spin tronic s
DESCRIPTION
spintronics = spin + electronicsTRANSCRIPT
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Spintronics Feb, 2013.
A
SEMINOR
ON
SPINTRONICS
Submitted to SEER AKADEMI of JNTU HYDERABAD
Submitted By
G. AMARENDHAR(11011J6009)
DEPARTMENT OF
VLSI AND EMBEDDED SYSTEMS DESIGN
SEER AKADEMI, JNTU-HYDERABAD
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Spintronics Feb, 2013.
(FEB 2013)
INDEX
* ABSTRACT 3
* HISTORY 4
* SPINTRONICS 5
* WHY IS IT GOING TO BE ONE OF THE RAPIDLY
EMERGING FIELDS? 6
* ELECTRON SPIN: FUNDAMENTALS OF SPIN 7
* GIANT MAGNETO RESISTANCE 8
* CONSTRUCTION OF GMR 9
* SPIN VALVE TECHNOLOGY 10
* MRAM (MAGNETO RESISTIVE RANDOM ACCESS
MEMORY) 11
* METALS-BASED SPINTRONIC DEVICES 13
* SEMICONDUCTOR-BASED SPINTRONIC DEVICES 15
* SPIN TRANSISTOR CONCEPT 16
* LATEST DEVELOPMENTS 17
* CURRENT RESEARCH 18
* CONCLUSION 19
* REFERNCES 20
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Spintronics Feb, 2013.
ABSTRACT
In this paper we will discuss about a field of Nanotechnology,
which is believed to replace conventional electronics in the near future,
i.e. “spintronics”. Research and technology developments in the field of
spintronics have grown tremendously in the past 10-15 years and already
have had a major impact on the data storage industry. The future looks
even brighter, as many new spintronic discoveries have been recently
made a promise of even bigger impact in the future. This paper
summarizes the past accomplishments, describes some of the major
discoveries that will have a lasting impact on the field, and discusses
some of the technologies that may revolutionize data storage in the next
decade.
“Spintronics” is an emergent NANO technology, which uses the
spin of an electron instead of or in addition to the charge of an electron.
Electron spin has two states either “up” or “down”. Aligning spins in
material creates magnetism. Moreover, magnetic field affects the passage
of spin-up and spin-down electrons differently. The paper starts with the
detail description of the fundamentals and properties of the spin of the
electrons. It proceeds with a note on magneto resistance, the development
of Giant Magneto Resistance (GMR) and devices like Magneto Random
Access Memory, which are new versions of the traditional RAM. It
describes how this new version of RAM can revolutionize the memory
industry. There is also detailed explanation of the way, how this
revolution can increase the data density in our memory systems. It is
followed by an account of new Spin Field Effect Transistors. It also
specifies the difference between electronic devices and spintronic
devices.
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Spintronics Feb, 2013.
History
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 magneto resistance
independently by Albert Fert et al. and Peter Grünberg et al. (1988). The
origins can be traced back further to the ferromagnetic/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.
Theory
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
Spin Up (0) Spin Down(1)
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Spintronics Feb, 2013.
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 trapping potentials (for instance, at impurities, where
lifetimes can be milliseconds).
SPINTRONICS:
Imagine a data storage device of the size of an atom working at a
speed of light. Imagine a microprocessor whose circuits could be changed
on the fly. One minute is could be optimized for data base access. The
next for transaction processing and the next for scientific number
crunching. Finally, imagine a computer memory thousands of times
denser and faster than today’s memories. The above-mentioned things
can be made possible with the help of an exploding
science–“spintronics”.
Spintronics is a NANO technology which deals with spin
dependent properties of an electron instead of or in addition to its charge
dependent properties, Conventional electronics devices rely on the
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Spintronics Feb, 2013.
transport of electric charge carries electrons. But there is other
dimensions of an electron other than its charge and mass i.e. spin. This
dimension can be exploited to create a remarkable generation of
spintronic devices. It is believed that in the near future spintronics could
be more revolutionary than any other thing that nanotechnology has
stirred up so far.
WHY IS IT GOING TO BE ONE OF THE RAPIDLY EMERGING
FIELDS?
As there is rapid progress in the miniaturization of semiconductor
electronic devices leads to a chip features smaller than 100 nanometers in
size, device engineers and physists are inevitable faced with a looming
presence of a quantum property of an electron known as spin, which is
closely related to magnetism. Devices that rely on an electron spin to
perform their functions from the foundations of spintronics. Information-
processing technology has thus far relied on purely charge based devices
ranging from the now quantum, vacuum tube today’s million transistor
microchips. Those conventional electronic devices move electronic
charges around, ignoring the spin that tags along that side on each
electron.
1951mercury memory(UNIVAC)
Today0.85”HDD, 4 GBytes, 12.5 MB/sec
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ELECTRON SPIN:
FUNDAMENTALS OF SPIN
1. In addition to their mass and electric charge, electrons have an intrinsic
quantity of angular momentum called spin, almost of if they were tiny
spinning balls.
2.Associated with the spin is magnetic field like that of a tiny bar magnet
lined up with the spin axis.
3.Scientists represent the spin with a vector. For a sphere spinning “west
to east”, the vector points “north” or “up”. It points “down” for the
opposite spin.
4. In a magnetic field, electrons with
“spin up” and “spin down” have
different energies.
5. In an ordinary electronic circuit
the spins are oriented at random and
have no effect on current flow.
6. Spintronic devices create spin-polarized currents and use the spin to
control current flow.
Electrons like all fundamental particles have a property called spin,
which can be oriented in one direction, or the other called spin-up or spin-
down. Magnetism is an intrinsic Physical property associated with the
spins. An intuitive notion of how an electron spins is suggested below.
Imagine a small electronically charged sphere spinning rapidly. The
circulating charges in the sphere amount to tiny loops of electric current
which creates a magnetic field.. a spinning sphere in an external magnetic
field changes its total energy according to how its spin vector is aligned
with the spin. In some ways, an electron is just like a spinning sphere of
charge, an electron has a quantity of angular momentum (spin) an
associated magnetism. In an ambient magnetic field and the spin
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Spintronics Feb, 2013.
changing this magnetic field can change orientation. Its energy is
dependent on how its spin vector is oriented. The bottom line is that the
spin along with mass and charge is defining characteristics of an
electron,. In an ordinary electric current, the spin points at random and
plays no role in determining the resistance of a wire or the amplification
of a transistor circuit.
GIANT MAGNETO RESISTANCE:
Magnetism is the integral part of the present day’s data storage
techniques. Right from the Gramophone disks to the hard disks of the
super computer magnetism plays an important role. Data is recorded and
stored as tiny areas of magnetized iron or chromium oxide. To access the
information, a read head detects the minute changes in magnetic field as
the disk spins underneath it. In this way the read heads detect the data and
sent it to the various succeeding circuits. The magneto resistant devices
can sense the changes in the magnetic field only to a small extent, which
is appropriate to the existing memory devices. When we reduce the size
and increase data storage density, we reduce the bits, so our sensor also
has to be small and maintain very, very high sensitivity. The thought gave
rise to the powerful effect called “GIANT MAGNETO
RESISTANCE” OR (GMR):
Giant magnetoresistance (GMR) came into picture in 1988, which
lead the rise of spintronics. It results from subtle electron-spin effects in
ultra-thin ‘ multilayer’ of magnetic materials, which cause huge changes
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in their electrical resistance when a magnetic field is applied. GMR is 200
times stronger than ordinary magnetoresistance. It was soon realized that
read heads incorporating GMR materials would be able to sense much
smaller magnetic fields, allowing the storage capacity of a hard disk to
increase from 1 to 20 gigabits.
CONSTRUCTION OF GMR:
The basic GMR device consists of
a three-layer sandwich of a magnetic
metal such as cobalt with a nonmagnetic
metal filling such as silver. Current
passes through the layers consisting of
spin-up and spin-down electrons. Those
oriented in the same direction as the
electron spins in a magnetic layer pass
through quite easily while those oriented
in the opposite direction are scattered. If
the orientation of one of the magnetic
layers can easily be changed by the
presence of a magnetic field then the device will act as a filter, or ‘spin
valve’, letting through more electrons when the spin orientations in the
two layers are the same and fewer when orientations are oppositely
aligned. The electrical resistance of the device can therefore be changed
dramatically. In an ordinary electric Current, the spin points at random
and plays no role in determining the resistance of a wire or the
amplification of a transistor circuit. Spintronic devices, in contrast, rely
on differences in the transport of “spin up” and “spin down” electrons.
When a current passes through the Ferro magnet, electrons of one spin
direction tend to be obstructed.
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A ferromagnet can even affect the flow of a current in a nearby
nonmagnetic metal. For example, in the present-day read heads in
computer hard drives, wherein a layer of a nonmagnetic metal is
sandwiched between two ferromagnetic metallic layers, the magnetization
of the first layer is fixed, or pinned, but the second ferromagnetic layer is
not. As the read head travels along a track of data on a computer disk, the
small magnetic fields of the recorded 1’s and 0`s change the second
layer’s magnetization back and forth parallel or antiparallel to the
magnetization of the pinned layer. In the parallel case, only electrons that
are oriented in the favoured direction flow through the conductor easily.
In the antiparallel case, all electrons are impeded. The resulting changes
in the current allow GMR read heads to detect weaker fields than their
predecessors; so that data can be stored using more tightly packaged
magnetized spots on a disk.
SPIN VALVE TECHNOLOGY
The first really significant technological discovery was the spin valve, 6 illustrated in Figure 1. This is a multilayer structure incorporating a “magnetically hard” or pinned, ferromagnetic layer on top (consisting of a bilayer of an anti ferromagnet strongly coupled to a ferromagnetic layer), a nonmagnetic conductor layer (typically copper) in the middle, and a “magnetically soft” or “free” layer on the bottom just
Basic heterostructure of a spin valve.
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above the substrate. The pinning of the top ferromagnetic layer significantly biases the switching field for this layer far away from zero fields, so it is not free to rotate at low fields. Thus, “pinning” means that this layer is always pointing in the same direction relative to the substrate. If the magnetic moments in the pinned and free layers are parallel, the current can flow easily throughout the structure, and the resistance is low. However, if the layers are magnetized oppositely, the current is impeded, and the resistance is high. Aspin valve can function as either a magnetic field sensor or a hysteretic memory device, depending on how easy it is to rotate the moment of the free layer from parallel to antiparallel with respect to the pinned layer. In the early 1990s, IBM started a project to develop such GMR devices as read-head sensors for magnetic disk drives and introduced them to the marketplace in 1998. This introduction had an almost immediate impact on disk drive capacity that has lasted to the present day.
MRAM (MAGNETORESISTIVE RANDOM ACCESS MEMORY):
An important spintronic device, which is supposed to be one of the
first spintronic devices that have been invented, is MRAM. Unlike
conventional random-access, MRAMs do not lose stored information
once the power is turned off...A MRAM computer uses power, the four
page e mail will be right there for you. Today’s pc use SRAM and
Figure 3. Schematic illustration of the operation of the toggling method of MRAM switching. Dark arrows represent the direction of magnetization of the film just above the tunnel barrier and determine the magnetoresistance of the tunnel junction. (a) Initial state of the structure (low resistance when the magnetization of the lower ferromagnet is aligned with the pinned layer on the other side of the tunnel barrier). (b) Current in the bit line is turned on, producing a field in the y-direction (Hy), as illustrated by the upper square wave in the lower part of the figure; the magnetizations of both layers rotate and “scissor,” producing a net moment in the y-direction. (c) Current in the word line is turned on (lower square wave in the figure), producing a field in the x-direction (Hx); the two layers scissor more and the direction of the net moment is rotated towards the x-direction. (d) When the bit line current is turned off, the net moment rotates to be aligned with the x-axis. (e) Finally, when the word line current is turned off, the layers have “toggled” a full 180°.
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Spintronics Feb, 2013.
DRAM both known as volatile memory. They can store information only
if we have power. DRAM is a series of Capacitors; a charged capacitor
represents 1 where as an uncharged capacitor represents 0. To retain 1
you must constantly feed the capacitor with power because the charge
you put into the capacitor is constantly leaking out.
MRAM is based on integration of magnetic tunnel junction (MJT).
Magnetic tunnel junction is a three-layered device having a thin
insulating layer between two metallic Ferro-magnets. Current flows
through the device by the process of quantum tunneling; a small number
of electrons manage to jump through the barrier even though they are
forbidden to be in the insulator. The tunneling current is obstructed when
the two ferromagnetic layers have opposite orientations and is allowed
when their orientations are the same. MRAM stores bits as magnetic
polarities rather than electric charges. When a big polarity points in one
direction it holds1, when its polarity points in other direction it holds 0.
These bits need electricity to change the direction but not to maintain
them. MRAM is non-volatile so, when you turn your computer off all the
bits retain their 1`s and 0`s.
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Metals-based spintronic devices
The simplest method of generating a spin-polarized current in a
metal is to pass the current through a ferromagnetic material. The most
common application of this effect is a giant magneto resistance (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: (1) current-in-
plane (CIP), where the electric current flows parallel to the layers and (2)
current-perpendicular-to-plane (CPP), where the electric current flows in
a direction perpendicular to the layers.
Figure 2. Photomicrographs showing the increasing density of prototype magnetic random-access memory (MRAM) chips. (a) IBM 1 mm _ 1.5 mm, 1 kbit chip with a 5.4-m2 twin cell in 0.25-m technology with approximately 3–10-ns access time (from Reference 22, with permission). (b) Motorola 3.9 mm _ 3.2 mm, 256 kbit chip with 7.1-m2 cell in 0.6-m technology with 50-ns access time (from Reference 23,with permission). (c) Motorola 4.25 mm _ 5.89 mm, 1 Mbit chip with 7.1-m2 cell in 0.6-m technology with 50-ns access time (fromReference 24, with permission). (d) Motorola 4.5 mm _ 6.3 mm, 4 Mbit chip with 1.55-m2 cell in 180-nm technology with 25-ns access time(from Reference 17, with permission). (e) IBM 7.9 mm _ 10 mm, 16 Mbit chip with 1.42-m2 cell in 180-nm technology with 30-ns access time
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Other metals-based Spintronics devices:
Tunnel Magneto resistance (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.
Applications
The storage density of hard drives is rapidly increasing along an
exponential growth curve, in part because Spintronics-enabled devices
like GMR and TMR sensors have increased the sensitivity of the read
head which measures the magnetic state of small magnetic domains (bits)
on the spinning platter. The doubling period for the areal density of
information storage is twelve months, much shorter than Moore's Law,
which observes that the number of transistors that can cheaply be
incorporated in an integrated circuit doubles every two years.
MRAM, or magnetic random access memory, uses a grid of
magnetic storage elements called magnetic tunnel junctions (MTJ's).
MRAM is nonvolatile (unlike charge-based DRAM in today's computers)
so information is stored even when power is turned off, potentially
providing instant-on computing. 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
(Everspin, 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.
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Spintronics Feb, 2013.
Another design in development, called Racetrack memory, encodes
information in the direction of magnetization between domain walls of a
ferromagnetic metal wire.
Semiconductor-based spintronic devices:
In early efforts, spin-polarized electrons are generated via optical
orientation using circularly-polarized photons at the band gap energy
incident on semiconductors with appreciable spin-orbit interaction (like
GaAs and ZnSe). Although electrical spin injection can be achieved in
metallic systems by simply passing a current through a Ferro magnet, the
large impedance mismatch between ferromagnetic metals and
semiconductors prevented efficient injection across metal-semiconductor
interfaces. A solution to this problem is to use ferromagnetic
semiconductor sources (like manganese-doped gallium arsenide
GaMnAs), increasing 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.
Applications
Advantages of semiconductor-based Spintronics applications are
potentially lower power use and a smaller footprint than electrical devices
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used for information processing. Also, applications such as
semiconductor lasers using spin-polarized 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.
SPIN TRANSISTOR CONCEPT:
Traditional transistors use on-and-off charge currents to create bits
—the binary zeroes and ones of computer information. “Quantum spin
field effect” transistor will use up-and-down spin states to generate the
same binary data. One can think of electron spin as an arrow; it can point
upward or downward; “spin-up and spin-down can be thought of as a
digital system, representing the binary 0 and 1. The quantum transistor
employs also called “spin-flip” mechanism to flip an up-spin to a
downspin, or change the binary state from 0 to 1. One proposed design of
a spin FET (spintronic field-effect transistor) has a source and a drain,
separated by a narrow semi conducting channel, the same as in a
conventional FET. In the spin FET, both the source and the drain are
ferromagnetic. The source sends spin polarized electrons in to the
channel, and this spin current flow easily if it reaches the drain unaltered
(top). A voltage applied to the gate electrode produces an electric field in
Parallel alignment → positive current Antiparallel alignment → negative current
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the channel, which causes the spins of fast-moving electrons to process,
or rotate (bottom). The drain impedes the spin current according to how
far the spins have been rotated. Flipping spins in this way takes much less
energy and is much faster than the conventional FET process of pushing
charges out of the channel with a larger electric filed.
Electronic Devices Spintronic devices
1. Based on properties of charge of
the electron
1. Based on intrinsic property spin
of electron
2. Classical property 2. Quantum property
3. Controlled by an external electric
field in modern electronics
3. Controlled by external magnetic
field
4. Materials: conductors and
semiconductors
4.Materials: ferromagnetic
materials
5. Based on the number of charges
and their energy
5. Two basic spin states; spin-up
and spin-down
6. Speed is limited and power
dissipation is high
6. Based on direction of spin and
spin coupling, high speed
LATEST DEVELOPMENTS:
Toshiba developed a spintronics-based MOSFET cell .[DEC 11,2009]
Researchers manipulated and detected spin at room temperature for
the first time .[NOV 27,2009]
Researchers developed a way to control electron spin using pure-
electric means .[OCT 29,2009]
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Spintronics Feb, 2013.
CURRENT RESEARCH:
Objective 1 : Magnetic FPGAs
The objective will be to design a magnetic FPGA which will incorporate finely distributed Magnetic Tunnel Junctions (MTJs) for non-volatile storage and configuration purposes above of a CMOS core circuit. In complement of existing high density FPGAs, it will provide better versatility with intrinsic re-configurability, instant on/off and energy saving. Such FPGAs can be used as general purpose standalone products. In the SPIN project, the FPGA will be targeted to provide intelligent processing of the magnetometers and sensors developed in objectives 2 and 3.
Objective 2: ultra-high sensitivity "spin valve" based magnetometers for biochips and medical applications, or "Biosensor"
The objective is to develop a new generation of ultra-high sensitivity integrated magnetometers. The highest demand now for this kind of sensors is for medical applications, mainly biochips, bio magnetism and MRI, but there is a large number of potential applications in magnetic imaging for non-destructive evaluation or field sensing for reliability testing in transport, electronics, etc.
Objective 3: highly integrated "spin valve" based current and voltage sensors, or "GMR sensor array"
The objective will be to fabricate a new generation of dense integrated sensors for current and voltage monitoring. One main output is the monitoring of fuel cells and batteries. Typical fuel cells for automotive applications will contain about 240 cells providing each one 1.3V. For the safety of the car and for efficient energy monitoring, it is necessary to follow in real time the voltage behavior of each cell with insulation between the control systems of at least 1.5kV.
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Spintronics Feb, 2013.
CONCLUSION
Interest in Spintronics arises, in part, from the looming problem of
exhausting the fundamental physical limits of conventional electronics.
However, complete reconstruction of industry is unlikely and Spintronics
is a “variation” of current technology. The spin of the electron has
attracted renewed interest because it promises a wide variety of new
devices that combine logic, storage and sensor applications.
So with this paper we have proved that the new generation of
computing and information technology is on its way to revolutionize the
21st century. We believe it makes sense instead to build on the extensive
foundations of conventional electronic semiconductor technology; we
exploit the spin of the electron and create new devices and circuits, which
could be more beneficial.
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Spintronics Feb, 2013.
REFERNCES
1. www.wikipidia.com
2. www.mrs.org/bulletin
3. www.dac.neu.edu
4. www.physics.udel.edu
5. www.nano.caltech.edu
6. www.spintronics-info.com
7. www.nanotech-now.com
8. www.technology24.com