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.
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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
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