Spintronics
                                                               Future of electronics

1 What is Spintronics?:

Spintronics can be fairly new term for you but the concept isn't so very exotic .This technological discipline aim to exploit subtle and mind bending esoteric quantum property of electron to develop a new generation of electronics devices. The formal definition of spintronics is “the study of role played by electron (and more generally nuclear) spin in solid state physics and possible devices that  specifically exploit spin properties instead or in addition to charge degree of freedom”. A  simpler definition is that spintronics is  a  “new branch of electronics in which electron spin in, addition to charge is manipulated to yield a desired outcome” .

Control over spins in the solid state forms the basis for nascent spintronics and quantum information technologies. There is a growing interest in the use of electronic and nuclear spins in semiconductor nanostructures as a medium for the manipulation and storage of both classical and quantum information.

Spin-based electronics offer remarkable opportunities for exploiting the robustness of quantum spin states by combining standard electronics with spin-dependent effects that arise from the interactions between Sections, nuclei, and magnetic fields. Here we provide an overview of recent developments in coherent electronic spin dynamics in semiconductors ant quantum structures, including a discussion of temporally- and spatially-resolved magneto-optical measurements that reveal an interesting interplay between electronic and nuclear spins. In particular, we present an electrical scheme for local spin manipulation based on g­tensor modulation resonance (g-TMR), functionally equivalent to electron spin resonance (ESR) but without the use of time dependent magnetic fields.

The technique of g-TMR enables three-dimensional control of electron spins in nanometer-scale geometries using a single voltage signal. These results provide a compelling proof of concept that quantum spin Information can be locally manipulated using high-speed electrical circuits. Furthermore, recent measurements of hybrid ferromagnet / semiconductor hetero structures under optical illumination reveal that nuclear spins become highly polarized at low temperatures.                                           

We explore the potential for exploiting this behavior to create complex nuclear domains and arrays in lithographically patterned structures. A time-resolved polarization microscope is used to directly image the nuclear landscape in hybrid nanostructures, demonstrating the ability to design and control polarization patterns in the semiconductor. These experiments investigate the electronic, photonic, and magnetic manipulation of electron and nuclear spins in a variety of semiconductor structures and focus on investigating the underlying physics for quantum information processing in the solid state.               

2WHY IS IT GOING TO BE ONE OF THE RAPIDLY EMERGING FIELDS?

Though the field of electronics is considered to be very vast,  even his field is attaining its limitations. The two main limitations which is propelling the scientists and researchers new technology are:                                                                                              

1)Moore’s Law

2)Gate Width

Moore’s Law:

  Moore, one of the co- founders of Intel Corporation, visualized in the early 1970’s that the number of transistors fabricated in a single chip will double for every 18 months. Now, after almost three decades, the number of transistors fabricated in a single chip is so large that it places severe demands on the material and fabrication technology used.

 

Gate Width:

 Some scientist and experts have predicted that by the year of 2008, the width of gate electrode in an FET will be around 45nm, which again places severe demands on the material and fabrication technology used. The figure below shows the variation of the gate electrode length over the years.   

 

Due to the above mentioned limitations many alternatives for electronics have been considered . Spintronics has gained prominence because of the fact that spin devices can be fabricated with small variations to present fabrication technology whereas other alternatives require complete replacement of present fabrication units.

ELECTRON SPIN       

An electron spin s = 1/2 is an intrinsic property of electrons. Electrons have intrinsic angular momentum characterized by quantum number 1/2. In the pattern of other quantized angular momenta, this gives total angular momentum

Spin "up" and "down" allows two electrons for each set of spatial quantum numbers.

s=

The resulting fine structure which is observed corresponds to two possibilities for the z-component of the angular momentum.

This causes an energy splitting because of the magnetic moment of the electron

Two types of experimental evidence which arose in the 1920s suggested an additional property of the electron. One was the closely spaced splitting of the hydrogen spectral lines, called fine structure. The other was the Stern-Gerlach experiment which showed in 1922 that a beam of silver atoms directed through an inhomogeneous magnetic field would be forced into two beams. Both of these experimental situations were consistent with the possession of an intrinsic angular momentum and a magnetic moment by individual electrons. Classically this could occur if the electron were a spinning ball of charge, and this property was called electron spin.

Quantization of angular momentum had already arisen for orbital angular momentum, and if this electron spin behaved the same way, an angular momentum quantum number s = 1/2 was required to give just two states. This intrinsic electron property gives:

The electron spin magnetic moment is important in the spin-orbit interaction which splits atomic energy levels and gives rise to fine structure in the spectra of atoms. The electron spin magnetic moment is also a factor in the interaction of atoms with external magnetic fields (Zeeman effect).

2.1 FUNDAMENTALS OF SPIN

1   In addition to their mass and electric charge, electrons have an intrinsic quantity of angular momentum called spin, almost as if they were tiny spinning balls.

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

3 Scientists represent the spin with a vector. For a sphere spinning "west to east" the vector points "north" or "up." It points "down" for the opposite spin.

4 In a magnetic field, electrons with "spin up" and "spin down" have different energies.

5  In an ordinary electric circuit the spins are oriented at random and have no effect on current flow.

6 Spintronic devices create spin-polarized currents and use the spin to control current flow.

 6.1 GIANT MAGNETO RESISTANCE

Giant Magneto resistance (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 Fert’s 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 also  known as magnetoresistance .

Figure1: 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 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.

When electron spins are aligned (all spin-up or aft spin-down), these create a large scale-net magnetic .moment as seen in magnetic materials like iron and cobalt. Magnetism is an intrinsic physical property associated with the spins of electrons in a material.

Magnetism is already exploited in recording devices such as computer hard disks. Data is recorded and stored as tiny areas of magnetised iron or chromium oxide. To access the information, a read head detects the minute changes in magnetic field as the disk spins underneath it. This induces corresponding changes in the head’s electrical resistance a phenomenon called magneto resistance.

Spintronics burst on the scene in 1988 when French and German physicists discovered much more powerful giant magneto resistance (GMR). It results from .subtle electron-spin effects in ultra-thin multilayers of magnetic materials which cause huge changes in their electrical resistance when; a magnetic field is applied.

GMR is 200 times stronger than ordinary magneto resistance. IBM soon realized that read heads incorporating GMR materials can sense much smaller magnetic fields, allowing the storage capacity of a hard disk to increase from 1 to 20 gigabytes. In 1997 it launched GMR read heads into the market worth about $1 billion a year.

CONSTRUCTION OF GMR

The basic GMR device is a three-layer sandwich of a magnetic metal (such as cobalt) with a nonmagnetic metal filling (such as silver). A current passes through the layers consisting of spin-up and spin-down electrons. The electrons oriented in the same direction as the electron spins in a magnetic layer pass through quite easily, while those oriented in the opposite direction are scattered.

If the orientation of one of the magnetic layers is changed by the presence of a magnetic field, the device will act as a filter or a spin valve, letting through more electrons when the spin orientations in the two layers are the same and fewer electrons when the spin orientations are oppositely aligned. The electrical resistance of the device can therefore be changed dramatically.

How magneto resistance works

6.2 MTJ: - [Magnetic Tunnel Junction]

This device is not yet used in the industries but will soon its application. The structure of MTJ is very simple with two Ferro magnetic layers separates by a semiconductor layer.   

           

   The figure shows the structure of MTJ. As said earlier the direction of spin decides the resistance of the device. The Semiconductor is often called Tunnel Barrier as it acts as the barrier between two ferro magnetic layers. If the resistance is high then the number electrons tunneling are low and if the resistance is low then the electrons tunneling are high.

                                                                

  6.3 MEMORY CHIPS

Physicists have been quick to see further possibilities of spin valves. -The spin valves are not only the highly sensitive magnetic sensors but these can also be made to act as switches by flipping the magnetization in one of the layers. This allows information to be stored as 0s and 1s (magnetisations of the layers parallel or anti parallel) as in a conventional transistor memory device. An obvious application is the magnetic version of the RAM used in your computer.

The advantage of magnetic random access memory (MRAM) is that it is nonvolatile, i.e. information isn’t lost when the system is switched off The main advantages of MRAM devices include lower cost, smaller size, faster speed, and less power consumption. These devices would be much more robust in extreme conditions such as high temperature and high level radiation or interference. The US electronics company Honeywell has already shown that arrays of linked MRAMs could be made to work. The potential market for MRAMs is worth $100 billion annually.

For the past three years or so, researchers around the world have been working hard on a range of MRAM devices. A particularly promising device is the magnetic tunnel junction that has two magnetic layers separated by an insulating metal-oxide layer Electrons can tunnel from one layer to the other only when magnetizations of the two layers in the same direction. Other wise the resistance is high in fact, a thousand times higher than in the standard spin valve.

Even-more interesting are the devices that combine the magnetic layers with semiconductors like silicon. The advantage of using silicon is that it is still a favorite with the electronics industry and is likely to remain so- Such hybrid devices could be made to behave more A Ttke conventional transistors. These could be used as non-volatile logic elements that could be reprogrammed using software during actual processing to create an entirely new type of very fast computing.

Inductive write/GMR read head

6.4 SENSORS

GMR sensors are already being developed in the UK. These have a wide range of applications and their market is worth $8 billion a year.

Applications include:

•    Fast and accurate position and motion sensing of mechanical components in precision engineering and  robotics .

•           All kinds of automotive sensors for fuel handling systems, electronicengine control, anti-skid systems, speed control, and navigation.

•           Missile guidance.

•           Position and motion sensing in computer video games.

•           Key-hole surgery and post-operative care.

    6.5  SPIN-VALVE TRANSISTOR

A new type of magnetic field sensor is the spin-valve transistor (Fig. 5). This transistor is based on the magneto resistance found in. multilayers (for example, in Co/Cu/Co). Usually, the resistance of a multiplayer is measured with the current-in-plane (CIP). The CIP configuration suffers from several drawbacks; for example, the CIP magneto resistance is diminished by shunting and diffusive surface scattering. Hence the fundamental parameters of the spin-valve effect, such as the relative contributions of interface and bulk spin-dependent scattering, are difficult to obtain using the CIP geometry items, mainly because the electrons cross all magnetic layers. But a practical difficulty is encountered: the perpendicular resistance of the ultra-thin multilayers is too small to be measured by ordinary techniques.

Band structure of the spin

Measuring with the current perpendicular to plane (CPP) solves most problems, mainly because the electrons crass all magnetic layers. But a practical difficulty is encountered; the perpendicular resistance of the ultra-thin multilayer is too small to be measured by ordinary techniques.

Schematic cross-section of the spin-valve transistor

Fabrication

The spin-valve transistor consists of silicon emitter, a magnetic multi-layer as the base and silicon collector (Fig. 6). Electrons are injected from the emitter, passing the first Schottky barrier (semiconductor-metal interface) into the base. Because of the thin base multilayer (10 nm), most electrons are not directed to the base contact and travel perpendicular through the multilayer across the second Schottky barrier. These electrons form the collector-current.

Figure 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 .   

A Co/Cu multilayer is sputtered on one of the two silicon 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 proves stronger than silicon.