Download an introduction to spintronics

रास्ट्रीय प्रद्योगिकी संस्ट्थान हमीरपरु
NATIONAL INSTITUTE OF TECHNOLOGY
HAMIRPUR
Centre for Materials Science and Engineering
AN INTRODUCTION TO SPINTRONICS
BY: SAMIR KUMAR
10M601
M.TECH 1ST YEAR
Center for Materials Science and Engineering
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Introduction
What do we mean by spin of an electron
Why Spintronics
Spintronic Effects
Phases in Spintronics
Materials of Spintronics
Conclusions
Acknowledgments
Outline
Electron has :
Mass
Charge
Spin
• One can picture an
electron as a charged
sphere rotating about
an axis.
What is spin?
•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
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.
Electron Spin
The component Sz along z axis:
S z  ms 
ms  1 / 2
(spin up)
ms  1 / 2
(spin down)
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.
Spintronics=spin based
electronics
What is Spintronics?
Moore’s Law
Why Spintronics?
Moore’s
Law states
that the
number of
transistors
on a
silicon
chip will
roughly
double
every
eighteen
months
Can Moore’s law keep going?
Power density (W/cm2)
Power dissipation=greatest obstacle for Moore’s 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.
500
500
400
400
Passive Power (Device Leakage)
300
300
200
200
Active Power
100
100
00
500
0.5
350
130 100
0.35 250
0.25 180
0.18 0.13
0.1
Technology node (nm)
70
50
0.07
0.05
•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
Advantages of Spintronics Devices
Spintronics Effects
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GMR (Giant MagnetoResistance)
FM-Metal-FM


MTJ (Magnetic Tunnel
Junction)
FM-Insulator-FM
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).
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
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

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

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:
through
spins go
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
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
Magnetic Tunnel Junction
Device
Ferromagnetic leads L & R
Parallel alignment (P)
Insulating spacer S
Antiparallel alignment (AP)
Measured: tunneling current I,
conductance G
Tunneling magneto-resistance (TMR)
I ( P)  I ( AP)
TMR 
I ( P)  I ( AP)
G( P)  G( AP)
TMR 
G( P)  G( AP)
Applications
• The read heads of modern hard disk drives.
• Is also the basis of MRAM, a new type of nonvolatile memory.
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

MRAM combines the best characteristics of Flash, SRAM
and DRAM
Phases in Spintronics
 SPIN
INJECTION
 SPIN
MANIPULATION
 SPIN
DETECTION
Spin injection
It is the transport or creating a non-equilibrium spin
population across interface
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Using a ferromagnetic electrode
Effective fields caused by spin-orbit interaction.
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Tunnel barrier could be used to effectively inject spins
into a semiconductor
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Tunneling spin injection via Schottky barrier
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By “hot” electrons
Spin Manipulation

To control electron spin to realize desired physical
operation efficiently by means of external fields
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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.
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
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.
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.
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.
Spintronic Research and Applications
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GMR - Giant magnetoresistance - HDD read heads
MTJ - Magnetic Tunnel Junction - HDD read heads+MRAM
MRAM - Magnetic RAM - nonvolitile memory
STT - Spin Transfer Torque - MRAM+oscillator
Solution
• Diluted Magnetic Semiconductor or (DMS).
Add Fe or Mn to
Si/GaAs
• Half-Metallic Ferromagnets
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•
•
Fe3O4 magnetite
CrO2
Heusler FM
Ni2MnGa
Co2MnAl
Diluted Magnetic Semiconductor
or (DMS)
One way to achieve FS is to
dope some magnetic impurity in
a semiconductor matrix. (Diluted
Magnetic Semiconductor )
Magnetic impurity
Semiconductor host atom
Various DMS displays room temperature ferromagnetism!
Theoretical
predictions
by Dietl, Ohno et al.
Science 287, 1019 (2000)
& PRB 63, 195205 (2001)
Curie Temperature — The temperature above which a ferromagnetic
material loses its permanent magnetism.
DMS materials I: (Ga,Mn)As

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First DMS material, discovered in 1996 by Ohno et
al.
Curie temperature 𝑻𝒄 = 𝟏𝟏𝟎 K at optimal doping
Max TC
~ 110K
x ~ .05
[Ohno et al., APL 69, 363 (1996)]
DMS materials II: (Ga,Mn)N

First room temperature
DMS discovered in 2001

High curie temperature
◦ Experiment: up to Tc =800 K
◦ Theory: up to Tc =940 K
Highest Tc
in Dietl’s
prediction
DMS materials III:
Transition metal doped oxide

Room temperature
ferromagnetism
discovered in Mn doped
ZnO in 2001
Material:
◦ Mn doped ZnO
Hysteresis curve at Room temperature
◦ Co doped TiO
for Mn doped ZnO(Sn)
 Reported Tc up to 400K

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.
Half-Metallic Ferromagnets
E.g.:
Chromium(IV) oxide
Fe3O4 magnetite
Heusler alloys
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.
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What happens to spin currents at boundaries
between different semiconductors?
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How long can a spin current retain its polarization in
a semiconductor?
THANK YOU
for your kind
attention
☺