ABSTRACT

“A New Electronics Field For a New Century: - Spintronics.”

Spintronics aims to exploit one of the subtle quantum properties of the electron, its spin to yield a desired outcome.

Electrons carry an intrinsic electric charge and most electronic and optical devices (i.e. lasers, detectors of optical radiation, etc.) operate by manipulating these charges through the use of electric voltages.

Spin is another important intrinsic property of electrons. Like all things that spin, electrons spin in a direction usually defined by the axis of the motion. In the case of an electron, spin can take only one of two positions with regard to some arbitrarily chosen direction -- spin-up or spin-down.

               

Conventional electronic devices use only the electron's charge. In them, spinning electrons point randomly in all directions and are not directly involved in device function. Spintronic devices, however aim to take advantage of both properties.

               

If spin can be manipulated, electrons will be able to perform new functions in data processing and storage. In fact, data processing and storage could be merged in the same basic component.

   

This would make it possible to produce, among other things, "Quantum computers" that would not have to relay on binary digits (1or 0) but could encode information in different spin states -- up, down or any of an infinite number of possibilities involving a mixture of both.

               

The consequence will be a fundamental change in the concept of electronic device design resulting not only in higher computing speed, but also in great improvements in such fields as photonics and data transmission and communication.

                 

In order to align or otherwise manipulate the orientation of spin, however, structures of metals or semiconductors must be developed that are capable of sensing electron spin direction and, based on this information, creating gateways for electrons. Such materials might allow spins pointing in one direction to pass, for instance, while spins pointing in the opposite direction might be turned away.

               

 It being aimed to develop magnetic conducting materials specifically those made of ferromagnetic heterostructure {ln (Mn) As / Ga (Mn) Sb / Al (Mn) Sb} – and novel devices from these materials. This group of materials has great potential for electronic devices and for optical devices to produce and detect infrared and far infrared signals (useful for, e.g., heat sensing and night vision).

                   

There being review of proposal of using electron spins in quantum-confined structures as qubits and the requirements for implementing a quantum computer. Description of several realizations of one- and two-qubit gates and of the read-in and read-out tasks. Discussions recently proposed schemes for using a single quantum dot as spin-filter and spin-memory device. Considering electronic EPR (Einsten-Podolsky- Rosen) pairs needed for quantum communication it has been shown that their spin entanglement can be detected in mesoscopic transport measurements using metallic as well as super conducting leads attached to the dots.

                               

                             

INDEX

                                        

                                                                         

  1. INTRODUCTION TO SPINTRONICS
  2. MECHANISM OF SPINTRONICS
  3. SPINTRONIC DEVICES
  4. SPIN BASED QUANTUM COMPUTERS
  5. ADVANTAGES
  6. SPIN WAVES
  7. ADVANTAGES OF SPIN WAVES
  8. CONCLUSION
  9. REFERENCES                                                                                                                                                                        

           

                   

          

                            

INTRODUCTION TO SPINTRONICS

Magnetoelectronics, Spin Electronics, and Spintronics are different names for the same thing.

Spintronics are devices that could one day replace today’s conventional transistors in microelectronics. Spintronics, also known as spin electronics, exploit an electron’s spin characteristic not just its charge.

What is Spintronics?

                     

This is a new technological discipline, which aims to exploit the subtle and mind-bendingly esoteric quantum properties of the electron to develop a new generation of electronic devices. Every electron exists in one of two states, spin-up or spin-down; it is possible to make a sandwich of gold atoms between two thin films of magnetic material that will act as a filter or valve that only permits electrons in one of the two states to pass. The filter can be changed from one state to the other using a brief and tiny burst of current. From this simple device it's hoped to make incredibly tiny chips that will act as super-fact memories whose contents will survive loss of power. The adjective is spintronic.

                             

Spintronics, or spin electronics, refers to the study of the role played by electron (and more generally nuclear) spin in solid-state physics, and possible devices that specifically exploit spin properties instead of or in addition to charge degrees of freedom. .The prototype device that is already in use in industry as a read head and a memory-storage cell is the giant-magneto resistive (GMR) sandwich structure which consists of alternating ferromagnetic and nonmagnetic metal layers. Depending on the relative orientation of the magnetizations in the magnetic layers, the device resistance changes from small (parallel magnetizations) to large (anti-parallel magnetizations). This change in resistance (also called magneto resistance) is used to sense changes in magnetic fields. Recent efforts in GMR technology have also involved magnetic tunnel junction devices where the tunneling current depends on spin orientations of the electrodes.                      

MECHANISM OF SPINTRONICS

Spintronics (slide) is a new branch of electronics in which electron spin, in addition to charge, is manipulated to yield a desired outcome. All spintronic devices act according to the simple scheme: 

(1.) Information is stored (written) into spins as a particular spin orientation (up or down).     

    

(2.) The spins, being attached to mobile electrons, carry the information along a wire.             

 

(3.) The information is read at a terminal, and orientation of conduction electrons survives for a relatively long time (nanoseconds, compared to tens of femtoseconds during which electron momentum decays), which makes spintronic devices particularly attractive for memory storage and magnetic sensors applications, and, potentially for quantum computing where electron spin would represent a bit (called qubit) of information.

Voltage Control of Spin Direction in Magnetic Memory Devices

                 Spintronic structures are shown, including giant magneto resistance (GMR), tunneling magneto resistance (TMR), and Argonne's new four-layer structure denoted VCR to stand for "voltage controlled rotation" of the magnetization. F1 and F2 are ferromagnetic layers, S is a metallic spacer, and I is an insulating layer.

Fig 1: -Spintronic structure

"Spintronics" is a fairly recent term, but the concept isn’t so very exotic. The particles we call electrons have both charge and spin. Conventional electronic devices use only the charge, while spintronic devices take advantage of both properties. When the spins of a material’s electrons are aligned along a common direction, rather than pointing randomly, it is said to be magnetized. Today, most of the information we deal with is processed and stored magnetically.

                           

SPINTRONIC DEVICES

1.DATTA DAS SPIN TRANSISTOR

Datta-Das spin transistor was the first spintronic device to be proposed for fabrication in the metal-oxide-semiconductor geometry familiar in conventional microelectronics. An electrode made of a ferromagnetic material (left) emits spin-aligned electrons (red spheres), which pass through a narrow channel (blue) controlled by the gate electrode (gray) 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 (bottom), 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. As yet, no convincingly successful application of this proposal has been demonstrated.



2. JOHNSON SPIN TRANSISTOR

Depending on the orientation of magnetization in the two ferromagnetic layers, the current in the collector circuit flows either from the base into the emitter (left) or from the emitter into the base (right).

3. SPIN POLARIZED SOLAR BATTERY

                                     

                                                  n

       

         p

Filtered solar light (circularly polarized) generates electron hole pairs in the depletion region. Only electrons carry the polarization if the semiconductor is -V, like GaAs. The resulting current flowing in an external circuit that connects the n and p regions is spin-polarized.

        

                         

                               

4. MAGNETIC FIELD EFFECT TRANSISTOR

                                     

                                                  n

       

         p

Magnetic field B is applied along the p-n junction. The current in the circuit connecting the junction in the transverse direction depends critically on the size of depletion layer (it is small for the larger layer and larger for a small layer). If the g factor of the electrons or holes is large, a change in B can lead to a large change in the width of the depletion layer and in the magnitude of the transverse current.

                        

                     SPIN BASED QUANTUM COMPUTERS

One of the most ambitious spintronic devices is the spin-based quantum Computer (QC) in solid-state structures. Using electron (or nuclear) spin for QC purposes is a manifestly obvious idea since a fermion with spin 1/2 is a natural and intrinsic qubit. Quantum computation requires both long quantum coherence time and precise external control. Because of the requirement of very long coherence time for a QC, both nuclear spin and electron spin have been proposed as qubit in a QC.

Quantum computers lead the race by their totally different and innovative algorithms harnessing the power of quantum effect in the nature.

 

 Key factors, which gives a Quantum Computers a definitive leap are-

EFFECTIVELY MORE STORAGE

                                     

If we choose an atom as a physical bit then quantum mechanics tells us that apart from the two distinct electronic states the atom can be also prepared in a coherent superposition of the two states. This means that the atom is both in state 0 and state 1. Now if we can push the idea of superposition of numbers a bit further, then we can easily understand that as the number of quantum bits increases, power of storage grows exponentially. Any classical register of 3 bit can store in a given moment of time only one out of eight different numbers while a quantum register composed of three qubits can store in a given moment all eight numbers in a single quantum superposition. If we keep adding qubits to the register we increase its storage capacity exponentially i.e. three qubits can store 8 different numbers at once, four qubits can store 16 different numbers at once, and so on; in general L qubits can store 2L numbers at once. And thus storage power keeps on increasing massively, with each successive addition of atom.

EXTREMELY FASTER PROCESSING

Once a quantum register is prepared in a superposition of different numbers we can at once perform operations on all of them. For example, if qubits are atoms then suitably tuned laser pulses affect atomic electronic states and evolve initial superposition of encoded numbers into different superpositions. During such evolution each number in the superposition -is affected and as the result we generate a massive parallel computation albeit in one piece of quantum hardware. This means that a quantum computer can in only one computational step perform the same mathematical operation on 2L different input numbers encoded in coherent superpositions of L qubits. In other words a quantum computer offers an enormous gain in the use of computational resources such as time and memory.

 

The figure shows how a particular circuit called a 'half-adder' can be made from a pattern composed of two kinds of cell. This type of architecture is very suitable for the nanometer-scale, where simple units form naturally. One way to make the cells would be using structures called quantum-dots, which are also known as 'artificial atoms'.

HIGHLY EFFICIENT ALGORITHMS

Quantum algorithms have the potential to be dramatically faster than their conventional counterparts. A good example is an algorithm for searching through lists. The problem is to find a person's name in a telephone directory, given his or her phone number. If the directory contains N entries, then on average, you would have to search through N/2 entries before you find it. Grover's quantum algorithm does much better. It finds the name after searching through only √ N entries, on average. So for a directory of 10,000 names, the task would require √ (10,000) = 100 steps, rather than 5000. The algorithm works by first creating a superposition of all 10,000 entries in which each entry has the same likelihood of appearing in response to a measurement made on the system. Then, to increase the probability of a measurement producing the required entry, the superposition is subjected to a series of quantum operations that recognize the required entry and increase its chances of appearing.

                         ADVANTAGES OF SPINTRONICS

                                   SPIN WAVES

About Spin waves:-

In a sample of pure iron, it is the unpaired electrons within the atomic lattice that are responsible for the material's magnetic properties. These electrons have spin and an associated magnetic moment creating individual magnetic dipoles through each electron's spin axis. Each electron's spin axis can also precess like a gyroscope and spin waves propagate through these precessing magnetic dipoles of the electrons as their individual magnetic fields interact with each other. In other magnetic materials a combination of both axial and orbital electron spin can create magnetism if the combination is not compensated for by equal and opposite combined spins of other electrons. Spin waves can propagate through the precession of these resulting uncompensated spins. Spin waves can also propagate through uncompensated nuclear spins but the magnetic moments and coupling between spins is weaker than with electron spins.

Spin waves can transport energy and information from one location to another.

Short and long range spin wave interactions can occur among particles causing fluctuations in the sea of standing waves among all matter.

                                                                       

SPIN WAVES

FUNCTION OF SPIN WAVES:-

1.      Signal        Processing:
There are 2 basic forms of motion of all electrically charged particles. Either they can change in position or change in spin axis orientation.
Much of electronics today is based on the first of these 2 forms of motion.
Signal processing electronics can be advanced by using devices based on the propagation of changing spin axis orientations.

  1. Communications:
    Almost all forms of radio communication today are based on the radiation of electromagnetic waves from electrically charged particles that move back and forth changing position Communications technology can be advanced by instead using electromagnetic waves radiated from the precessional rotation of electrically charged particals. Fluctuations can be induced in and propagate through the sea of electromagnetic standing waves among all matter in the universe.

3. Power Generation 

It should be possible to build spin wave lasers and spin wave electrical power generation devices.It should be possible to absorb some of the energy present in standing waves among all matter and convert it to electricity to power electrical motors and appliances.This can be accomplished through spin wave interactions with the electromagnetic standing waves among all matter.

4. Propulsion:

It should be possible to create and sustain spin wave processes within a flying vehicle that utilizes a metallic resonant cavity, also serving as a “Faraday cage”These spin wave processes can be used to shift the phase of the precessional motions of all atomic particles of the vehicle and its contents relative to the phase of standing waves radiated to and from the precessional motions of all external matter.This can create electromagnetic forces between vehicle and these standing waves among all external matter. Force vectors can be controlled to lift and propel the vehicle at very high speeds.

  1. ADVANTAGES OF SPIN WAVES

     

CONCLUSION

                 

“Spin much like mass and charge is an intrinsic property of electron which has several states –“up”, “down” or somewhere in between.

                 

Today’s computers rely on silicon-based microchips to process data in a binary form – which allows only for “on” and “off” states. Quantum computers however, will be able to examine data using spins, which has can have many different states. Next generation Quantum computers” will be able to process information much faster than the conventional microchip machines and the capacity can be increased by factor of many thousands.

                   

An inherent advantage of spintronics over electronics- the fact that magnets tend to sty magnetized – is sparking industry in replacing computer’s semiconductor based components with magnetic ones, starting with the RAM. Cut off an electronic device’s power, and the information stored via electronic charge is lost. That is why, before turning a computer off, the user has to save new work to a disk. A computer with all magnetic RAM would always retain the information put into it. But most important, there would be no “boot-up” waiting period when the power is turned on – a great advantage, especially for the laptop user.

REFERENCES