Spin does not replace charge current just provide extra control Using suitable materials, many different “bit” states can be interpreted
New technology has been proposed which would involve a complete set of new materials, new handling and processing techniques, and altered circuit design. Such developments include single-electron transistors and molecular-electronic devices based on organic materials or carbon nanotubes.
Charge state can be destroyed by interactions with impurities or other charges
Think of optical polarizers
Rashba effect – consequence of spin orbit interaction, proportional to electric field in a structure with inversion asymmetry
IntroductionConventional electronic devices ignorethe spin property and rely strictly on thetransport of the electrical charge ofelectronsAdding the spin degree of freedomprovides new effects, new capabilities andnew functionalities
Disadvantages of electronics High power consumption. High heat dissipation. Electronics memory is volatile. Takes up higher space on chip, thus less compact. Electron manipulation is lower , so poor read & write speed.Electronics require unique and specialized semiconductors materials. Common metals such as Fe, Al, Ag , etc. can’t be used.
Future DemandsMoore’s Law states that the number of transistors on a silicon chip willroughly double every eighteen monthsBy 2008, it is projected that the width of the electrodes in amicroprocessor will be 45nm acrossAs electronic devices become smaller, quantum properties of the wavelike nature of electrons are no longer negligibleSpintronic devices offer the possibility of enhanced functionality, higherspeed, and reduced power consumption
Computational benefitsSimple device structure for high degree of integration and high process yield.Large magnetocurrent for high speed operationHigh transconductance for high speed operationHigh amplification capability (V, I, and/or power)Hyperthreading enchancmentBit vs. qubit
Electronics v/s Spintronics One of the main advantage of spintronics over electronics isthe magnets tend to stay magnetize which is sparking in theindustry an interest for replacing computer’s semiconductorbased components with magnetic ones, starting with the RAM. With an all-magnetic RAM, it is now possible to have acomputer that retains all the information put into it. Mostimportantly, there will be no ‘boot-up’ waiting period whenpower is turned on.
Another promising feature of spintronics is that it doesn’trequire the use of unique and specialized semiconductor, thereby allowing it to work with common metals like Cu, Al, Ag. Spintronics will use less power than conventional electronics,because the energy needed to change spin is a minute fractionof what is needed to push charge around.
Conventional ElectronicsMetal Gaten+ n+Ohmic contact Ohmic ContactP-type SiOxideElectronInversion layerMetal Oxide Semiconductor Field Effect TransistorMOSFETGate Voltage changes electron densitychanges conductivity
SpintronicsInject polarized spin from one FM contact -- modulate current bymodifying spin precession via Rashba effectSpin TransistorSchottky GateFM Metal FM MetalInGaAsModulation Doped AlGaAs2DEGSpinAnalyzerBSpinInjector
Principal of spintronics Spintronics is based on the spin of electrons rather than its charge. Every electron exist in one of the two states- spin-up and spin-down,with spins either positive half or negative half. In other words, electrons can rotate either clock wise or anti-clockwisearound its own axis with constant frequency. The two possible spin states represent ‘0’ and ‘1’ in logical operations.
Combining the best of both worldsFerro magnets• Stable Memory• Fast switching• High ordering temp• Spin transport• Technological base(magnetic recordings)Semiconductors• Bandgap engineering• Carrier density & type• Electrical gating• Long spin lifetime• Technological base(Electronics)develop spin based transistors ,switches and logic circuits?create control propagate spininformation in semiconductorstructures?
Advantages of Spin Information is stored into spin as one of two possible orientationsSpin 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
Spin is a characteristic that makes an electron a tiny magnet with north andsouth poles. The orientation of north-south axis depends on the particle’s axis of spin. In ordinary materials, the up magnetic moments cancel the down magneticmoment so no surplus moment piles up. Ferro-magnetic materials like iron, cobalt and nickel is needed fordesigning of spin electronic devices.
Giant MagnetoResistive (GMR)1988 France, GMR discovery is accepted as birth ofspintronicsA Giant MagnetoResistive device is made of at leasttwo ferromagnetic layers separated by a spacer layerWhen the magnetization of the two outside layers isaligned, lowest resistanceConversely when magnetization vectors areantiparallel, high RSmall fields can produce big effectsparallel and perpendicular current
Spin ValveSimplest and most successful spintronic deviceUsed in HDD to read information in the form of smallmagnetic fields above the disk surface
Datta Das Spin TransistorThe Datta Das Spin Transistor was firstspin device proposed for metal-oxidegeometry, 1989Emitter and collector are ferromagneticwith parallel magnetizationsThe gate provides magnetic fieldCurrent is modulated by the degree ofprecession in electron spin
Spin TransferVThe spin of theconduction electronis rotated by itsinteraction with themagnetization.This implies the magnetization exerts a torque on the spin. ByConservation of angular momentum, the spin exerts an equal andOpposite torque on the magnetization.<S>v2M1M
Experimental Proof of Spin TransferIPredicted theoreticallyby Slonczewksi andBerger in 1996IIPAP
Anisotropic magnetoresistance (AMR) Property of a material in which a dependence of electrical resistanceon the angle between the direction of electric current and direction ofmagnetization is observed
Tunnel magnetoresistance(TMR) Tunnel magnetoresistance (TMR) is a magnetoresistive effect thatoccurs in a magnetic tunnel junction (MTJ), which is a componentconsisting of two ferromagnets separated by a thin insulator.
Click to edit Master text stylesSecond level● Third level● Fourth level● Fifth levelWHICH MATERIAL GIVESOPTIMISTIC VALUE
(MRAM)Magneto resistive RAMReading process(ON-State) Measurement of the bit cellresistance by applying acurrent in the ‘bit line’ Comparison with a referencevalue mid-way between thebit high and low resistancevalues
Writing process(Off-State) Currents applied in both lines : 2magnetic fields Both fields are necessary toreverse the free layermagnetization When currents are removed :Same configuration
Spintronics capabilities(1) Hard drives up to 1.2 petabytes with compact capability
Conclusion Interest in spintronics arises, in part, from the looming problemof exhausting the fundamental physical limits of conventionalelectronics. However, complete reconstruction of industry is unlikely andspintronics is a “variation” of current technology The spin of the electron has attracted renewed interest because itpromises a wide variety of new devices that combine logic,storage and sensor applications. Moreover, these "spintronic" devices might lead to quantumcomputers and quantum communication based on electronicsolid-state devices, thus changing the perspective of informationtechnology in the 21st century.