1. Introduction to Spintronics

2. Electron has : Mass Charge Spin Spintronics=spin based electronics information is carried by spin not by charge ferromagnetic metallic alloy based devices transport in fm materials is spin polarized

3. Introduction Conventional electronic devices ignore the spin property
As electronic devices become smaller, quantum properties of the wavelike nature of electrons are no longer negligible.
Adding the spin degree of freedom provides new effects, new capabilities and new functionalities
Information is stored into spin as one of two possible orientations
Spin does not replace charge current just provide extra control
Using suitable materials, many different “bit” states can be interpreted

4. Advantages of spintronics
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

5. Phases in Spintronics
SPIN INJECTION
SPIN TRANSFER
SPIN DETECTION

6. Spin injection Using a ferromagnetic electrode
effective fields caused by spin-orbit interaction.
a vacuum tunnel barrier could be used to effectively inject spins into a semiconductor
back biased Fe/AlGaAs Schottky diode has been reported to yield a spin injection efficiency of 30%
By “hot” electrons

7. Spin Transfer
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

8. Spin Transfer Torque The spin of the conduction electronv v
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.

9. Spin detection
Optical detection techniques using magnetic resonance force microscopy
Electrical sensing techniques-through quantum dots and quantum point contact

10. SPIN RELAXATION Leads to spin equilibration
T1-Spin-lattice relaxation time
T2-Spin-spin relaxation time
Neccesary condition 2T1>=T2.

11. Application GMR(Giant magnetoresistance)
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
Think of optical polarizers

12. Parallel current GMR
Perpendicular current GMR

13. Spin Valve Simplest and most successful spintronic device
Used in HDD to read information in the form of small magnetic fields above the disk surface

14. Tunnel Magnetoresistance
Tunnel Magnetoresistive effect combines the two spin channels in the ferromagnetic materials and the quantum tunnel effect
TMR junctions have resistance ratio of about 70%
MgO barrier junctions have produced 230% MR
230% with sputtering deposition

15. MRAM MRAM uses magnetic storage elements
Tunnel junctions are used to read the information stored in MRAM
Non volatile, instant-on computers

16. MRAM
Attempts were made to control bit writing by using relatively large currents to produce fields
This proves unpractical at nanoscale level

17. 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

18. MRAM MRAM promises: Density of DRAM Speed of SRAM
Non-volatility like flash
DRAM stores one bit in only one capacitor and transistor, very dense but very power hungry because capacitor loses charge, must be frequently refreshed
SRAM stores one bit in six transistors, faster than DRAM but less dense

19. 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

20. 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
Rashba effect – consequence of spin orbit interaction, proportional to electric field in a structure with inversion asymmetry

21. Current Research
Ferromagnetic transition temperature in excess of 100 K
Spin injection from ferromagnetic to non-magnetic semiconductors and long spin-coherence times in semiconductors.
Ferromagnetism in Mn doped group IV semiconductors.
Room temperature ferromagnetism
Large magnetoresistance in ferromagnetic semiconductor tunnel junctions.
“The crystalline quality, surface topography, and thermal stability of the films indicate the possibility of growing epitaxial Ge on top of Mn5Ge3 so that epitaxial trilayers or ‘spin valves’ and perhaps even multilayer structures can be fabricated for spintronics research and applications.”

22. Future Outlook High capacity hard drives Magnetic RAM chips
Spin FET using quantum tunneling
Quantum computers

23. limitations Controlling spin for long distances
Difficult to INJECT and MEASURE spin.
Interfernce of fields with nearest elements
Control of spin in silicon is difficult

24. THANK YOU!