crucial issue now is to find a material that can inject a spin-polarized current efficiently into a semiconductor at room temperature. To get round this problem, most research groups have created short bursts of spin-polarized electrons by illuminating the surface of the semiconductor with pulses of circularly polarizedlight. But the ultimate goal is to inject spins electrically.To date, two different approaches have been taken to solve the problem. The first involves growing additional spinaligning layers of a magnetic semiconductor on top of the existing material using molecular beam epitaxy.The second approach involves injecting spin-polarized electrons from a ferromagnetic metal like cobalt, nickel or iron, but this has proved difficult because layers containing randomly oriented spins form between the metal and the semiconductor.The spins are able to quantum-mechanically tunnel through the so-called Schottky barrier that had formed between the iron and the gallium arsenide. Yet the spin-injection efficiency remained far below the bulk spin polarization of the iron film,which is about 40%. Several microscopic effects might explain the shortage of spin in the semiconductor, including “spin-flip” scattering at the metal–semiconductor interface or spin dephasing in the semiconductor heterostructure. Spin transfer between a metallic ferromagnet and a semiconductor therefore remains a challenge.However, a recent spin-injection experiment using a scanning-probe technique may provide new insights into the problem. Vincent LaBella and colleagues at the University of Arkansas in the US have scanned the surface of gallium arsenide with a sharp tip consisting of a wire made from a single crystal of nickel. By injecting a 100% spin-polarized current into the material, the Arkansas team was able to correlate the spin-injection efficiency with surface features on the semiconductor. They found that while 92% of the electrons injected into flat terraces kept the same polarization, the situation changed dramatically near sharp steps. Most of the electrons flipped their spins within a few nanometres of a step edge, thereby disruptingthe flow of spin.Experts have argued that these ferromagnet– semiconductor hybrid structures may also suffer from parasitic magnetoresistance phenomena. Unlike the metallic or oxide interlayers in GMR and TMR devices, semiconductor channels are very sensitive to magnetic fields. Indeed, the stray field due to a single ferromagnetic nanostructure is often sufficient to deflect electrons and create additional resistance. Andrey Geim and colleagues at Manchester University in the UK, our group and others have studied stray fields in detail.The results have shown the importance of the shape of the ferromagnetic domains in the source and the drain. Indeed micromagnetic simulations and magnetic imaging have been crucial for understanding all-electrical spin-injection experiments

.

Fig: Spins are injected into an indium-arsenide crystal and then detected by this test chip, which was fabricated by Christopher Schierholz at Hamburg University using photo- and electron-beam lithography. Electron transport is studied by means of superconducting leads, which are hardly affected by noise at low temperature. The inset shows the ferromagnetic source and drain measuring 1 μm across separated by a semiconductor channel just 150 nmin length. The gate electrode that completes this MOSFET has been omitted.

V. Spintronics and Semiconductors

Incorporating spintronics into semiconductor technology would not only provide an avenue for making present devices smaller, but it would also provide a plethora of opportunities to develop new types of devices. By taking advantage of a semiconductor’s good optical properties and amplification abilities, novel devices such as ultra high speed switches, multi-functional logic gates, and fully programmable all-spintronic microprocessors have the potential to be realized. One of the earliest concepts of a potential spintronic device is the Spin Field-effect Transistor. The idea of a spin FET is shown in Figure . It consists of source and drain contacts that are made out of ferromagnetic materials so as to inject and detect spin-polarized electrons transported in a high-mobility channel. The source terminal would serve as a source for the spin-polarized current. By applying a voltage to the gate terminal that is carefully chosen with respect to the magnetization of the drain, the spin of the electrons being transported through the channel would rotate. This effectively would change the conductance of the channel. Therefore, by independently controlling the magnetization of the terminals such as in techniques presented in the MRAM concept, the spin FET can serve as a nonvolatile and reprogrammable device that has more than one mode of usage. Ideas such as this would incorporate additional functionality into traditional logic gates consequently creating a potential new realm of versatile integrated circuits.

Fig: Concept of SPIN FET

VI. Spintronic Technologies

1) Giant Magneto Resistive Effect.

Giant Magnetoresistanceis a change in resistance dueto a change in the magnetization orientation distribution in a system. Resistance changes are often very large (giant) because the Zeemanpotentials are so strongThe large changes of resistance occur in weak fields because of the (relatively) small anisotropy energies. They can occur very abruptly because the magnetization orientation usually behaves like a classical field that can be suddenly switched between distinct local-minimum configurations by tuning the system parameters througha stability region boundary.

The GMR is observed in thin-film materials composed of alternate ferro magnetic and nonmagnetic layers. The resistance of the material is lowest when the magnetic moments in ferromagnetic layers are aligned in the same direction, and highest when they are antialigned. This is because the spin-aligned currents from one layer are scattered strongly when they encounter a layer that is magnetically aligned in the opposite direction, creating additional resistance. But when the magnetic fields are oriented in the same direction, the spin-aligned currents pass through easily.

Current GMR materials operate at room temperature and exhibit significant changes in resistivity when subjected to relatively small external magnetic fields. Thus they can be used as magnetic field sensors. The imposed magnetic field changes the magnetic orientation of one of the two layers, disrupting their relative orientation and thus changing the resistivity. The first GMR-based magnetic field sensor was created in 1994, and high-performance disk drives utilizing GMR-based read heads to detect magnetic fields were realized in 1997 and now are ubiquitous. These read heads are responsible for the very rapid growth in magnetic storage densities that has occurred in the last decade.

1988 France, GMR discovery is accepted as birth of spintronics

A Giant MagnetoResistive device is made of at least two ferromagnetic layers separated by a spacer layer

When the magnetization of the two outside layers is aligned, lowest resistance

Conversely when magnetization vectors are antiparallel, high R

Small fields can produce big effects parallel and perpendicular current

In alternate FM/nonmagnetic layered system,

R is low when the magnetic moments in the FM layers are aligned,

R is high when the magnetic moments in the FM layers are antialigned.

2) The Spin-Valve Transistor

A new type of magnetic field sensor is the spin-valve transistor. This transistor is based on the spin-valve effect, a type of magnetoresistance that is found in multi-layers, for example in (Co/Cu/Co). Usually, the resistance of the multi-layer is measured with the current-in-plane (CIP). The CIP configuration suffers from several drawbacks: the CIP magnetoresistance (MR) is diminished for example by shunting and diffusive surface scattering. The fundamental parameters of the effect, such as the relative contributions of interface and bulk spin dependent scattering are difficult to obtain using the CIP geometry. Measuring with the current perpendicular to the planes (CPP) solves most of these problems, mainly because the electrons cross all magnetic layers, but a practical difficulty is encountered: the perpendicular resistance of the ultra thin multi-layers is too small to be measured by ordinary techniques.We found a novel approach to measure CPP-GMR: by employing a metal base transistor structure with a magnetic multi-layer functioning as the base. A schematic structure of the device is shown in figure.

Fig. Schematic cross-section of the spin-valve transistor. A Co/Cu/CO sandwich base is sputtered on a silicon substrate. Vacuum bonding is done while sputtering the Pt layer. The picture on the bottom shows the band-structure of the spin-valve transistor.

The spin-valve transistor consists of a silicon emitter, a magnetic multi-layer as the base and a silicon collector. Electrons are injected from the emitter, passing the first Schottky barrier (semiconductor-metal interface) into the base. Because of the thin base multi-layer(10nm), most of the electrons are not directed to the base contact, but travel perpendicular through the multi-layer across the second Schottky barrier. These electrons form the collectorcurrent. A Co/Cu multi-layer is sputtered on one of the two silicon substrates. The last layer is sputtered on both substrates and these are pressed together at the last second of the sputter deposition. Because of the smoothness and freshness of the metal surfaces spontaneous adhesion occurs at room temperature. A metal layer between two crystalline semiconductors is accomplished and the bond proved stronger then the silicon. Through lithographical processes and wet chemical etching of the top substrate and the metal base, spin-valve transistors are fabricated.

Magnetic sensitivity

The number of electrons that reach the collector increases exponentially with the mean free path of the electrons in the base. The mean free path varies with the applied magnetic field, hence the collector current becomes strongly magnetic field dependent. In figure 7 a plot of the collector current against magnetic field is shown in fig.

Fig. Collector current variation versus magnet field at a emitter current of 25mA, temperature 100, 200 and 300K, range from -240 to 240 kA/m. The inset on the left-hand side accentuates the small field behavior from -24 to 24kA/m.

The collector current variation at low temperatures was more then 400%. Even larger variations are expected with higher quality bases. (The hysteresis is caused by the hysteresis of the magnetic layers.) Fundamental properties of the spinvalve effect can now be examined in a complete new way. The extreme magneto-sensitivity makes the transistor a interesting device for high-tech readheads for high-density hard disks and magnetic RAMs.

3) Spintronic FET ( DATTA- DAS TRANSISTOR)

Ideal use of MRAM would utilize control of the spin channels of the current Spin transistors would allow control of the spin current in the same manner that

conventional transistors can switch charge currents Using arrays of these spin transistors, MRAM will combine storage, detection, logic and

communication capabilities on a single chip This will remove the distinction between working memory and storage, combining

functionality of many devices into one

The first scheme for a spintronic device based on the metal-oxide-semiconductor technology familiar to microelectronics designers was the field effect spin transistor proposed in 1989 By Supriyo Datta and Biswajit Das of Purdue University. In a conventional field effect transistor, electric charge is introduced via a source electrode and collected at a drain electrode. A third electrode, the gate, generates an electricfield that changes the size of the channel through which the source-drain current can flow, akin to stepping on a garden hose. This results in a very small electric field being able to control large currents.

In the Datta-Das device, a structure made from indium-aluminum-arsenide and indium-gallium-arsenide provides a channel for two-dimensional electron transport between two ferromagnetic electrodes. One electrode acts as an emitter, the other a collector (similar, in effect, to the source and drain, respectively, in a field effect transistor).The emitter emits electrons with their spins oriented along the direction of the electrode’s magnetization, while the collector (with the same electrode magnetization) acts as a spin filter and accepts electrons with the same spin only. In the absence of any changes to the spins during transport, every emitted electron enters the collector. In this device, the gate electrode produces a field that forces the electron spins toprecess, just like the precession of a spinning top under the force of gravity. The electron current is modulated by the degree of precession in electron spin introduced by the gate field: An electron passes through the collector if its spin is parallel, and does not if it is antiparallel, to the magnetization. The Datta-Das effect should be most visible for narrow band-gap semiconductors such as InGaAs, which have relatively large spin-orbit interactions (that is, a magnetic field introduced by the gate current has a relatively large effect on electron spin).

Figure. Datta-Das spin transistor was the first spintronic device to be proposed for fabrication in a metal-oxide-semiconductor geometry familiar in conventional microelectronics. An electrode made of a ferromagnetic material (purple) emits spin-aligned electrons (red spheres), which pass through a narrow channel (blue) controlled by the gate electrode (gold) and are collected by another ferromagnetic electrode (top). With the gate voltage off, the aligned spins pass through the channel and are collected at the other side (middle). With the gate voltage on, the field produces magnetic interaction that causes the spins to precess, like spinning tops in a gravity field. If the spins are not aligned with the direction of magnetization of the collector, no current can pass. In this way, the emitter-collector current is modulated

by the gate electrode.

Figure: Spintronic FET

While fast nonvolatile memories could be very important to increasing computer capabilities, a key bottleneck is moving information between memories and logic circuits. Ideally, if individual devices could both process and store information, transfer delays could be eliminated, at least for data in immediate use. A spinbased device that could accomplish this dual task is a spin-polarized field-effect transistor (spin FET). In a conventional FET, when a bias voltage is applied, a conducting channel is created between the source and the drain regions, allowing the transistor to act as a switch. If source and drain contacts are madefrom ferromagnetic materials, the electrons emitted Spintronics from each contact have a preferential spin. Thus the current can be controlled either by applying a bias voltage as in a conventional FET, or by changing the orientation of the spins as they move from the source to the drain by rotation or by electric-field-controlled scattering. There is, however, a serious difficulty that has so far prevented the development of practical spin FETs. The conductivity of ferromagnetic materials, generally metals, is much higher than that of the semiconductors that make up the rest of the FET. This means that there are far more mobile electrons in the ferromagnet than in the semiconductors, so only a few of the spin-aligned electrons are ableto enter the semiconductor. For a large transfer of spin-aligned electrons, the conductivity of the ferromagnets and the semiconductors must be closely matched, or there must be a tunneling contact between the ferromagnet and the semiconductor to match the conductivities. One way to achieve this match is to utilize ferromagnetic semiconductors asthe source and the drain. The first ferromagnetic semiconductors with Curie temperatures (TC) above 50 K (−223◦C or −370◦F), developed in 1996, were diluted magnetic semiconductors—alloys in which some atoms are randomly replaced by magnetic atoms, such as manganese.However, these early materials still had to be cooled to cryogenic temperatures to exhibit ferromagnetism. Subsequent research has shown that other types of semiconductors can exhibit ferromagnetism at much higher temperatures. In 1998 ferromagnetic behavior of GaMnAs was reported with a Curie temperature of about 110 K (−163◦C or −262◦F), which was subsequently raised to nearly 200K(−73◦Cor−100◦F). In 2000 room-temperatureferromagnetism in TiCoO2 was discovered in Japan.

4) SMT-MRAM (SPIN MOMENTUM TRANSFER MAGNETIC RANDOM ACCESS MEMORY)

Spin Momentum Transfer EffectThe angular momentum carried by a spin-polarized current can exert a torque on the magnetization of a magnetic film that is magnetized in any nonparallel direction. This effect, also known as spin torque effect.

Spintronics is also paving the way for more refined storage technologies. Magnetic random-access memory, or MRAM, is a developing technology that has been projected to hit the market within the next few years. MRAM will have the advantage of being a nonvolatile form of memory (like EEPROM and flash memory) that has rewritability rates challenging conventional volatile RAM. Initial plans were to eventually use MRAM as a replacement for other types of nonvolatile memory in order to increase rewriting speeds, but because of these high rewriting speeds it has also been suggested that MRAM replace conventional RAM which is typically volatile. This would allow the entire state of a computer’s software to be stored after the computer has been turned off. Upon the turning on of one’s computer, all of the software configurations would already be present. Consequently, the utilization of MRAM would virtually eliminate boot-up time. In the most common types of memory, such as DRAM, a bit is stored as a voltage on a capacitor. The switching of this voltage from high to low or low to high is done by means of a transistor. Early concepts of MRAM used hysteresis by using two or more current carrying wires to store a data bit. This involves applying a magnetic field to a ferromagnetic material to magnetize it. This would represent a “1”. After the material has been magnetized for the first time, it requires not only the absence of the previous field to demagnetize the material but the application of a field with the opposite polarity. The demagnetized material would represent a “0.” The applying of magnetic fields is done via two or more current carrying wires. The magnetization of the material would only occur if the material is exposed to some threshold field. Suppose that two wires are interfaced to each cell as shown in Figure 3. The wires are then interfaced to the material in such a way that if both wires are chosen to carry current, the threshold field is surpassed, and the material is altered in magnetization. Most current MRAM concepts still utilize this write concept.

FIG. Two-dimensional MRAM cell array. Ix or Iy alone is not sufficient to write to the cell. Both currents, Ix and Iy, together are required.

New techniques in reading schemes for cell arrays are being developed. Previous ideas involved probing the magnetized material inductively, but this proved to be insufficient. With the discovery of the GMR effect, researchers became interested in ways to utilize GMR as a means of reading from an MRAM cell. One of the latest techniques to use GMR is by means of a Spin Dependent Tunneling (SDT) device. Such devices are typically called Magnetic Tunnel Junctions (MTJ), and they are an enhancement of the so called “sandwich” structure as described in the latter discussion regarding GMR sensors. The idea is basically the same, except now the layer preventing the magnetic coupling between the free layer and the pin layer is a thin dielectric, or insulating material. This effectively eliminates conventional current. Therefore, the only current passing through the structure is tunneling current. When the free layers are parallel to each other, the tunneling probability is high, and there is a relatively low resistance. When the free layers are anti-parallel with respect to each other, the tunneling probability is very low. A very high resistance is the result. The latest studies have indicated that SDT devices result in over a 40% magnetoresistance. In other words, the material’s resistance changes by 40% when changing states (Ref. 6). This is far superior to sandwich structures or Pseudo-Spin Valve structures (another type of enhanced GMR sandwich). Such a large percentage in change results in stronger signals and hence faster read speeds. There are several other advantageous of using an SDT device as an MRAM cell. It is relatively easy to manipulate the magnetoresistance through varying the thickness of the tunneling barrier in SDT devices. Also, a large magnetoresistance can be maintained when the area of the junction is reduced to the nanometer

scale. In addition, SDT devices have a wide frequency response and are low power. These versatile attributes make the SDT device a promising building block for MRAM .

Figure: Motorola’s MTJ MRAM cell.

Conventional MRAM utilizes currentgenerated magnetic fields to rotate the magnetization in the free layer. In spite of advances in the switching methodology to make the switching robust to disturbances, increase the yield, and lower the switching current by magnetic cladding of the word and bit lines, SMT potentially offers orders-ofmagnitude lower switching currents and concomitantly much lower energy per bit to write. Apparently SMT switching can significantly improve the performance of MRAM and make it a truly universal memory. Key areas of research to be addressed in the near future include using MRAM in embedded memory and for multibit memory (stacked memory).

Figure: A summary of the projected performance of MRAM and SMT-MRAM

5) Qubits and Quantum Computing

Quantum computing. Another avenue for using the spins of elementary particles comes from the rapidly developing field of quantum computing. The states of spin of electrons or other spin-1/2 particles can be used as an implementation of a qubit (quantum bit, the unit of quantum information). Information can be encoded using the polarization of the spin, manipulation (computation) can be done using external magnetic fields or laser pulses, and readout can be done by measuring spin-dependent transport.Quantum computers execute a series of simple unitary operations (gates) on one or two qubits at a time. The computation on a quantum computer is a sequence of unitary transformations of an initial state of a set of qubits. After the computation is performed, the qubits can be measured, and the outcome of the measurement is the result of the quantum computation. Quantum effects such as interference and entanglement are used as computational resources and make quick solutions to hard problems possible. For some very special problems, suchas factorization of large prime numbers or exhaustive database searches, quantum-computing algorithms have been developed that show a very significant speed-up in computation time and a reduction in complexity. For certain calculations that find global properties of functions such as factoring and discrete logarithms, the speed-up for a quantum processor is dramatic. For these operations, a 30-logical-qubit quantum processor can perform the same calculationin the same time as a 109-bit classical computer. Scientists are searching for quantum mechanical two-state systems with long dephasing times, which would provide the ability to carry out computations before stored information is lost. It must be possible to readily farbricate and scale these quantum systems if they are to perform quantum algorithms.One very viable candidate for quantum information is electron spins in coupled quantum dots. However, other two-level systems have been proposed for implementing qubits, and include nuclear magnetic resonance (NMR), which involves nuclear spins in special molecules; excited states of ions in traps; cavity quantum electrodynamic systems; Josephsonjunctions; and SQUIDs (superconducting quantum interference devices). The potential uses of quantum qubit systems range from quantum key distribution, quantum encryption, and quantum dense coding to quantum teleportation and ultraprecise clock synchronization.

The Qubit as a Quantum Bit

It has been proposed that an electron or another elementary particle could serve as a quantum bit. This is because of the bipolar nature of an elementary particle’s spin state. For instance, a particle in its spin up state could represent a “1” while a particle in its spin down state could represent a “0.” In order to prepare for the theory of the superposition principle, the spin up and spin down states of a proton will be demonstrated by means of the Stern-Gerlach experiment. In the Stern-Gerlach experiment (conducted in the 1920’s), a beam of protons was sent through a static, nonuniform magnetic field such that0≠∇Br. Classically it is assumed that the beam consisted of protons whose magnetic dipole moments,pμr, are distributed in some random fashion in direction and magnitude. The force exerted on a proton by the field would then E= -μp ∇B Now, because is random, F is random. Therefore, the beam was expected to smear out. However, it was observed that instead of smearing out, the beam split into two separate beams. Furthermore, a proton’s magnetic dipole moment could have only two different orientations: in the direction of the field or in the opposite direction. This is a consequence of the protons having

two discreet spin states. Each of these spin states corresponded to an energy level. If μp is in the opposite direction of the field, then the proton has an energy of E= +μp B . If it is oriented in the direction of the field, then the proton’s energy is E= -μp B . These correspond to “high” and “low” energies and can therefore be thought of as the bit states “1” and “0.” From now on these states will be denoted in the following way: |o>,

|1>

This is in accordance with state vector theory, where ψ is a “ket vector.” All ket vectors will be will represent a quantum state. The general idea of the qubit is now defined.

VII. Spintronics: The Future

In spite of the current difficulties with ferromagnet– semiconductor hybrid structures, one of the beauties of these devices is that the spin can be controlled in many different ways. Experiments have already shown that electron spins can be manipulated optically, as well as with magnetic and electric fields. And there are hints that spin amplification might be possible in semiconductors. Moreover, spin can even be controlled at the nanometre level using nanomagnets, which produce very localized magnetic fields. Even the phase of a coherent spin current can be adjusted at the interface between two dissimilar semiconductors. In the case of electric-field control, our group has recently developed a theory to explain electron transport in a realistic spin-MOSFET in which spins are injected and detected electrically. Our model takes into account the material characteristics, spin-dependent transmission across the ferromagnet– semiconductor interface, and the dependence of the density of charge carriers and the Rashba field on the gate voltage. It predicts that the magnetoresistance of the MOSFET changes with voltage in a similar way to the spin- FET devised by Datta and Das. Various groups have shown that a spin transistor comprising a semiconductor sandwiched between a gate, a source and a drain made from conventional metallic ferromagnets works in principle, but progress has been hampered because the spin-injection efficiency is low. One way round this problem might be to use semiconductors that are ferromagnetic at room temperature. However, several research groups are taking a different approach and are investigating the growth of so-called Heusler alloys. These materials are made of metals that are only partially aligned in their pure state but have all their spins aligned at room temperature in alloy form. In principle,we can boost magnetoresistance effects to 100% if we fabricate sources and drains from these materials.

Modern lithography and deposition techniques now allow us to fabricate devices sufficiently small that electrons travel through them ballistically, i.e. without being scattered. As a result, the critical factor for spin injection is spindependent scattering at interfaces . Calculations by George Kirczenow, by Peter Dederich’s group and others now suggest that the interface between a semiconductor and a conventional metallic ferromagnet grown by molecular beam epitaxy could “filter” the spins to provide a fully spin-polarized current. Indeed, interface engineering is currently a hot topic and the race to reach high spin-injection efficiencies is on.Very recently, research groups at the Naval Research Lab at Buffalo reported an efficiency of 30% for spin injection from iron into a gallium-arsenide heterostructure after they improved the Schottky tunnelling barrier. The recent developments in spin transport and spin injection may herald a new era of semiconductor spintronics that could potentially transform the microelectronics industry. Most revolutionary is the idea that a genuinely quantummechanical system like electron spin could be used to encode information in quantum systems. Since the spin can be in a superposition of different quantum states, it can be used as a quantum bit or “qubit” in quantum computation and communication. The implementation of realistic qubits is an ambitious and long-term research goal that will go on fascinating solid-state physicists long after Dirac’s 100th anniversary.

VIII. Hindrances In The Path Of Development Of Spintronics As An Industry

Before practically pursuing the construction of spintronic devices such as the spin FET, fundamental questions regarding the integration of spintronic effects into semiconductors must be answered. For instance, can economical methods of combining ferromagnetic materials and semiconductors into integrated chips be achieved? Can ferromagnetic semiconductors be maintained at room temperature? Can spin-polarized currents be injected successfully into semiconductors? These types of questions must be thoroughly dealt with before the field of spintronics can truly take off as an industry. Questions such as these commonly fall under the following four categories of requirements:

1. Efficient electrical injection of spin polarized carriers into a semiconductor 2. Adequate spin diffusion lengths and lifetimes for transport within the device

3. Effective control and manipulation of the spin system 4. Efficient detection of the spin system to determine the output

Great progress has been made in the latter three issues. However, efficient injection of spin polarized currents continues to be the main obstacle in developing spintronic semiconductor devices. This injection relies heavily on two key components: the type of material used for the source of spin-polarized currents and the interface between these materials and semiconductors. The following will explore some of the concepts relating to these components.

Of the materials considered for the construction of a terminal that sources spin polarized current, the most common suggestion is the usage of ferromagnetic metals. Ferromagnetic metals have high Curie temperatures which makes them attractive as potential sources. The Curie temperature is defined as the temperature at which a ferromagnet ceases to exhibit exchange-coupling. In other words, as a result of thermal agitation, the magnetic dipole alignment of the material decomposes when above the Curie temperature. Because devices typically run at temperatures above 300K, it is important to have as high of Curie temperature as possible to ensure proper functioning. A fundamental problem with the using of ferromagnetic metals is that when interfaced to a semiconductor, the mismatch in conductivities results in spin-flip scattering which effectively limits the spin injection efficiency.

IX. Conclusion

The realm of spintronics has a promising future. As quantum mechanics becomes an influential factor in small-scale electronics, utilizing an electron’s (or proton’s) spin states will allow a wide array of new possibilities never before conceived of. Research being done in the field of spintronics is technically still in its infancy, but strong gains have been made in many sub-areas of the field. The effect of Giant Magnetoresistance is a spin technology that has already been incorporated into modern technology, and the release of MRAM into the market is pending. Significant progress has been made into researching possible materials for spintronic semiconductors. The prospect of incorporating ferromagnetic materials into semiconductors will provide a new positive direction for the semiconductor industry. In addition, the concepts of the qubit and quantum computing provide exciting ideas about the possibilities of what computers of future would be able to accomplish. In sum, the field of spintronics has the potential to provide

continuation and seemingly limitless growth of technology in the 21st century.