IntroductionConventional electronic devices ignore the spin

property

As electronic devices become smaller, quantum properties of the wavelike nature of electrons are no longer negligible.

Adding the spin degree of freedom provides new effects, new capabilities and new functionalities

Information is stored into spin as one of two possible orientations

Advantages of spintronicsNon-volatile memory performance improves with smaller devices Low power consumption Spintronics does not require unique and

specialised semiconductorsDissipation less transmission Switching time is very lesscompared to normal RAM chips, spintronic

RAM chips will:– increase storage densities by a factor of three– have faster switching and rewritability rates smaller

Phases in Spintronics SPIN INJECTION

SPIN TRANSFER

SPIN DETECTION

Spin injectionUsing a ferromagnetic electrode

effective fields caused by spin-orbit interaction.

a vacuum tunnel barrier could be used to effectively inject spins into a semiconductor

back biased Fe/AlGaAs Schottky diode has been reported to yield a spin injection efficiency of 30%

By “hot” electrons

Spin TransferCurrent passed through a magnetic field becomes

spin polarized

This flipping of magnetic spins applies a relatively large torque to the magnetization within the external magnet

This torque will pump energy to the magnet causing its magnetic moment to precess

If damping force is too small, the current spin momentum will transfer to the nanomagnet, causing the magnetization to flip

Spin Transfer Torque

The spin of the The spin of the conduction electron conduction electron is rotated by its is rotated by its interaction with the interaction with the magnetization.magnetization.

This implies the magnetization exerts a torque on the spin. By This implies the magnetization exerts a torque on the spin. By Conservation of angular momentum, the spin exerts an equal and Conservation of angular momentum, the spin exerts an equal and Opposite torque on the magnetization.Opposite torque on the magnetization.

2M1M

<S><S>

v v

Spin detection Optical detection techniques using

magnetic resonance force microscopy

Electrical sensing techniques-through

quantum dots and quantum point contact

SPIN RELAXATIONLeads to spin equilibration

T1-Spin-lattice relaxation time

T2-Spin-spin relaxation time

Neccesary condition 2T1>=T2.

ApplicationGMR(Giant magnetoresistance)

Discovered in 1988 France

a multilayer GMR consists of two or more ferromagnetic layers separated by a very thin (about 1 nm) non-ferromagnetic spacer (e.g. Fe/Cr/Fe)

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

Conversely when magnetization vectors are antiparallel, high R

Parallel current GMR

Perpendicular current GMR

Spin Valve

Simplest and most successful spintronic device

Used in HDD to read information in the form of small magnetic fields above the disk surface

Tunnel MagnetoresistanceTunnel Magnetoresistive effect combines

the two spin channels in the ferromagnetic materials and the quantum tunnel effect

TMR junctions have resistance ratio of about 70%

MgO barrier junctions have produced 230% MR

MRAMMRAM uses magnetic storage elements

Tunnel junctions are used to read the information stored in MRAM

MRAM

Attempts were made to control bit writing by using relatively large currents to produce fields

This proves unpractical at nanoscale level

MRAM

The spin transfer mechanism can be used to write to the magnetic memory cells

Currents are about the same as read currents, requiring much less energy

MRAMMRAM promises:

Density of DRAMSpeed of SRAMNon-volatility like flash

Spin 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

Datta Das Spin TransistorThe Datta Das Spin

Transistor was first spin device proposed for metal-oxide geometry, 1989

Emitter and collector are ferromagnetic with parallel magnetizations

The gate provides magnetic field

Current is modulated by the degree of precession in electron spin

Current ResearchFerromagnetic transition temperature in

excess of 100 K Spin injection from ferromagnetic to non-

magnetic semiconductors and long spin-coherence times in semiconductors.

Ferromagnetism in Mn doped group IV semiconductors.

Room temperature ferromagnetism Large magnetoresistance in ferromagnetic

semiconductor tunnel junctions.

Future OutlookHigh capacity hard drivesMagnetic RAM chipsSpin FET using quantum tunnelingQuantum computers

limitationsControlling spin for long distancesDifficult to INJECT and MEASURE spin.Interfernce of fields with nearest elementsControl of spin in silicon is difficult

THANK YOU!