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SPINTRONICS & ITS APPLICATION MADESH GURIKAR U
SPINTRONICS & ITS APPLICATION
MADESH GURIKAR U
Contents
Introduction & History. Spintronics. GMR. TMR. MRAM. Other applications. Future of spintronics.
Introduction & History
Spintronics- concerned with manipulation,
storage and transfer of information by means of electron spin along with the electron charge.
History – Introduced in 1996Coined by Dr S.Wolf.Originally was Defense Advanced
Research Project Agency (DARPA) program.
Spintronics
A multidisciplinary field
Defination:
Spintronics also known as magnetoelectroni-cs, 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.
Concept:
Electrons are spin-1/2 fermions and therefore constitute a two-state system with spin "up" and spin "down".
Instead of solely relying on the electron’s negative charge to manipulate electron motion or to store information, spintronic devices would further rely on the electron’s spin degree of freedom
Theory:
Spin angular momentum, or simply spin, is one of the Fundamental properties of electrons.
Electrons with different spins experience different resistance in a magnetized conductor.
This phenomenon causes giant magnetoresistance effect in ferromagnetic metal layers.
Nature of electron spin
The direction of spin is specified by the component of S Along particular axis given by
S = mhm= spin magnetic numberm= ½ or -1/2.h=planck’s constant.
When m s =1/2 the electron is said to “spin up”, and “spin down” otherwise.
The binary directional state of spin seems to make electron spin a perfect quantity for computer information storage and processing.
One bit of information is a binary state, represented usually by one and zero.
we let spin up represent one and spin down zero (or the other way around), then one electron can carry one bit of information.
Spin Measurement:
The magnetic moment µ associated with spin enables to manipulate and measure spin.
µ = spin magnetic moment
µ = γ sγ = -2.00232 e/2m.s = spin angular momentum.
The interaction between spin magnetic moment and an external magnetic field provides a way to measure S
The force F on a particular atom depends on S of that atom’s valence electron, and is given by
F = γ α s.α = a is a constant whose value
depends on how inhomogeneous the magnetic field is.
The entire set up --Stern-Gerlach apparatus
Primary requirments:
SPIN INJECTOR -Generate a current of spin-polarized electrons comprising more of one spin species—up or down.
SPIN DETECTOR -Separate system that is sensitive to the spin polarization of the electrons.
Manipulation of the electron spin during transport between injector and detector
GMR:
Gaint Magnetoresistance.
It is a quantum mechanical magnetoresistance effect observed in thin film structures composed of alternating ferromagnetic and nonmagnetic layers.
Result- significant decrease (typically 10–80%) in electrical resistance in the presence of a magnetic field.
In the absence of an external magnetic field resistance offered is very very high.
Types of GMR:
Multilayer GMR:Two or more ferromagnetic layers are separated by a very thin (about 1 nm) non-ferromagnetic spacer.e.g. Fe/Cr/Fe
Spin Valve GMR:Multilayer structure incorporating a “magnetically hard,” or pinned, ferromagnetic layer on top.
pSeudo Spin Valve:
The significant difference is the coercivities of the ferromagnetic layers.
A soft magnet will be used for one layer; where as a hard ferromagnet will be used for the other
Granular GMR:
Occurs in solid precipitates of a magnetic material
To date, granular GMR has only been observed in matrices of copper containing cobalt granules
TMR:
Tunnel Magnetoresistance - occurs in magnetic tunnel junctions (MTJs).
A component consisting of two ferromagnets separated by a thin insulator.
If the insulating layer is thin enough (typically a few nanometers), electrons can tunnel from one ferromagnet into the other.
Phenomenological DescriptionThe direction of the two
magnetizations of the ferromagnetic films can be switched individually by an external magnetic field.
If the magnetizations are in a parallel orientation it is more likely that electrons will tunnel through the insulating film than if they are in the oppositional (antiparallel) orientation.
Physical Explanation
The relative resistance change—or effect amplitude—is defined as
Rap - electrical resistance in the anti-parallel
state. Rp - resistance in the parallel state.
The spin polarization P is calculated from the spin dependent density of states (DOS) at the Fermi energy
MRAM (magnetoresistive RAM)MRAM data is not stored as electric
charge or current flows, but by magnetic storage elements.
The elements are formed from two ferromagnetic plates, each of which can hold a magnetic field, separated by a thin insulating layer.
One of the two plates is a permanent magnet set to a particular polarity, the other's field will change to match that of an external field.
READ:accomplished by measuring the electrical resistance of the cell.
A particular cell is (typically) selected by powering an associated transistor which switches current from a supply line through the cell to ground.
WRITE: each cell lies between a pair of write lines arranged at right angles to each other, above and below the cell.
When current is passed through them, an induced magnetic field is created at the junction, which the writable plate picks up.
ADVANTAGES:
Non volatile.High speed.Low voltage operation.Unlimited Endurance.Reliable.
DISADVANTAGE:
As the device is scaled down in size, there comes a time when the induced field overlaps adjacent cells over a small area, leading to potential false writes – HALF SELECT.
Complication:
Tunnel Barrier: Is very thin(<2nm):Resistance and the
change in resistance R and delta R depends on Barrier thickness.
MicroMagnetic Effects.Thermal Stability issues for small
bits. Caution for Half-selected bits switching
via thermal activation.
1D Magnetic Selection
A high current of either polarity (plus current for a "1" and negative current for a "0" is passed through a select transistor and through the memory cell to write.
Only a small current can be used to read the cell.
FUTURE:
In the future we will have devices that are hybrid charge and spin based devices.
Spintronics holds the hope for the development of Quantum Computers.
Spintronics will produce batteries which last much longer, recharge times will be reduced drastically, and devices like laptops will have instant ‘ON’.
References:
S A Wolf, D D Awschalom, P A Buhrman, J M Daughton, M L von Molna Roukes, A Y Chtchelnakova and D M Treger, Spintronics : a spin-based electronics vision for the future, Science , Vol.294, p.1488,2001.
S. Das Sarma. “Spintronics,” American Scientist, Vol. 89, pp518, Nov.-Dec. 2001.
http://physik.kfunigraz.ac.at/~jaf/research/spintronics/spintronics.html
http://www.physics.umd.edu/rgroups/spin/intro.html http://whatis.techtarget.com/definition/0,,sid9_gci1131718,0
0.html http://www.wun.ac.uk/spintronics/ http://www.worldscibooks.com/nanosci/7281.html http://www.wisegeek.com/what-is-spintronics.htm http://news.stanford.edu/pr/03/zhang820.html http://newton.ex.ac.uk/research/emag/spintronics/
