Paper Presentation

ABSTRACT

“Spintronics” is an emergent Nanotechnology, which uses the spin of an electron instead of or in addition to the charge of an electron. Electron spin has two states either “up” or “down”. Aligning spins in material creates magnetism. Moreover, magnetic field affects the passage of spin-up and spin-down electrons differently the paper starts with the detail description of the fundamentals and properties of the spin of the electrons. It proceeds with a note on magneto résistance, the development of Giant Magneto résistance (GMR) and devices like Magneto Random Access Memory, which are the new version of the traditional RAMs. It describe how this new version of RAMs which can revolutionize the memory industry. There is also detailed explanation of the way, how this revolution can increase the data density in our memory systems. It is followed by an account of new Spin Field Effect Transistors.

A Ferro magnet can even affect the flow of a current in a nearby nonmagnetic metal. For example, in the present-day read heads in computer hard drives, wherein a layer of a nonmagnetic metal is sandwiched between two ferromagnetic metallic layers, the magnetization of the first layer is fixed, or pinned, but the second ferromagnetic layer is not. As the read head travels along a track of data on a computer disk, the small magnetic fields of the recorded 1’s and 0‘s change the second layer’s magnetization back and forth parallel or antiparallel to the magnetization of the pinned layer. In the parallel case, only electrons that are oriented in the favored direction flow through the conductor easily. In the antiparallel case, all electrons are impeded. The resulting changes in the current allow GMR read heads to detect weaker fields than their predecessors; so that data can be stored using more tightly packaged magnetized spots on a disk.

Keywords: Nano technology, spintronics, Ferro magnet, Giant magneto resistance, RAMs, conductors.

Conclusion:
So, the new generation of computing and information technology is on its way to revolutionize the 21st century. Spintronics is a technology with a fast track from the discovery of GMR and MTJ materials to the incorporation of these materials in commercial devices. Spintronics read heads dominate the hard-disk market. Magnetic Sensors based on spintronics are making inroads in markets where some combination of high resolution, high sensitivity, small size, and low power are required. Digital data couplers and displacing opt isolators in many applications and are making inroads into new markets heretofore unavailable. MRAM devices are on the horizon and offer the promise of laptop computers that do not need to boot up and cell phones with increased battery time and increased capabilities. It makes sense instead to build on the extensive foundations of conventional electronic semiconductor technology; we exploit the spin of the electron and create new devices and circuits, which could be more beneficial. Finally it ends with a note on why we should switch on this technology

INTRODUCTION

SPINTRONICS:

Imagine a data storage device of the size of an atom working at a speed of light. Imagine a microprocessor whose circuits could be changed on the fly. One minute is could be optimized for data base access. The next for transaction processing and the next for scientific number crunching. Finally, imagine a computer memory thousands of times denser and faster than today’s memories


The above-mentioned things can be made possible with the help of an exploding science – “spintronics”. Spintronics is a NANO technology which deals with spin dependent properties of an electron instead of or in addition to its charge dependent properties,.
Conventional electronics devices rely on the transport of electric charge carries-electrons. But there is other dimension of an electron other than its charge and mass i.e. spins. This dimension can be exploited to create a remarkable generation of spintronic devices. It is believed that in the near future spintronics could be more revolutionary than any other thing that nanotechnology has stirred up so far.


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

As there is rapid progress in the miniaturization of semiconductor electronic devices leads to a chip features smaller than 100 nanometers in size, device engineers and physists are inevitable faced with a looming presence of a quantum property of an electron known as spin, which is closely related to magnetism. Devices that rely on an electron spin to perform their functions from the foundations of Spintronics. Information-processing technology has thus far relied on purely charge based devices ranging from the now quantum, vacuum tube today’s million transistor microchips. Those conventional electronic devices move electronic charges around, ignoring the spin that tags along that side on each electron.

ELECTRON SPIN, FUNDAMENTALS OF SPIN:

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




2. Associated with the spin is 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 electronic 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. Electrons like all fundamental particles have a property called spin, which can be oriented in one direction, or the other called spin-up or spin-down. Magnetism is an intrinsic Physical property associated with the spins.




An intuitive notion of how an electron spins is suggested below.


Imagine a small electronically charged sphere spinning rapidly. The circulating charges in the sphere amount to tiny loops of electric current which creates a magnetic field. A spinning sphere in an external magnetic field changes its total energy according to how its spin vector is aligned with the spin. In some ways, an electron is just like a spinning sphere of charge, an electron has a quantity of angular momentum (spin) an associated magnetism. In an ambient magnetic field and the spin changing this magnetic field can change orientation. Its energy is dependent on how its spin vector is oriented. The Bottom line is that the spin along with mass and charge is defining characteristics of an electron,. In an ordinary electric current, the spin points at random and plays no role in determining the resistance of a wire or the amplification of a transistor circuit. Spintronic devices in contrast rely on the differences in the transport of spin-up and spin-down electrons.

GIANT MAGNETO RESISTANCE:

Magnetism is the integral part of the present day’s data storage techniques. Right from the Gramophone disks to the hard disks of the super computer magnetism plays an important role. Data is recorded and stored as tiny areas of magnetized iron or chromium oxide. To access the information, a read head detects the minute changes in magnetic field as the disk spins underneath it. In this way the read heads detect the data and sent it to the various succeeding circuits. The magneto resistant devices can sense the changes in the magnetic field only to a small extent, which is appropriate to the existing memory devices. When we reduce the size and increase data storage density, we reduce the bits, so our sensor also has to be small and maintain very, very high sensitivity. The thought gave rise to the powerful effect called “GIANT MAGNETORESISTANCE” OR
(GMR).
Giant magneto resistance (GMR) came into picture in 1988, which lead the rise of spintronics. It results from subtle electron-spin effects in ultra-thin ‘multilayer’ of magnetic materials, which cause huge changes in their electrical resistance when a magnetic field is applied. GMR is 200 times stronger than ordinary magnetoresistance. It was soon realized that read heads incorporating GMR materials would be able to sense much smaller magnetic fields, allowing the storage capacity of a hard disk to increase from 1 to 20 gigabits.




CONSTRUCTION OF GMR:

The basic GMR device consists of a three-layer sandwich of a magnetic metal such as cobalt with a nonmagnetic metal filling such as silver. Current passes through the layers consisting of spin-up and spin-down electrons. Those oriented in the same direction as the electron spins in a magnetic layer pass through quite easily while those oriented in the opposite direction are scattered. If the orientation of one of the magnetic layers can easily be changed by the presence of a magnetic field then the device will act as a filter, or ‘spin valve’, letting through more electrons when the spin orientations in the two layers are the same and fewer when orientations are oppositely aligned. The electrical resistance of the device can therefore be changed dramatically. In an ordinary electric Current, the spin points at random and plays no role in determining the resistance of a wire or the amplification of a transistor circuit.


Spintronic Devices, in contrast, rely on differences in the transport of “spin up” and “spin down” electrons. When a current passes through the Ferro magnet, electrons of one spin direction tend to be obstructed. A Ferro magnet can even affect the flow of a current in a nearby nonmagnetic metal. For example, in the present-day read heads in computer hard drives, wherein a layer of a nonmagnetic metal is sandwiched between two ferromagnetic metallic layers, the magnetization of the first layer is fixed, or pinned, but the second ferromagnetic layer is not. As the read head travels along a track of data on a computer disk, the small magnetic fields of the recorded 1’s and 0‘s change the second layer’s Magnetization back and forth parallel or antiparallel to the magnetization of the pinned layer. In the parallel case, only electrons that are oriented in the favored direction flow through the conductor easily. In the antiparallel case, all electrons are impeded. The resulting changes in the current allow GMR read heads to detect weaker fields than their predecessors; so that data can be stored using more tightly packaged magnetized spots on a disk.




MRAM (MAGNETORESISTIVE RANDOM ACCESS MEMORY):

An important spintronic device, which is supposed to be one of the first spintronic devices that have been invented, is MRAM.

.256K MRAM

Unlike conventional random-access, MRAMs do not lose stored information once the power is turned off...A MRAM computer uses power, the four page e mail will be right there for you. Today pc use SRAM and DRAM both known as volatile memory. They can store information only if we have power. DRAM is a series of Capacitors; a charged capacitor represents 1 where as an uncharged capacitor represents 0. To retain 1 you must constantly feed the capacitor with power because the charge you put into the capacitor is constantly leaking out. MRAM is based on integration of magnetic tunnel junction (MJT). Magnetic tunnel junction is a three-layered device having a thin insulating layer between two metallic ferromagnetism. Current flows through the device by the process of quantum tunneling; a small number of electrons manage to jump through the barrier even though they are forbidden to be in the insulator. The tunneling current is obstructed when the two ferromagnetic layers have opposite orientations and is allowed when their Orientations are the same. MRAM stores bits as magnetic polarities rather than electric charges. When a big polarity points in one direction it holds1, when its polarity points in other direction it holds 0. These bits need electricity to change the direction but not to maintain them. MRAM is non volatile so, when you turn your computer off all the bits retain their 1‘s and 0‘s.




SPIN TRANSISTOR CONCEPT:


Traditional transistors use on-and-off charge currents to create bits—the binary zeroes and ones of computer information. “Quantum spin field effect” transistor will use up-and-down spin states to generate the same binary data. One can think of electron spin as an arrow; it can point upward or downward; “spin up and spin-down can be thought of as a digital system, representing the binary 0 and 1. The quantum transistor employs also called “spin-flip” mechanism to flip an up-spin to a downspin, or change the binary state from 0 to 1. One proposed design of a spin FET (spintronic field-effect transistor) has a source and a drain, separated by a narrow semi conducting channel, the same as in a conventional FET. In the spin FET, both the source and the drain are ferromagnetic. The source sends spin-polarized electrons in to the channel, and this spin current flow easily if it reaches the drain unaltered (top). A voltage applied to the gate electrode produces an electric field in the channel, which causes the spins of fast moving electrons to process, or rotate (bottom). The drain impedes the spin current according to how far the spins have been rotated. Flipping spins in this way takes much less energy and is much faster than the conventional FET process of pushing charges out of the channel with a larger electric filed.

COMPARISION BETWEEN ELECTRONICS AND SPINTRONICS DEVICES:

1. Based on properties of charge of the electron
1. Based on intrinsic property spin of electron

2.Quantum property
2. Classical property
3. Controlled by an external electric field in modern electronics

3. Controlled by external magnetic field

4. Materials: conductors and semiconductors
4.Materials: ferromagnetic materials
5. Based on the number of charges and their energy

5. Two basic spin states; spin-up and spin down

6. Speed is limited and power dissipation is
high
6. Based on direction of spin and spin
coupling, high speed


QUANTUM COMPUTER:

In a quantum computer, the fundamental unit of information (called a quantum bit or qubit). This qubit property arises as a


directs consequence of its adherence to the laws of quantum mechanics. A qubit can exist not only in a state corresponding to the logical state 0 or 1 as in a classical bit, but also in states corresponding to a blend or superposition of these classical states. In other words, a qubit can exist as a zero, a one or simultaneously as both 0 and 1, with a numerical coefficient representing the probability for each state. Each electron spin can represent a bit; for instance, a 1 for spin up and 0 for spin down. With conventional computers, engineers go to great lengths to ensure that bits remain in stable, well-defined states. A quantum computer, in contrast, lies on encoding information within quantum bits, or qubits, each of which can exist in a superposition of 0 and 1. By having a large number of qubits in superposition of alternative states, a quantum computer intrinsically contains a massive parallelism. A typical disruption would effectively change a superposition of 0 and 1 randomly into either a 0 or a 1, as process called decoherence. State-of-the-art qubits based on the charge of electrons in a semiconductor remain coherent for a few picoseconds at best and only at temperatures too low for practical applications. The rapid decoherence occurs because the electric force between charges is strong and long range. In traditional semiconductor devices, this strong interaction is beneficial, permitting delicate control of current flow with small electronic fields. To quantum coherent devices, however, it is disadvantage. As a result, an experiment was conducted on the qubits, which are based on the electron-spin. Electron-spin qubits interact only weakly with the environment surrounding them, principally through magnetic fields that are non-uniform in space or changing in time. Such fields can be effectively shielded. The goal of the experiment was to create some of these coherent spin states in a semiconductor to see how long they could survive. Much to the surprise, the optically excited spin states in ZnSe remained coherent for several nanoseconds at low temperatures—1,000 times as long as charge-based qubits. The states even survived for a few nanoseconds at room temperature. Subsequent studies of electrons in gallium arsenide (GaAs) have shown that, under optimal conditions, spin coherence in a semiconductor is possible.

ADVANTAGES:

Ø With lack of dissipation, spintronics may be the best mechanism for creating ever smaller device.
Ø Spintronics does not require and specialized semiconductor, therefore it can be implemented or worked with common metals such as copper, aluminum and silver.
Ø Spintronics devices would consume less power compared to conventional electronics, because energy needed to change spin is a easy compared to energy needed to push the charge around.
Ø Since spin does not change when power is turned off, since the memory is non-volatile.
Ø Spintronics devices is also used for multi purpose (amplifiers)
Ø Spintronic device doesn’t require electric current.

DISADVANTAGES:

Ø Spintronic devices unable to controlling the spin for long distances.
Ø Difficult to inject and measure spin in silicon
Ø Silicon causes electrons to lose their spin state

CONCLUSION:

It makes sense instead to build on the extensive foundations of conventional electronic semiconductor technology; we exploit the spin of the electron and create new devices and circuits, which could be more beneficial.

REFERENCE:

Ø IBM RD 50-1 spintronics_A retrospective and perspective.
Ø www.prola.aps.org/pdf/prl
Ø www.google/spintronice/nanotech