AN INTRODUCTION TO SPINTRONICS

BY: SAMIR KUMAR10M601

M.TECH 1ST YEARCenter for Materials Science and Engineering

NATIONAL INSTITUTE OF TECHNOLOGYHAMIRPUR

Centre for Materials Science and Engineering

रा�स्ट्री�य प्रद्योगि�की सं�स्था�न हमी�रापु�रा

Outline

IntroductionWhat do we mean by spin of an electronWhy SpintronicsSpintronic EffectsPhases in Spintronics Materials of SpintronicsConclusionsAcknowledgments

Electron has :Mass

ChargeSpin

INTRODUCTION

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

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.

sz mS

down)(spin

up)(spin

21

21

/

/

s

s

m

m

The component Sz along z axis:

Electron Spin

• Conventional electronic devices ignore the spin property.

• Random spins have no effect on current flow.

SPINTRONICS = SPIN + ELECTRONICS

What is Spintronics?

Spintronics=spin based electronics

Spintronic devices create spin-polarized currents and use the spin to control current flow.

Moore’s LawMoore’s Law states that the number of transistors on a silicon chip will roughly double every eighteen months

Why Spintronics?

Can Moore’s law keep going?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.

0

100

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0.5 0.35 0.25 0.18 0.13 0.1 0.07 0.05

Active Power

Passive Power (Device Leakage)

350 250 180 130 100 70 50

500

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100

0

Technology node (nm)

Po

we

r d

ensi

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m)

2

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

Spintronics Effects

GMR (Giant Magneto-Resistance)

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 GMREasier 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: spins go through

Applications of GMR

It is used in Hard Drives

0.5 MB← 1975

1997 (before GMR) : 1 Gbit/in2 , 2007 : GMR heads ~ 300 Gbit/in2

100 GB hard disc (Toshiba), →

soon in portable digital audio-players

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.

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

Ferromagneticelectrodes

Magnetic Tunnel JunctionDevic

e

( ) ( )

( ) ( )

I P I APTMR

I P I AP

( ) ( )

( ) ( )

G P G APTMR

G P G AP

Parallel alignment (P) Antiparallel alignment (AP)

Ferromagnetic leads L & R Insulating spacer S

Measured: tunneling current I, conductance G

Tunneling magneto-resistance (TMR)

Applications

• The read heads of modern hard disk drives.

• Is also the basis of MRAM, a new type of non-volatile memory.

Magnetoresistive Random Access MemoryMRAM 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

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

Spin ManipulationTo 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.

Spin Transfer Torque

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.

2M1M

S

v v

Spin DetectionTo 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

• 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.

Problems

GMR - Giant magnetoresistance - HDD read headsMTJ - Magnetic Tunnel Junction - HDD read heads+MRAMMRAM - Magnetic RAM - nonvolitile memorySTT - Spin Transfer Torque - MRAM+oscillator

Spintronic Research and Applications

Solution

• Diluted Magnetic Semiconductor or (DMS).

Add Fe or Mn toSi/GaAs

• Half-Metallic Ferromagnets

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 )

Semiconductor host atom

Magnetic impurity

Theoretical predictions

by Dietl, Ohno et al.

Various DMS displays room temperature ferromagnetism!

Curie Temperature — The temperature above which a ferromagnetic material loses its permanent magnetism.

Science 287, 1019 (2000) & PRB 63, 195205 (2001)

DMS materials I: (Ga,Mn)AsFirst DMS material, discovered in 1996 by

Ohno et al.Curie temperature K at optimal doping

Max TC ~ 110Kx ~ .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◦Co doped TiO

Reported Tc up to 400K

Hysteresis curve at Room temperature for Mn doped ZnO(Sn)

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)

oxideFe3O4 magnetite Heusler alloys

Future Outlook

High capacity hard drivesMagnetic RAM chipsSpin FET using quantum tunnelingQuantum computers

LimitationsProblems 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?

THANK YOU for your kind

attention☺