19747894 Introduction to Spintronics

8/3/2019 19747894 Introduction to Spintronics

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Electron has :

Mass

Charge

Spin

Spintronics=spin based electronics

information is carried by spin not by

charge

ferromagnetic metallic alloy baseddevices

transport in fm materials is spin

polarized



Introduction

y Conventional electronic devices ignore the spin property

y As electronic devices become smaller, quantumproperties of the wavelike nature of electrons are nolonger negligible.

y Adding the spin degree of freedom provides new effects,new capabilities and new functionalities

y Information is stored into spin as one of two possibleorientations



Advantages of spintronics

y Non-volatile memory

y performance improves with smaller devices

y Low power consumption

y Spintronics does not require unique and specialisedsemiconductors

y Dissipation less transmission

y Switching time is very less

y compared to normal RAM chips, spintronic RAMchips will:

increase storage densities by a factor of three

have faster switching and rewritability rates smaller



Phases in SpintronicsySPIN INJECTION

ySPIN TRA NSFER

y

SPIN DETECTION



Spin injectiony Using a ferromagnetic electrode

y effective fields caused by spin-orbit interaction.

y a vacuum tunnel barrier could be used to effectively inject spins into a semiconductor

y back biased Fe/AlGaAs Schottky diode has beenreported to yield a spin injection efficiency of 30%

y By hot electrons



Spin Transfer

y Current passed through a magnetic field becomes spinpolarized

y This flipping of magnetic spins applies a relatively large torque

to the magnetization within the external magnet

y This torque will pump energy to the magnet causing itsmagnetic moment to precess

y If damping force is too small, the current spin momentum willtransfer to the nanomagnet, causing the magnetization to flip



Spin Transfer Torque

The spin of theThe spin of the

conduction electronconduction electron

is rotated by itsis rotated by its

interaction with theinteraction with the

magnetization.magnetization.

This implies the magnetization exerts a torque on the spin. ByThis implies the magnetization exerts a torque on the spin. By

Conservation of angular momentum, the spin exerts an equal andConservation of angular momentum, the spin exerts an equal and

Opposite torque on the magnetization.Opposite torque on the magnetization.

2

M1

M

<S><S>

v v



Spin detectionOptical detection techniques using magnetic

resonance force microscopy

Electrical sensing techniques-through quantum

dots and quantum point contact



SPIN RELAXATIONy Leads to spin equilibration

y T1-Spin-lattice relaxation time

y T2-Spin-spin relaxation time

y Neccesary condition 2T1>=T2.



Application

GMR(Giant magnetoresistance)y Discovered in 1988 France

y 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)

y When the magnetization of the two outsidelayers is aligned, resistance is low

y Conversely when magnetization vectors areantiparallel, high R



Parallel current GMR

Perpendicular current GMR



Spin Valve

y Simplest and most successful spintronic device

y Used in HDD to read information in the form of small magnetic fields above the disk surface



Tunnel Magnetoresistance

y Tunnel Magnetoresistive effect combines the twospin channels in the ferromagnetic materials andthe quantum tunnel effect

y TMR junctions have resistance ratio of about 70%

y MgO barrier junctions have produced 230% MR



MRAMy

MRAM uses magnetic storage elements

y Tunnel junctions are used to read the informationstored in MRAM



MRAM

y Attempts were made to control bit writing by using relatively large currents to produce fields

y This proves unpractical at nanoscale level



MRAM

y The spin transfer mechanism can be used to write to the magnetic memory cells

y Currents are about the same as read currents,

requiring much less energy



MRAMy

MRAM promises:y Density of DRAM

y Speed of SRAM

y Non-volatility like flash



Spin Transistor

y Ideal use of MRAM would utilize control of thespin channels of the current

y Spin transistors would allow control of the spin

current in the same manner that conventionaltransistors can switch charge currents

y Using arrays of these spin transistors, MRAM willcombine storage, detection, logic and

communication capabilities on a single chipy This will remove the distinction between working

memory and storage, combining functionality of many devices into one



Datta Das Spin Transistor

y The Datta Das SpinTransistor was first spindevice proposed for metal-oxide geometry, 1989

y Emitter and collector areferromagnetic with parallelmagnetizations

y The gate provides magnetic

fieldy Current is modulated by the

degree of precession inelectron spin



Current Researchy

Ferromagnetic transition temperature in excess of 100 K

y Spin injection from ferromagnetic to non-magneticsemiconductors and long spin-coherence times insemiconductors.

y Ferromagnetism in Mn doped group IV semiconductors.

y Room temperature ferromagnetism

y Large magnetoresistance in ferromagnetic

semiconductor tunnel junctions.



Future Outlooky High capacity hard drives

y Magnetic RAM chips

y Spin FET using quantum tunnelingy Quantum computers



limitationsy Controlling spin for long distances

y Difficult to INJECT and ME ASUR E spin.

y Interfernce of fields with nearest elementsy Control of spin in silicon is difficult



THANK YOU!

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19747894 Introduction to Spintronics