19747894 introduction to spintronics
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8/3/2019 19747894 Introduction to Spintronics
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Electron has :
Mass
Charge
Spin
Spintronics=spin based electronics
information is carried by spin not by
charge
ferromagnetic metallic alloy baseddevices
transport in fm materials is spin
polarized
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Introduction
y Conventional electronic devices ignore the spin property
y As electronic devices become smaller, quantumproperties of the wavelike nature of electrons are nolonger negligible.
y Adding the spin degree of freedom provides new effects,new capabilities and new functionalities
y Information is stored into spin as one of two possibleorientations
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Advantages of spintronics
y Non-volatile memory
y performance improves with smaller devices
y Low power consumption
y Spintronics does not require unique and specialisedsemiconductors
y Dissipation less transmission
y Switching time is very less
y compared to normal RAM chips, spintronic RAMchips will:
increase storage densities by a factor of three
have faster switching and rewritability rates smaller
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Phases in SpintronicsySPIN INJECTION
ySPIN TRA NSFER
y
SPIN DETECTION
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Spin injectiony Using a ferromagnetic electrode
y effective fields caused by spin-orbit interaction.
y a vacuum tunnel barrier could be used to effectively inject spins into a semiconductor
y back biased Fe/AlGaAs Schottky diode has beenreported to yield a spin injection efficiency of 30%
y By hot electrons
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Spin Transfer
y Current passed through a magnetic field becomes spinpolarized
y This flipping of magnetic spins applies a relatively large torque
to the magnetization within the external magnet
y This torque will pump energy to the magnet causing itsmagnetic moment to precess
y If damping force is too small, the current spin momentum willtransfer to the nanomagnet, causing the magnetization to flip
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Spin Transfer Torque
The spin of theThe spin of the
conduction electronconduction electron
is rotated by itsis rotated by its
interaction with theinteraction with the
magnetization.magnetization.
This implies the magnetization exerts a torque on the spin. ByThis implies the magnetization exerts a torque on the spin. By
Conservation of angular momentum, the spin exerts an equal andConservation of angular momentum, the spin exerts an equal and
Opposite torque on the magnetization.Opposite torque on the magnetization.
2
M1
M
<S><S>
v v
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Spin detectionOptical detection techniques using magnetic
resonance force microscopy
Electrical sensing techniques-through quantum
dots and quantum point contact
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SPIN RELAXATIONy Leads to spin equilibration
y T1-Spin-lattice relaxation time
y T2-Spin-spin relaxation time
y Neccesary condition 2T1>=T2.
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Application
GMR(Giant magnetoresistance)y Discovered in 1988 France
y 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)
y When the magnetization of the two outsidelayers is aligned, resistance is low
y Conversely when magnetization vectors areantiparallel, high R
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Parallel current GMR
Perpendicular current GMR
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Spin Valve
y Simplest and most successful spintronic device
y Used in HDD to read information in the form of small magnetic fields above the disk surface
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Tunnel Magnetoresistance
y Tunnel Magnetoresistive effect combines the twospin channels in the ferromagnetic materials andthe quantum tunnel effect
y TMR junctions have resistance ratio of about 70%
y MgO barrier junctions have produced 230% MR
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MRAMy
MRAM uses magnetic storage elements
y Tunnel junctions are used to read the informationstored in MRAM
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MRAM
y Attempts were made to control bit writing by using relatively large currents to produce fields
y This proves unpractical at nanoscale level
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MRAM
y The spin transfer mechanism can be used to write to the magnetic memory cells
y Currents are about the same as read currents,
requiring much less energy
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MRAMy
MRAM promises:y Density of DRAM
y Speed of SRAM
y Non-volatility like flash
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Spin Transistor
y Ideal use of MRAM would utilize control of thespin channels of the current
y Spin transistors would allow control of the spin
current in the same manner that conventionaltransistors can switch charge currents
y Using arrays of these spin transistors, MRAM willcombine storage, detection, logic and
communication capabilities on a single chipy This will remove the distinction between working
memory and storage, combining functionality of many devices into one
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Datta Das Spin Transistor
y The Datta Das SpinTransistor was first spindevice proposed for metal-oxide geometry, 1989
y Emitter and collector areferromagnetic with parallelmagnetizations
y The gate provides magnetic
fieldy Current is modulated by the
degree of precession inelectron spin
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Current Researchy
Ferromagnetic transition temperature in excess of 100 K
y Spin injection from ferromagnetic to non-magneticsemiconductors and long spin-coherence times insemiconductors.
y Ferromagnetism in Mn doped group IV semiconductors.
y Room temperature ferromagnetism
y Large magnetoresistance in ferromagnetic
semiconductor tunnel junctions.
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Future Outlooky High capacity hard drives
y Magnetic RAM chips
y Spin FET using quantum tunnelingy Quantum computers
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limitationsy Controlling spin for long distances
y Difficult to INJECT and ME ASUR E spin.
y Interfernce of fields with nearest elementsy Control of spin in silicon is difficult
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