Seminar Report 2011 Spintronics

Seminar Report 2011 SpintronicsIntroduction Spintronics (a neologism meaning "spin transport electronics", also known as magnetoelectronics, is an emerging technology that exploits the intrinsic spin of the electron and its associated magnetic moment, in addition to its fundamental electronic charge, in solid-state devices. They rely completely on magnetic moment of the electron. Electrons are spin-1/2 fermions and therefore constitute a two-state system with spin "up" and spin "down". Electrons have a property that they occupy only one quantum state at a given time. To make a spintronic device, the primary requirements are a system that can generate a current of spin-polarized electrons comprising more of one spin species—up or down—than the other (called a spin injector), Spin process can be accomplished using real external magnetic fields or effective fields caused by spin-orbit interaction.Dept of EEE 18 S.S.M Polytechnic, Tirur Seminar Report 2011 SpintronicsAdvantages of spintronics Non-volatile memory. Performance improves with smaller devices. Low power consumption. Spintronics does not require unique and specialized semiconductors. Dissipation less transmission. Switching time is very less compared to normal RAM chips, spintronicRAM chips will: o Increase storage densities by a factor of three, o Have faster switching and rewritability rates smaller.Limitations Controlling spin for long distances. Difficult to Inject and Measure spin. Interfernce of fields with nearest elements. Control of spin in silicon is diffic.Dept of EEE 18 S.S.M Polytechnic, Tirur Seminar Report 2011 SpintronicsHistorical PerspectiveThe research field of spintronics emerged from experiments on spin- dependent electron transport phenomena in solid-state devices done in the 1980s, including the observation of spin-polarized electron injection from a ferromagnetic metal to a normal metal by Johnson and Silsbee (1985), and the discovery of giant magnetoresistance independently by Albert Fert and PeterGrünberg. The origins can be traced back further to the ferromagnet/superconductor tunneling experiments pioneered by Meservey and Tedrow, and initial experiments on magnetic tunnel junctions by Julliere in the 1970s. The use of semiconductors for spintronics can be traced back at least as far as the theoretical proposal of a spin field-effect-transistor by Datta and Das in 1990.Dept of EEE 18 S.S.M Polytechnic, Tirur Seminar Report 2011 SpintronicsWorkingElectrons are spin-1/2 fermions and therefore constitute a two-state system with spin "up" and spin "down". To make a spintronic device, the primary requirements are a system that can generate a current of spin-polarized electrons comprising more of one spin species—up or down—than the other (called a spin injector), and a separate system that is sensitive to the spin polarization of the electrons (spin detector). Manipulation of the electron spin during transport between injector and detector (especially in semiconductors) via spin precession can be accomplished using real external magnetic fields or effective fields caused by spin-orbit interaction.Spin polarization in non-magnetic materials can be achieved either through the Zeeman Effect in large magnetic fields and low temperatures, or by non-equilibrium methods. In the latter case, the non-equilibrium polarization will decay over a timescale called the "spin lifetime". Spin lifetimes of conduction electrons in metals are relatively short (typically less than 1 nanosecond) but in semiconductors the lifetimes can be very long (microseconds at low temperatures), especially when the electrons are isolated in local trapping potentials (for instance, at impurities, where lifetimes can be milliseconds).All spintronic devices act according to the simple scheme:Dept of EEE 18 S.S.M Polytechnic, Tirur Seminar Report 2011 Spintronics(1) Information is stored (written) into spins as a particular spin orientation (up or down). (2) The spins, being attached to mobile electrons, carry the information along a wire, and (3) The information is read at a terminal.Dept of EEE 18 S.S.M Polytechnic, Tirur Seminar Report 2011 SpintronicsGiant Magnetoresistance (GMR)The simplest method of generating a spin-polarised current in a metal is to pass the current through a ferromagnetic material. The most common application of this effect is a giant magnetoresistance (GMR) device. A typicalGMR device consists of at least two layers of ferromagnetic materials separated by a spacer layer. When the two magnetization vectors of the ferromagnetic layers are aligned, the electrical resistance will be lower (so a higher current flows at constant voltage) than if the ferromagnetic layers are anti-aligned. This constitutes a magnetic field sensor.Two variants of GMR have been applied in devices: current-in-plane (CIP), where the electric current flows parallel to the layers and current-perpendicular-to-plane (CPP), where the electric current flows in a direction perpendicular to the layers.Other metals-based spintronics devices: Tunnel Magnetoresistance (TMR), where CPP transport is achieved by using quantum-mechanical tunneling of electrons through a thin insulator separating ferromagnetic layers.Dept of EEE 18 S.S.M Polytechnic, Tirur Seminar Report 2011 Spintronics Spin Torque Transfer, where a current of spin-polarized electrons is used to control the magnetization direction of ferromagnetic electrodes in the device.MRAMMRAM uses magnetic storage elements. Tunnel junctions are used to read the information stored in MRAM. Attempts were made to control bit writing by using relatively large currents to produce fields. This proves unpractical at nanoscale level. 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.Dept of EEE 18 S.S.M Polytechnic, Tirur Seminar Report 2011 SpintronicsMRAM Promises:  Density of DRAM  Speed of SRAM  Non-volatility like flashSpin TransistorIdeal 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.Dept of EEE 18 S.S.M Polytechnic, Tirur Seminar Report 2011 SpintronicsCurrent ResearchFerromagnetic 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. Future Outlook High capacity hard drives, the future Plastic data storage. Magnetic RAM chips Spin FET using quantum tunneling Quantum computersDept of EEE 18 S.S.M Polytechnic, Tirur Seminar Report 2011 SpintronicsApplicationsMotorola has developed a 1st generation 256 kb MRAM based on a single magnetic tunnel junction and a single transistor and which has a read/write cycle of under 50 nanoseconds (Ever spin, Motorola's spin-off, has since developed a 4 Mbit version. There are two 2nd generation MRAM techniques currently in development: Thermal Assisted Switching (TAS) which is being developed by Crocus Technology, and Spin Torque Transfer (STT) on whichCrocus, Hynix, IBM, and several other companies are working.Another design in development, called Racetrack memory, encodes information in the direction of magnetization between domain walls of a ferromagnetic metal wire.Dept of EEE 18 S.S.M Polytechnic, Tirur Seminar Report 2011 SpintronicsSemiconductor-based spintronic devicesFerromagnetic semiconductor sources (like manganese-doped gallium arsenide GaMnAs), increase the interface resistance with a tunnel barrier, or using hot-electron injection.Spin detection in semiconductors is another challenge, which has been met with the following techniques: Faraday/Kerr rotation of transmitted/reflected photons Circular polarization analysis of electroluminescence Nonlocal spin valve (adapted from Johnson and Silsbee's work with metals) Ballistic spin filtering The latter technique was used to overcome the lack of spin-orbit interaction and materials issues to achieve spin transport in silicon, the most important semiconductor for electronics. Because external magnetic fields (and stray fields from magnetic contacts) can cause large Hall effects and magnetoresistance in semiconductors (which mimic spin-valve effects), the only conclusive evidence of spin transport in semiconductors is demonstration of spinDept of EEE 18 S.S.M Polytechnic, Tirur Seminar Report 2011 Spintronics precession and dephasing in a magnetic field non-collinear to the injected spin orientation. This is called the Hanle effect.Dept of EEE 18 S.S.M Polytechnic, Tirur Seminar Report 2011 SpintronicsSpin DetectionSpin detection in semiconductors is another challenge, which has been met with the following techniques: Faraday/Kerr rotation of transmitted/reflected photons. Circular polarization analysis of electroluminescence.Dept of EEE 18 S.S.M Polytechnic, Tirur Seminar Report 2011 SpintronicsDepicting Spin Of Electrons Concerns To devise economic ways to combine ferromagnetic metals and semiconductors in integrated circuits. To find an efficient way to inject spin-polarized currents, or spin currents, into a semiconductor. To maximize the time period for spin current to retain its polarization in a semiconductor. To make semiconductors that are ferromagnetic at room temperature and don’t lose their property even at high temperature To minimize spin currents at boundaries between different semiconductors so as to minimize the loss.Dept of EEE 18 S.S.M Polytechnic, Tirur Seminar Report 2011 SpintronicsApplicationsApplications such as semiconductor lasers using spin-polarized electrical injection have shown threshold current reduction and controllable circularly polarized coherent light output. Future applications may include a spin- based transistor having advantages over MOSFET devices such as steeper sub- threshold slope.Motorola has developed a 1st generation 256 kb MRAM based on a single magnetic tunnel junction and a single transistor and which has a read/write cycle of under 50 nanoseconds.There are two 2nd generation MRAM techniques currently in development: Thermal Assisted Switching (TAS) which is being developed by Crocus Technology, and  Spin Torque Transfer (STT) on which Crocus, Hynix, IBM, and several other companies are working.Semiconductor lasers using spin-polarized electrical injection have shown threshold current reduction and controllable circularly polarized coherent light output.Dept of EEE 18 S.S.M Polytechnic, Tirur Seminar Report 2011 SpintronicsSpintronic CouplersUsing the same sputtering technology, it is possible to build a thin film on-chip coil. A current in this coil, when combined with an on-chip GMR magnetic sensor separated by an insulating layer, can couple a signal across the insulator achieving galvanic isolation. Like the sensor, these components can be combined with other semiconductor functions to produce a very high-speed digital isolator. In a spintronic coupler, four GMR resistors form a Wheatstone bridge(see Figure 7). A thin polymer dielectric barrier layer provides several thousand volts of isolation from the input coil. A magnetic field proportional to the input current signal is generated beneath the coil winding. The resulting magnetic field flips the spin of electrons in the GMR resistors, changing their resistance. A magnetic shield protects the sensor from external fields. There are two spins (UP spin and DOWN Spin).This spintronic scanning technique is an efficient technique used in the medical field to detectcancer cells.Cancer cells are easy to be identified only when they are large in number. These cells when matured results in formation of tumor, which has to be removed by surgery. After surgery there may be presence of even a single cancer cell, which would result in growth of tumor in effected part of the body. TheDept of EEE 18 S.S.M Polytechnic, Tirur Seminar Report 2011 Spintronics spintronic scanning is an efficient technique to detect cancer cells even when they are less in number.A Patient is exposed to a strong magnetic field so that his body cell gets magnetized. A beam of electrons with polarized spin is introduced on the uneffected part of the body and the change in spin is detected by a polarimeter. A beam of electrons with polarized spin is introduced on the part which had undergone surgery.The difference in spin of electrons when introduced to normal area and abnormal area indicates whether cancer cells have been removed from the body. If not, it indicates the presence of traces of cancer cells and it has to be treated again for ensuring complete safety to the patient. Thus this technique efficiently identifies the presence of cancer cells in that part of the body that has undergone surgery to prevent any further developmentDept of EEE 18 S.S.M Polytechnic, Tirur Seminar Report 2011 SpintronicsConclusionSpintronics is a technology with a fast track from the discovery of GMR andMTJ materials to the incorporation of these materials in commercial devices.Spintronics read heads dominate the hard-disk market. Magnetic sensors based on spintronics are making inroads in markets where some combination of high resolution, high sensitivity, small size, and low power are required. Digital data couplers and displacing opto isolators in many applications and are making inroads into new markets heretofore unavailable. MRAM devices are on the horizon and offer the promise of laptop computers that do not need to boot up and cell phones with increased battery time and increased capabilities.Dept of EEE 18 S.S.M Polytechnic, Tirur

Seminar Report 2011 Spintronics

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