Introduction to Spintronics

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

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

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

Phases in Spintronics SPIN INJECTION SPIN TRANSFER SPIN DETECTION

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

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

Spin Transfer Torque The spin of the conduction electron 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.

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

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

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

Parallel current GMR Perpendicular current GMR

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

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

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

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

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 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

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

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.”

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

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

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