Download Introduction to Spintronics

Electron has :
Mass
Charge
Spin
Spintronics=spin based electronics
information is carried by spin not by
charge
ferromagnetic metallic alloy based
devices
transport in fm materials is spin
polarized
Introduction
 Conventional 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 spintronics
 Non-volatile memory
 performance improves with smaller devices
 Low power consumption
 Spintronics does not require unique and specialised
semiconductors
 Dissipation less transmission
 Switching time is very less
 compared 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 injection
 Using 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 Transfer
 Current 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
<S>
v
v
M1
M2
The spin of the
conduction electron
is rotated by its
interaction with the
magnetization.
This implies the magnetization exerts a torque on the spin. By
Conservation of angular momentum, the spin exerts an equal and
Opposite torque on the magnetization.
Spin detection
Optical detection techniques using magnetic
resonance force microscopy
Electrical sensing techniques-through quantum
dots and quantum point contact
SPIN RELAXATION
 Leads to spin equilibration
 T1-Spin-lattice relaxation time
 T2-Spin-spin relaxation time
 Neccesary condition 2T1>=T2.
Application
GMR(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 Magnetoresistance
 Tunnel 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
MRAM
 MRAM 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
MRAM
 MRAM promises:
 Density of DRAM
 Speed of SRAM
 Non-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 Transistor
 The Datta Das Spin
Transistor was first spin
device proposed for metaloxide 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 Research
 Ferromagnetic 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 Outlook
 High capacity hard drives
 Magnetic RAM chips
 Spin FET using quantum tunneling
 Quantum computers
limitations
 Controlling spin for long distances
 Difficult to INJECT and MEASURE spin.
 Interfernce of fields with nearest elements
 Control of spin in silicon is difficult
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