रास्ट्रीय प्रद्योगिकी संस्थान हमीरपुर

NATIONAL INSTITUTE OF TECHNOLOGY HAMIRPUR Centre for Materials Science and Engineering

AN INTRODUCTION TO SPINTRONICS

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

        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

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 10 32 radian/s would be required to match the observed angular momentum that is velocity of the order of 10 20 m/s.

Electron Spin

The component

S

z along z axis:

S z



m s



m s m s

  1

/

2 (spin up)   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 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.

Passive Power (Device Leakage) Active Power

Technology node (nm)

•

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

 

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 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/in 2 , 2007 : GMR heads ~ 300 Gbit/in 2

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 Insulating spacer S Parallel alignment (P) Antiparallel alignment (AP) Measured: tunneling current

I

, conductance

G

Tunneling magneto-resistance (TMR)

TMR TMR

      ) ) ) )

Applications

• •

The read heads of modern hard disk drives.

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

M

agnetoresistive

R

andom

A

ccess

M

emory



MRAM uses magnetic storage elements instead of electric used in conventional RAM



Tunnel junctions are used to read the information stored Memory, in Magnetoresistive typically a magnetization state and ”0” Random for zero Access point “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 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.

Spin Transfer Torque

S v v M 1 M

2

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 induces of voltage non-equilibrium or changes spin either 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

   

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

  • • 

Fe 3 O 4 CrO 2 magnetite Heusler FM Ni 2 MnGa Co 2 MnAl

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

 

First DMS material, discovered in 1996 by Ohno et al.

Curie temperature

𝑻 𝒄 = 𝟏𝟏𝟎

K at optimal doping

Max T

C

~ 110K x ~ .05

[Ohno et al., APL 69, 363 (1996)]

DMS materials II: (Ga,Mn)N



First room temperature DMS discovered in 2001

Highest T c in Dietl’s prediction 

High curie temperature

◦

Experiment : up to T c =800 K

◦

Theory : up to T c =940 K

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 T c Hysteresis curve at Room temperature for Mn doped ZnO(Sn) 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 Fe 3 O 4 magnetite Heusler alloys

Future Outlook



High capacity hard drives



Magnetic RAM chips



Spin FET using quantum tunneling



Quantum computers

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?

THANK YOU for your kind attention

☺