SpintronicsWhere the spin-around happens….

Shuvan Prashant T.Seminar Presentation as a part of

Nanotechnology PHY 1002

Outline

• Ferromagnetism Anything Special about it ?• Electron Spin Freedom to do Things Differently• Magnetoresistance

– OMR– AMR– GMR– TMR

• Applications:– Spin Valve– Tunnel Valve– MRAM

Electronic States

• Superconductor– Perfect R=0

• Metal– Good Conductor

• Semimetal– So-so conductor

• Semiconductor– Poor Conductor

• Insulator– Bad Conductor

Magnetic States

Ferromagnet Parallel Moments

Antiferromagnet Antiparallel Moments

Ferrimagnet Two different moments

Paramagnet Positive susceptibility

Diamagnet Negative susceptibility

B

Stern Gerlach Experience

• Under the influence of inhomogeneous magnetic field, the electrons split into two directions.

• Electron can assume two states of intrinsic angular momentum

• Spin UP or Spin DOWN• First experiment to prove

Spin really exists

e-

SN

FERROMAGNETISM

• Spin governs Electron motion in solids• Each atom has electromagnetic moment and all

moments are aligned in same directions• Exchange interaction H = -2 Je S1. S2 • Spin Alignment Bulk Magnetization Domains• Domain sizes adjust as domain walls shift • If walls are not easily movable , coercive field must be

large.• Remnant Magnetization Saturation • Hard Ferromagnet and Soft Ferromagnet

To be or not to be !!

METAL <M>=0

FERROMAGNETIC <M> != 0

N(E) N(E)

E E

EF

Exchange Splitting

Ferromagnet

N(E)

E

EF

Spin dependent Transport - Scattering

N(E)

E

EF

N(E)

E

EF

majorityminorityLow Resistance

High Resistance

So, What is spintronics ?

Electronics

Magnetics

Magnetoelectronics Spintronics

e

s

What’s the DEAL about?

Microprocessors will double their power every 18 months

• Silicon roadmap has come to an end …

• Moore’s law will not be possible when the circuit elements reach the atomic level

• By 2008, it is projected that the width of the electrodes in a microprocessor will be 45nm across

Spintronics

• Conventional electronic devices ignore the spin property and rely strictly on the transport of the electrical charge of electrons

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

• As electronic devices become smaller, quantum properties of the wavelike nature of electrons are no longer negligible

• Spintronic devices offer the possibility of – enhanced functionality, – higher speed, and – reduced power consumption

The Spin Advantage

• Information is stored into spin as one of two possible orientations• Spin lifetime is relatively long, on the order of nanoseconds • Spin currents can be manipulated• Spin devices may combine logic and storage functionality eliminating

the need for separate components• Magnetic storage is nonvolatile• Binary spin polarization offers the possibility of applications as qubits

in quantum computers

Ordinary MR

• Magnetoresistance dependence of electrical resistance on magnetic field (ΔR/R)B

• Caused by the carrier trajectories in a magnetic field

• MR α H2

• Cyclotron Motion• OMR ~ 0.1%

B

v

F= q v x B

R

H

Anisotropic MR

M

I

Depends on the relative direction of the current and magnetizationAMR = 2%

Giant MR and tunnel MR

• Ferromagnet– Fe, Co, Ni, CoFe,NiFe– Pinned Layer with high

Hc– Free layer with low Hc

FM FM

Space layerConductor : Cr, Ru, Cu, Au etc Giant Magnetoresistance (GMR)

Insulator: Al2O3 , MgO Tunnel Magnetoresistance(TMR)

Giant MR

Parallel Anti-parallel

GMR Effect

• Change of electron mobility observed between spin up and spin down electrons in traversing a ferromagnet

• If spin of the electron is parallel to the spins in the ferromagnet, the scattering(resistivity) is less than if the spin traverses a ferromagnetic region of antiparallel spins

• Regions with small dimensions so that electron spin directon is preserved during the traversal

• Inelastic mean path for the spin reversal must exceed the device dimensions

GMR- The Origins The Nobel Prize in Physics 2007

"for the discovery of Giant Magnetoresistance“GMR was discovered in 1988 in Fe/Cr/Fe trilayers by a research team led by Peter Grünberg of the Jülich Research Centre(DE), who owns the patent. It was also simultaneously but independently discovered in Fe/Cr multilayers by the group of Albert Fert of the University of Paris-Sud (FR)

Albert Fert Peter Grünberg

IBM Flash

IBM Flash2

GMR Spin ValveA nanophysical Magnetoresistance sensor

•Spin Valve a Sandwich comprising a• hard magnet(Co) , • a conductor like Cu

and • a soft Magnet like

NiFe. •The voltage across this current in plane sandwich is read to measure the fringing magnetic fields that point up or down at the joins between the magnetic domains written into the disk.

MR = (RAP – RP ) / RP

Tunnel Valve A nanophysical Magnetic Field Sensor

• Tunnel Current density prop to product of Density of initial states times the density of final states at the same energy

• Flip the magnetization of soft magnetic layer, its DOS for spin up and spin down from the hard magnet layer are interchanged

• A large change in tunneling current at fixed voltage bias leads to large TMR

• Spin direction is preserved during the tunneling process.

•Magnetoresistive Circuit Element•Metal Insulator Metal tunnel Junction • Soft magnet like Fe-Ni • Insulating Tunneling Barrier like

MgO • Hard magnet like Co

•TMR~200%

Tunnel MR (Perpendicular Current)

Think of one FM layer as spin polarizer and other as detector

Magnetic RAM

• 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

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

• This proves unpractical at nanoscale level

Spin Transfer (ST-RAM)

• Current passed through a magnetic field becomes spin polarized

• This flipping of magnetic spins applies a relatively large torque to the magnetization within the external magnet

• This torque will pump energy to the magnet causing its magnetic moment to precess

• If damping force is too small, the current spin momentum will transfer to the nanomagnet, causing the magnetization to flip

• Unwanted effect in spin valves• Possible applications in memory writing

MRAM

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

• Currents are about the same as read currents, requiring much less energy

MRAM

• MRAM promises:– Density of DRAM– Speed of SRAM– Non-volatility like flash

Spin Transistor

• Ideal use of MRAM would utilize control of the spin channels of the current

• Spin transistors would allow control of the spin current in the same manner that conventional transistors can switch charge currents

• Using arrays of these spin transistors, MRAM will combine storage, detection, logic and communication capabilities on a single chip

• This will remove the distinction between working memory and storage, combining functionality of many devices into one

Datta Das Spin Transistor

• The Datta Das Spin Transistor was first spin device proposed for metal-oxide geometry, 1989

• Emitter and collector are ferromagnetic with parallel magnetizations

• The gate provides magnetic field

• Current is modulated by the degree of precession in electron spin

The Spintronics Products

– iPod Nano– Mobile Phones – Laptop Hard Drives– Digital Cameras

"A computer hard disk reader that uses a GMR sensor is equivalent to a jet flying at a speed of 30,000 kilometers (19,500 miles) per hour ... at a height of just one meter above the ground, and yet being able to see and catalogue every single blade of grass it passes over,“ [Ben Murdin, a physics professor at the University of Surrey in southeast England] said.Source : Reuters

What All You Can Do With Spin

Conductor• GMR Spin valve

• GMR Head• Spin Transistor

Insulator• MRAM• STT-MRAM

Semiconductor• Spin- FET

• Hall Effect Device• Logic Device

Superconductor• Andreev Reflection • Spin Switch

Space Layer

Conclusion

• Need for Spintronics --Exhaustion of Physical Limits of Conventional Electronics

• Complete reconstruction of industry unlikely (spintronics is just a variation” of current technology)

• SPIN --- combines LOGIC, STORAGE AND SENSOR APPS• Spintronic Devices Prequel to Quantum computers and

Quantum Communication Systems Based on Solid State Electronics

Bibliography

• www.wikipedia.com• Presentations available on the Web– Introduction to Spintronics, Joanna Meador and

Farrukh Gondal – Introduction to Spintronics, Josh

Schaefferkoetter,February 27, 2007– Spintronics, Myung-Hwa Jung, Department of

Physics, Sogang University, Seoul

Q/A

THANKS FOR YOUR PATIENT LISTENING

SPECIAL THANKS TO PROF A. M. RAO