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Spintronics
Seminar Report 2011
Introduction
Spintronics
(a
neologism
meaning
"spin
transport
electronics", also known as magnetoelectronics, is an emerging
technology that exploits the intrinsic spin of the electron and its
associated magnetic moment, in addition to its fundamental
electronic charge, in solid-state devices. They rely completely on
magnetic moment of the electron. Electrons are spin-1/2 fermions
and therefore constitute a two-state system with spin "up" and spin
"down". Electrons have a property that they occupy only one quantum
state at a given time. To make a spintronic device, the primary
requirements are a system that can generate a current of spinpolarized electrons comprising more of one spin species—up or
down—than the other (called a spin injector), Spin process can be
accomplished using real external magnetic fields or effective fields
caused by spin-orbit interaction.
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S.S.M Polytechnic, Tirur
Spintronics
Seminar Report 2011
Advantages of spintronics

Non-volatile memory.

Performance improves with smaller devices.

Low power consumption.

Spintronics
does
not
require
unique
and
specialized
semiconductors.

Dissipation less transmission.

Switching time is very less compared to normal RAM chips,
spintronic RAM chips will:
o
Increase storage densities by a factor of three,
o
Have faster switching and rewritability rates smaller.
Limitations

Controlling spin for long distances.

Difficult to Inject and Measure spin.

Interfernce of fields with nearest elements.

Control of spin in silicon is diffic.
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S.S.M Polytechnic, Tirur
Spintronics
Seminar Report 2011
Historical Perspective
The research field of spintronics emerged from experiments
on spin-dependent electron transport phenomena in solid-state
devices done in the 1980s, including the observation of spin-polarized
electron injection from a ferromagnetic metal to a normal metal by
Johnson
and
Silsbee
(1985),
and
the
discovery
of
giant
magnetoresistance independently by Albert Fert and Peter Grünberg.
The origins can be traced back further to the ferromagnet
/superconductor tunneling experiments pioneered by Meservey and
Tedrow, and initial experiments on magnetic tunnel junctions by
Julliere in the 1970s. The use of semiconductors for spintronics can
be traced back at least as far as the theoretical proposal of a spin
field-effect-transistor by Datta and Das in 1990.
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S.S.M Polytechnic, Tirur
Spintronics
Seminar Report 2011
Working
Electrons are spin-1/2 fermions and therefore constitute a
two-state system with spin "up" and spin "down". To make a
spintronic device, the primary requirements are a system that can
generate a current of spin-polarized electrons comprising more of one
spin species—up or down—than the other (called a spin injector), and
a separate system that is sensitive to the spin polarization of the
electrons (spin detector). Manipulation of the electron spin during
transport between injector and detector (especially in semiconductors)
via spin precession can be accomplished using real external magnetic
fields or effective fields caused by spin-orbit interaction.
Spin polarization in non-magnetic materials can be
achieved either through the Zeeman Effect in large magnetic fields
and low temperatures, or by non-equilibrium methods. In the latter
case, the non-equilibrium polarization will decay over a timescale
called the "spin lifetime". Spin lifetimes of conduction electrons in
metals are relatively short (typically less than 1 nanosecond) but in
semiconductors the lifetimes can be very long (microseconds at low
temperatures), especially when the electrons are isolated in local
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S.S.M Polytechnic, Tirur
Spintronics
Seminar Report 2011
trapping potentials (for instance, at impurities, where lifetimes can be
milliseconds).
All spintronic devices act according to the simple scheme:
(1)
Information is stored (written) into spins as a particular spin
orientation (up or down).
(2)
The spins, being attached to mobile electrons, carry the
information along a wire, and
(3)
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The information is read at a terminal.
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S.S.M Polytechnic, Tirur
Spintronics
Seminar Report 2011
Giant Magnetoresistance (GMR)
The simplest method of generating a spin-polarised current
in a metal is to pass the current through a ferromagnetic material.
The
most
common
application
of
this
effect
is
a
giant
magnetoresistance (GMR) device. A typical GMR device consists of at
least two layers of ferromagnetic materials separated by a spacer
layer. When the two magnetization vectors of the ferromagnetic
layers are aligned, the electrical resistance will be lower (so a higher
current flows at constant voltage) than if the ferromagnetic layers are
anti-aligned. This constitutes a magnetic field sensor.
Two variants of GMR have been applied in devices:

current-in-plane (CIP), where the electric current flows
parallel to the layers and

current-perpendicular-to-plane
(CPP),
where
the
electric
current flows in a direction perpendicular to the layers.
Other metals-based spintronics devices:
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S.S.M Polytechnic, Tirur
Spintronics
Seminar Report 2011

Tunnel Magnetoresistance (TMR), where CPP transport is
achieved by using quantum-mechanical tunneling of electrons
through a thin insulator separating ferromagnetic layers.

Spin Torque Transfer, where a current of spin-polarized
electrons is used to control the magnetization direction of
ferromagnetic electrodes in the device.
MRAM
MRAM uses magnetic storage elements. Tunnel junctions
are used to read the information stored in MRAM. Attempts were
made to control bit writing by using relatively large currents to
produce fields. This proves unpractical at nanoscale level. 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.
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S.S.M Polytechnic, Tirur
Spintronics
Seminar Report 2011
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 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.
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S.S.M Polytechnic, Tirur
Spintronics
Seminar Report 2011
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
ferromagnetism.
IV
Large
semiconductors.
Room
magnetoresistance
in
temperature
ferromagnetic
semiconductor tunnel junctions.
Future Outlook

High capacity hard drives, the future Plastic data storage.

Magnetic RAM chips

Spin FET using quantum tunneling

Quantum computers
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S.S.M Polytechnic, Tirur
Spintronics
Seminar Report 2011
Applications
Motorola has developed a 1st generation 256 kb MRAM
based on a single magnetic tunnel junction and a single transistor
and which has a read/write cycle of under 50 nanoseconds (Ever
spin, Motorola's spin-off, has since developed a 4 Mbit version. There
are two 2nd generation MRAM techniques currently in development:
Thermal Assisted Switching (TAS) which is being developed by
Crocus Technology, and Spin Torque Transfer (STT) on which Crocus,
Hynix, IBM, and several other companies are working.
Another design in development, called Racetrack memory,
encodes information in the direction of magnetization between
domain walls of a ferromagnetic metal wire.
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S.S.M Polytechnic, Tirur
Spintronics
Seminar Report 2011
Semiconductor-based spintronic devices
Ferromagnetic semiconductor sources (like manganesedoped gallium arsenide GaMnAs), increase the interface resistance
with a tunnel barrier, or using hot-electron injection.
Spin detection in semiconductors is another challenge,
which has been met with the following techniques:

Faraday/Kerr rotation of transmitted/reflected photons

Circular polarization analysis of electroluminescence

Nonlocal spin valve (adapted from Johnson and Silsbee's
work with metals)

Ballistic spin filtering
The latter technique was used to overcome the lack of
spin-orbit interaction and materials issues to achieve spin transport
in silicon, the most important semiconductor for electronics. Because
external magnetic fields (and stray fields from magnetic contacts) can
cause large Hall effects and magnetoresistance in semiconductors
(which mimic spin-valve effects), the only conclusive evidence of spin
transport in semiconductors is demonstration of spin precession and
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S.S.M Polytechnic, Tirur
Spintronics
Seminar Report 2011
dephasing in a magnetic field non-collinear to the injected spin
orientation. This is called the Hanle effect.
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S.S.M Polytechnic, Tirur
Spintronics
Seminar Report 2011
Spin Detection
Spin detection in semiconductors is another challenge, which
has been met with the following techniques:

Faraday/Kerr rotation of transmitted/reflected photons.

Circular polarization analysis of electroluminescence.
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S.S.M Polytechnic, Tirur
Spintronics
Seminar Report 2011
Depicting Spin Of Electrons Concerns

To devise economic ways to combine ferromagnetic metals
and semiconductors in integrated circuits.

To find an efficient way to inject spin-polarized currents, or
spin currents, into a semiconductor.

To maximize the time period for spin current to retain its
polarization in a semiconductor. To make semiconductors
that are ferromagnetic at room temperature and don’t lose
their property even at high temperature

To minimize spin currents at boundaries between different
semiconductors so as to minimize the loss.
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S.S.M Polytechnic, Tirur
Spintronics
Seminar Report 2011
Applications
Applications such as semiconductor lasers using spinpolarized electrical injection have shown threshold current reduction
and controllable circularly polarized coherent light output. Future
applications may include a spin-based transistor having advantages
over MOSFET devices such as steeper sub-threshold slope.
Motorola has developed a 1st generation 256 kb MRAM
based on a single magnetic tunnel junction and a single transistor
and which has a read/write cycle of under 50 nanoseconds.
There are two 2nd generation MRAM techniques currently
in development:

Thermal Assisted Switching (TAS) which is being developed
by Crocus Technology, and

Spin Torque Transfer (STT) on which Crocus, Hynix, IBM, and
several other companies are working.
Semiconductor
lasers
using
spin-polarized
electrical
injection have shown threshold current reduction and controllable
circularly polarized coherent light output.
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S.S.M Polytechnic, Tirur
Spintronics
Seminar Report 2011
Spintronic Couplers
Using the same sputtering technology, it is possible to
build a thin film on-chip coil. A current in this coil, when combined
with an on-chip GMR magnetic sensor separated by an insulating
layer, can couple a signal across the insulator achieving galvanic
isolation. Like the sensor, these components can be combined with
other semiconductor functions to produce a very high-speed digital
isolator. In a spintronic coupler, four GMR resistors form a
Wheatstone bridge (see Figure 7). A thin polymer dielectric barrier
layer provides several thousand volts of isolation from the input coil.
A magnetic field proportional to the input current signal is generated
beneath the coil winding. The resulting magnetic field flips the spin of
electrons in the GMR resistors, changing their resistance. A magnetic
shield protects the sensor from external fields.
There are two spins (UP spin and DOWN Spin).This
spintronic scanning technique is an efficient technique used in the
medical field to detectcancer cells.
Cancer cells are easy to be identified only when they are
large in number. These cells when matured results in formation of
tumor, which has to be removed by surgery. After surgery there may
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S.S.M Polytechnic, Tirur
Spintronics
Seminar Report 2011
be presence of even a single cancer cell, which would result in growth
of tumor in effected part of the body. The spintronic scanning is an
efficient technique to detect cancer cells even when they are less in
number.
A Patient is exposed to a strong magnetic field so that his
body cell gets magnetized. A beam of electrons with polarized spin is
introduced on the uneffected part of the body and the change in spin
is detected by a polarimeter. A beam of electrons with polarized spin
is introduced on the part which had undergone surgery.
The difference in spin of electrons when introduced to
normal area and abnormal area indicates whether cancer cells have
been removed from the body. If not, it indicates the presence of traces
of cancer cells and it has to be treated again for ensuring complete
safety to the patient. Thus this technique efficiently identifies the
presence of cancer cells in that part of the body that has undergone
surgery to prevent any further development
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S.S.M Polytechnic, Tirur
Spintronics
Seminar Report 2011
Conclusion
Spintronics is a technology with a fast track from the discovery
of GMR and MTJ materials to the incorporation of these materials in
commercial devices. Spintronics read heads dominate the hard-disk
market. Magnetic sensors based on spintronics are making inroads in
markets where some combination of high resolution, high sensitivity,
small size, and low power are required. Digital data couplers and
displacing opto isolators in many applications and are making
inroads into new markets heretofore unavailable. MRAM devices are
on the horizon and offer the promise of laptop computers that do not
need to boot up and cell phones with increased battery time and
increased capabilities.
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