ELL 834: Special Topics in Integrated Electronic Circuits- II:Nanomagnetism and Spintronics

Debanjan Bhowmik

Department of Electrical Engineering

Indian Institute of Technology Delhi

Course Contents

• Module 1- Motivation:Overview of application of spintronics in current and future industry:Storage, Memory and Logic

• Module 2- Theory of magnetism: 2A. Paramagnetism : Spin and orbital angular momentum, Curie

paramagnetism,Pauli paramagnetism2B. Ferromagnetism: Exchange integral, Mean field theory, Curie Weiss

Law2C. Phenomenological Theory of Ferromagnetism: Anisotropy, Stoner

Wolfarth Model, Micromagnetics, Domain wall, Skyrmion

• Module 3- Applications of nanomagnetism in details3A. Hard disk drive 3B. Magnetic Random Access Memory3C. Racetrack Memory (Domain wall, skyrmion)3D. Nanomagnetic Logic3E. Need to understand spin transport phenomena (Reading and Writing

Mechanisms) : Magnetoresistance, Spin Current

Course Contents

• Module 4- Spin transport:

4A. Anisotropic Magneto-Resistance (AMR)

4B. Giant Magneto-Resistance (GMR)

4C. Tunneling Magneto-Resistance (TMR)

4D. Spin transfer torque (STT)

4E. Spin orbit torque (SOT): Rashba effect, spin Hall effect

4F. Magnetization dynamics due to STT and SOT

4G. Improvement of nanomagnetic/ spintronic devices using STT and SOT

• Module 5- Other applications of spintronics5A. Spin transfer torque oscillators

5B. Spintronic neural networks

• Course Project

References

• ELL834 lecture notes (to be uploaded on Moodle

every week 2-3 days before the class)

• IEEE Special Issues on Spintronics 2016:

Proceedings of the IEEE 104, 2016

• Magnetism in Condensed Matter- Blundell (Oxford University Press)

• Modern Magnetic Materials- Robert O’ Handley

• Spin current- Maekawa, Valenzuala, Saito, Kimura

(Oxford University Press)

Spintronics

Sensors Memory Logic

Hard disk for data storage

Magnetic Random Access Memory (MRAM)

NanomagneticLogic

Domain wall basedRacetrack memory

Non-volatile memory integrated with logic

Electrons have quantum mechanical property of spin,which can be used

to expand the functionality of electronic devices.

Major Applications:

Ohno et al. Proceedings of the IEEE 104 (10), 2016

Data storage (Hard disk)

Dramatic increase in storage density and reduction in price is due toadvent of magnetoresistive read head (sensor to read data) around 1991.

Fullerton et al. Proceedings of the IEEE 104 (10), 2016

Data storage (Hard disk)

Before 1991, a coil that could generate and detect magnetic field was used simultaneouslyas write head and read head. After 1991, magnetoresistive heads started being used as read heads. Magnetoresistive heads use magnetoresistance (spin transport effect) to detect magnetic field.

Fullerton et al. Proceedings of the IEEE 104 (10), 2016

Magnetoresistive Head

Anisotropic Magneto-Resistance (AMR):

Resistance is proportional toM cos2(θ), where M is the manetizationand θ is the angle between magnetization directionand the current.

Giant Magneto-Resistance (GMR):

Resistance is proportional to M cos(θ), where θ is the angle between the magnetization of the free layer andthe pinned layer.

Fullerton et al. Proceedings of the IEEE 104 (10), 2016

Spin valve

Magnetoresistive Head

Tunneling Magneto-Resistance (TMR)based read head

Fullerton et al. Proceedings of the IEEE 104 (10), 2016

TMR can be 100% or higher.

TMR> GMR> AMR

Tunnel junction

Origin of TMR

Cache and Main Memory (MRAM)

Wong, P.H.S. et al. Nature Nano. 10 (2015)

Cache and Main Memory (MRAM)

SRAM DRAM Flash

High density, cheapAccess time is low.

High power Consumption (refresh cycle)

Very high speed,Access time very low

Low density, expensive

CheapNon-volatile Write time is highPower for writing is highLow endurance

Wong, P.H.S. et al. Nature Nano. 10 (2015)

Flip flop circuit with 4Transistors; Leftmost and rightmosttransistors are for reading and writing

Transistor charges aCapacitor, every time capacitorIs read it has to be refreshed.

Floating gate is charged up, based on charge in floating gate, transistor is on or off when a voltageis applied on control gate

Memory (MRAM)

Organization of Magnetic Random Access Memory (MRAM)

Apalkov et al. Proceedings of the IEEE 104 (10), 2016

Switching due to Oersted field driven by current is not attractive due to Joule heating and unscalability.

Efficient way to switch the magnet:Spin Transfer Torque (STT)

Apalkov et al. Proceedings of the IEEE 104 (10), 2016

As conduction electrons pass through the polarizer (fixed magnetic layer) they becomespin polarized. Then in the free layer (FL), they transfer spin angular momentum tothe magnetization (M).

First generation MRAM: Switching driven by Oerstedfield due to current

Apalkov et al. Proceedings of the IEEE 104 (10), 2016

Second generation MRAM: Spin Transfer Torque MRAM(STTMRAM)

Efficient way to switch the magnet:Spin Transfer Torque (STT)

Alternative way to switch the magnet: Spin Orbit Torque (SOT)

Miron, I. M. et al. Nature 476 (2011)Liu, L. et al. Phys. Rev. Lett. 109 (2012)

Instead of two ferromagnetic layersseparated by a spacer layer, in this case, ferromagnetic metallayer - heavy metal layer interface is needed.

Advantages of STTMRAM/ SOTMRAM

Comparison with: MRAM’s advantages

SRAM Similar performance (MRAM slightly slower)Higher density than SRAM, hence cheaper

DRAM Similar density as DRAMLower power consumption

Flash Non-volatile like FlashWrite time is lowerEnergy consumption in writing is lower

Hence STTMRAM is considered Universal Memory; but currently its biggest potential use is in replacing SRAM for lower levels of cache (ITRS, 2015).

Racetrack Memory (Domain Wall or Skyrmions based)

http://www-03.ibm.com/ibm/history/ibm100/us/en/icons/racetrack/

Logic Devices/ Computing

Technological Progress by Rosen, Ritchie

Transistor density doubles every 18 months. (Moore’s Law)

Logic Devices/ Computing

Power dissipation in CMOS microprocessor= N C V2 f; N= density of transistors, C= capacitance, V= operating voltage of a transistor and f= clock frequency.Vdd cannot be scaled down because the off current becomes high (Subthreshold swing of 60 mV/decade)

MOSFET - ON to OFF state transition is gradual (Boltzmann limit of 60mV/decade)

Ferain et al. ,Nature 479, 2011

Logic Devices/ Computing

Clock frequency of microprocessor has saturatedto keep the power dissipationconstant.

Danowitz et al. Communications ofthe ACM 10(4), 1-18 (2012).

To reduce the energy consumption for computing, alternatives to transistor are being explored:Spintronics offers low power computing solutions.

Nanomagnetic Logic

Input 1 Input 2 output

0() 0() 1()

0() 1() 0()

1() 0() 0()

1() 1() 0()

Input 1 Input 2 output

0() 0() 1()

0() 1() 1()

1() 0() 1()

1() 1() 0()

Since magnet is an interacting system,it consumes low power for switching. Imre et al, Science 311(2006), Bhowmik et al. Nature Nano 9 (2014)

Beyond Von-Neumann architecture:Memoryintegrated computing

Wong, P.H.S. et al. Nature Nano. 10 (2015)

Spintronic memories can be fabricated on the top of CMOS processor. Latency and energy cost due toshuttling data between memoryand processor throughinterconnects is avoided.

One more look at the course contents!!

• Module 1- Motivation:Overview of application of spintronics in current and future industry:Storage, Memory and Logic

• Module 2- Theory of magnetism: 2A. Paramagnetism : Spin and orbital angular momentum, Curie

paramagnetism,Pauli paramagnetism2B. Ferromagnetism: Exchange integral, Mean field theory, Curie Weiss

Law2C. Phenomenological Theory of Ferromagnetism: Anisotropy, Stoner

Wolfarth Model, Micromagnetics, Domain wall, Skyrmion

• Module 3- Applications of nanomagnetism in details3A. Hard disk drive 3B. Magnetic Random Access Memory3C. Racetrack Memory (Domain wall, skyrmion)3D. Nanomagnetic Logic3E. Need to understand spin transport phenomena (Reading and Writing

Mechanisms) : Magnetoresistance, Spin Current

• Module 4- Spin transport:

4A. Anisotropic Magneto-Resistance (AMR)

4B. Giant Magneto-Resistance (GMR)

4C. Tunneling Magneto-Resistance (TMR)

4D. Spin transfer torque (STT)

4E. Spin orbit torque (SOT): Rashba effect, spin Hall effect

4F. Magnetization dynamics due to STT and SOT

4G. Improvement of nanomagnetic/ spintronic devices using STT and SOT

• Module 5- Other applications of spintronics5A. Spin transfer torque oscillators

5B. Spintronic neural networks

• Course Project

One more look at the course contents!!