Introduction to Spintronics

1. Introduction to Spintronics

2. Electron has :MassChargeSpin Spintronics=spin based electronicsinformation is carried by spin not by chargeferromagnetic metallic alloy based devicestransport 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

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 <S> v 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. ApplicationGMR(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

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

15. MRAM • MRAM uses magnetic storage elements • Tunnel junctions are used to read the information stored in MRAM

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

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

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.

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!