AN INTRODUCTION TO SPINTRONICS

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AN INTRODUCTION TO SPINTRONICS
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NATIONAL INSTITUTE OF TECHNOLOGY HAMIRPUR Centre for Materials Science and Engineering

• BY SAMIR KUMAR
• 10M601
• M.TECH 1ST YEAR
• Center for Materials Science and Engineering

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Outline

• Introduction
• What do we mean by spin of an electron
• Why Spintronics
• Spintronic Effects
• Phases in Spintronics
• Materials of Spintronics
• Conclusions
• Acknowledgments

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INTRODUCTION
Electron has MassChargeSpin
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What is spin?

• One can picture an electron as a charged sphere
  rotating about an axis.
• The rotating charged sphere will produce magnetic
  moment in that can be either up or down depending
  upon whether the rotation is anticlockwise or
  clockwise

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Electron Spin is a Quantum phenomenon

• A spinning sphere of charge can produce a
  magnetic moment.
• Considering Electrons size to be of the order of
  10-12 m at that size a high spin rate of some
  1032 radian/s would be required to match the
  observed angular momentum that is velocity of the
  order of 1020 m/s.

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Electron Spin
The component Sz along z axis
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SPINTRONICS SPIN ELECTRONICS

• Conventional electronic devices ignore the spin
  property.
• Random spins have no effect on current flow.

Spintronic devices create spin-polarized currents
and use the spin to control current flow.
Spintronicsspin based electronics
What is Spintronics?
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Moores Law

• Moores Law states that the number of transistors
  on a silicon chip will roughly double every
  eighteen months

Why Spintronics?
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Can Moores law keep going?
Power dissipationgreatest obstacle for Moores
law! Modern processor chips consume 100W of
power of which about 20 is wasted in leakage
through the transistor gates. The traditional
means of coping with increased power per
generation has been to scale down the operating
voltage of the chip but voltages are reaching
limits due to thermal fluctuation effects.

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Advantages of Spintronics Devices

• 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
• Promises a greater integration between the logic
  and storage devices

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Spintronics Effects

• GMR (Giant Magneto-Resistance)
• FM-Metal-FM

• MTJ (Magnetic Tunnel Junction)
• FM-Insulator-FM

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Giant Magneto-Resistance (GMR)

• The 2007 Nobel prize for physics was award
  jointly to Fert and Grunberg for giant
  magnetoresistance (GMR) discovered independently
  in 1988.

This discovery led to development of the spin
valve and later the tunnel magnetoresistance
effect (TMR) which found application in advanced
computer hard drives, and more recently
magneto-resistive random access memory (MRAM)
(which is non-volatile).
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Giant Magneto-Resistance (GMR)

• 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

Condition for GMR layer thickness nm
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Parallel Current GMR

• Current runs parallel between the ferromagnetic
  layers
• Most commonly used in magnetic read heads
• Has shown 200 resistance difference between zero
  point and antiparallel states

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Perpendicular Current GMR

• Easier to understand theoretically, think of one
  FM layer as spin polarizer and other as detector
• Has shown 70 resistance difference between zero
  point and antiparallel states
• Basis for
• Tunneling
• MagnetoResistance

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Concept of the Giant Magnetoresistance (GMR)

• 1) Iron layers with opposite magnetizations
  spin up and spindown are stopped ? no current
  (actually small current only)

2) If a magnetic field aligns the magnetizations
spins go through
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Applications of GMR
It is used in Hard Drives
0.5 MB ? 1975
100 GB hard disc (Toshiba), ? soon in
portable digital audio-players
1997 (before GMR) 1 Gbit/in2 , 2007 GMR heads
300 Gbit/in2
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Magnetic Tunnel Junction

• A magnetic tunnel junction (MTJ) consists of two
  layers of magnetic metal, such as cobalt-iron,
  separated by an ultrathin layer of insulator.

Ferromagnetic electrodes

• Tunnel Magnetoresistive effect combines the two
  spin channels in the ferromagnetic materials and
  the quantum tunnel effect

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Magnetic Tunnel Junction

• Device

Ferromagnetic leads L R
Insulating spacer S
Parallel alignment (P)
Antiparallel alignment (AP)
Measured tunneling current I, conductance
G Tunneling magneto-resistance (TMR)
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Applications

• The read heads of modern hard disk drives.
• Is also the basis of MRAM, a new type of
  non-volatile memory.

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Magnetoresistive Random Access Memory

• MRAM uses magnetic storage elements instead of
  electric used in conventional RAM
• Tunnel junctions are used to read the information
  stored in Magnetoresistive Random Access Memory,
  typically a 0 for zero point magnetization
  state and 1 for antiparallel state

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MRAM combines the best characteristics of Flash,
SRAM and DRAM
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Phases in Spintronics

• SPIN INJECTION
• SPIN MANIPULATION
• SPIN DETECTION

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Spin injection

• It is the transport or creating a non-equilibrium
  spin population across interface

• Using a ferromagnetic electrode
• Effective fields caused by spin-orbit
  interaction.
• Tunnel barrier could be used to effectively
  inject spins into a semiconductor
• Tunneling spin injection via Schottky barrier
• By hot electrons

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Spin Manipulation

• To control electron spin to realize desired
  physical operation efficiently by means of
  external fields
• Mechanism for spin transfer implies a spin
  filtering process.
• Spin filtering means that incoming electrons with
  spin components perpendicular to the magnetic
  moment in the ferromagnet are being filtered out.
• Spin-polarized current can transfer the angular
  momentum from carriers to a ferromagnet where it
  can change the direction of magnetization This
  effect is equivalent to a spin transfer torque.

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Spin Transfer Torque
S
v
v
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.
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Spin Detection
To measure the physical consequences of spin
coherent states in Spintronics devices.
The injection of non-equilibrium spin either
induces voltage or changes resistance
corresponding to buildup of the non-equilibrium
spin. This voltage can be measured in terms of
change in resistance by potentiometric method.
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Spin Detection Technique
An ultrasensitive silicon cantilever with a SmCo
magnetic tip positioned 125nm above a silica
specimen containing a low density of unpaired
electron spins. At points in the specimen where
the condition for magnetic resonance is
satisfied, the magnetic force exerted by the spin
on the tip.
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Materials of Spintronics
Problems

• Currently used materials in conventional
  electronics are usually non-magnetic and only
  charges are controllable.
• Existing metal-based devices do not amplify
  signals.
• Whereas semiconductor based spintronic devices
  could in principle provide amplification and
  serve, in general, as multi-functional devices.
• All the available ferromagnetic semiconductor
  materials that can be used as spin injectors
  preserve their properties only far below room
  temperature, because their Curie temperatures
  (TC) are low.

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Spintronic Research and Applications

• GMR - Giant magnetoresistance - HDD read heads
• MTJ - Magnetic Tunnel Junction - HDD read
  headsMRAM
• MRAM - Magnetic RAM - nonvolitile memory
• STT - Spin Transfer Torque - MRAMoscillator

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Solution

• Diluted Magnetic Semiconductor or (DMS).
• Add Fe or Mn to
• Si/GaAs
• Half-Metallic Ferromagnets
• Fe3O4 magnetite
• CrO2
• Heusler FM
• Ni2MnGa
• Co2MnAl

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Diluted Magnetic Semiconductor or (DMS)
One way to achieve FS is to dope some magnetic
impurity in a semiconductor matrix. (Diluted
Magnetic Semiconductor )
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Various DMS displays room temperature
ferromagnetism!
Science 287, 1019 (2000) PRB 63, 195205 (2001)
Theoretical predictions by Dietl, Ohno et al.
Curie Temperature The temperature above which a
ferromagnetic
material loses its permanent magnetism.
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DMS materials I (Ga,Mn)As

• First DMS material, discovered in 1996 by Ohno et
  al.
• Curie temperature ?? ?? ?????? K at optimal
  doping

Ohno et al., APL 69, 363 (1996)
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DMS materials II (Ga,Mn)N
Highest Tc in Dietls prediction

• First room temperature DMS discovered in 2001
• High curie temperature
• Experiment up to Tc 800 K
• Theory up to Tc 940 K

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DMS materials III Transition metal doped oxide

• Room temperature ferromagnetism discovered in Mn
  doped ZnO in 2001
• Material
• Mn doped ZnO
• Co doped TiO
• Reported Tc up to 400K

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Half-Metallic Ferromagnets
Half metals are ferromagnets with only one type
of conduction electron, either spin up, ?, or
spin down, ?
The valence band related to one type of these
electrons is fully filled and the other is
partially filled. So only one type of electrons
(either spin up or spin down) can pass through it.
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Half-Metallic Ferromagnets
E.g. Chromium(IV) oxide Fe3O4 magnetite Heusler
alloys
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Future Outlook

• High capacity hard drives
• Magnetic RAM chips
• Spin FET using quantum tunneling
• Quantum computers

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Limitations

• Problems that all the engineers and scientists
  may have to overcome are
• 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 create long relaxation time for effective spin
  manipulation.
• What happens to spin currents at boundaries
  between different semiconductors?
• How long can a spin current retain its
  polarization in a semiconductor?

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THANK YOU for your kind attention ?