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    Spintronics report Spintronics report Document Transcript

    • A Seminar Report On “Spintronics Technology” Session 2010-2011Submitted To: Submitted By:Dr. R.S. Meena Shailendra Kumar SinghMr. Pankaj Shukla C.R.No. 07/126Dept. of Electronics Engg. Final Year, ECEUCE, RTU, Kota UCE, RTU, Kota Department of Electronics and Communication Engg. University College of Engineering Rajasthan Technical University, Kota Page 1
    • CERTIFICATE This is certify that the Seminar report titled “Spintronics Technology”has been submitted in partial fulfilment of the requirement for the awardof Bachelor of Technology in Electronics & Communication Engineeringby following student of final year B.Tech. Shailendra Kumar Singh C.R.No:- 07/126 B.TECH. FINAL YEAR UCE, RTU, KOTASeminar Coordinators: Head of the Department:Dr R S Meena & Mr Pankaj Shukla Dr Rajeev Gupta(Associate Professors) ProfessorDept. Of Electronics Engg. Dept. Of Electronics Engg.UCE, RTU, Kota UCE, RTU, Kota Page 2
    • ACKNOWLEDGEMENT It gives me great pleasure to present my seminar report on “SpintronicsTechnology”. No work , however big or small, has ever been done without thecontributions of others. It would be a great pleasure to write a few words, which would although notsuffice as the acknowledgement of this long cherished effort, but in the absence of whichthis report would necessarily be incomplete. So these words of acknowledgement comeas a small gesture of gratitude towards all those people, without whom the successfulcompletion of this project would not have been possible. I would like to express deep gratitude towards Dr. R S Meena (AssociateProfessor of Electronics Engineering Dept., UCE, Kota) & Mr. Pankaj Shukla(Associate Professor of Electronics Engineering Dept., UCE, Kota) who gave metheir valuable suggestions, motivation and the direction to proceed at every stage.Theyare like a beam of light for us. Their kind guidance showed us the path of life and isunforgettable. They extended towards their valuable guidance, indispensable help andinspiration at times in appreciation I offer them my sincere gratitude. Last but not least we would like to thank the Department of ElectronicsEngineering, UCE, Kota for providing me with the facilities to lab, and all staff members ofcommunication lab, it would have been impossible for me to complete my project withouttheir valuable guidance & prompt cooperation. I have tried my level best to make this seminar report error free ,but I regret forerrors , if any. SHAILENDRA KUMAR SINGH C.R.NO. - 07/126 B. TECH. FINAL YEAR, ECE UCE, RTU, KOTA Page 3
    • CONTENTSS. No Chapters Page No 1. Introduction 07 2. Basic Principle 08 3. Gaint Magnetoresistance 10 4. Construction of GMR 12 5. Memory Chips 14 6. GMR Sensors 15 7. Spin Valve GMR 16 8. Spintronic Devices 17 9. MRAM 18 10. Spin Transistors 19 11. Spintronic Scanner 22 12. Conclusion 26 13. Reference 27 Page 4
    • List of FiguresS No. Figure Name Page No. 1. Electron spinning 08 2. Magnetic Orientation of electrons. 09 3. A GMR read head 10 4. A GMR Device 13 5. A General Magnetic Field Sensor 14 6. Spintronic Sensor 15 7. Standard Geometry for GMR based Spin Valves 16 8. GMR based Spin Valves for read head In hard drives 16 9. 256 K MRAM 18 10. Spin Transistor 19 11. Spin Polarised Field Effect Transistor 20 Page 5
    • ABSTRACT Spintronics is an emergent technology that exploits the quantum propensity of theelectrons to spin as well as making use of their charge state. The spin itself is manifested as adetectable weak magnetic energy state characterised as ―spin up‖ or ―spin down‖. Conventional electronic devices rely on the transport of electrical charge carriers –electrons – in a semiconductor such as silicon. Now, however, device engineers and physicists areinevitably faced the looming presence of quantum mechanics and are trying to exploit the spin ofthe electron rather than its charge. Devices that rely on the electron‘s spin to perform theirfunctions form the foundations of spintronics (short for spin-based electronics), also known asmagnetoelectronics. Spintronics devices are smaller than 100 nanometre in size, more versatile andmore robust than those making up silicon chips and circuit elements. The potential market is worthhundreds of billions of dollar a year. Spintronics burst on the scene in 1988 when French and German physicists discovereda very powerful effect called Giant Magnetoresistance (GMR). It results from subtle electron-spineffects in ultra thin multilayers of magnetic materials, which cause huge changes in their electricalresistance when a magnetic field is applied. This resulted in the first spintronic device in the formof the spin valve. The incorporation of GMR materials into read heads allowed the storage capacityof a hard disk to increase from one to 20 gigabits. In 1997, IBM launched GMR read heads, into amarket worth around a billion dollars a year. The field of spintronics is relatively young and it is difficult to predict how it willevolve. New physics is still being discovered and new materials being developed, such as magneticsemiconductors and exotic oxides that manifest an even more extreme effect called ColossalMagnetoresistance. Page 6
    • Chapter 1 INTRODUCTIONConventional electronic devices rely on the transport of electrical charge carriers –electrons in asemiconductor such as silicon. Now, however, physicists are trying to exploit the ‗spin‘ of theelectron rather than its charge to create a remarkable new generation of ‗spintronic‘ deviceswhich will be smaller, more versatile and more robust than those currently making up siliconchips and circuit elements.Imagine a data storage device of the size of an atom working at a speed of light. Imagine acomputer memory thousands of times denser and faster than today‘s memories and also imaginea scanner technique which can detect cancer cells even though they are less in number. Theabove-mentioned things can be made possible with the help of an exploding science –―Spintronics‖.Spintronics is a technology which deals with spin dependent properties of an electron instead ofor in addition to its charge dependent properties. Conventional electronics devices rely on thetransport of electric charge carries-electrons. But there is other dimensions of an electron otherthan its charge and mass i.e. spin. This dimension can be exploited to create a remarkablegeneration of spintronic devices. It is believed that in the near future spintronics could be morerevolutionary than any other technology.As there is rapid progress in the miniaturization of semiconductor electronic devices leads to achip features smaller than 100 nanometers in size, device engineers and physicists are inevitablefaced with a looming presence of a quantum property of an electron known as spin, which isclosely related to magnetism. Devices that rely on an electron spin to perform their functionsform the foundations of spintronics.Information-processing technology has thus far relied on purely charge based devices rangingfrom the now quantum, vacuum tube today‘s million transistor microchips. Those conventionalelectronic devices move electronic charges around, ignoring the spin that tags along that side oneach electron. Page 7
    • Chapter 2 BASIC PRINCIPLEThe basic principle involved is the usage of spin of the electron in addition to mass and charge ofelectron. Electrons like all fundamental particles have a property called spin which can beorientated in one direction or the other – called ‗spin-up‘ or ‗spin-down‘ –like a top spinninganticlockwise or clockwise. Spin is the root cause of magnetism and is a kind of intrinsic angularmomentum that a particle cannot gain or lose. The two possible spin states naturally represent‗0‘and ‗1‘in logical operations. Spin is the characteristics that makes the electron a tiny magnetcomplete with north and south poles .The orientation of the tiny magnet ‗s north-south polesdepends on the particle‘s axis of spin.Fundamentals of spin:1. In addition to their mass, electrons have an intrinsic quantity of angular momentum called spin, almost of if they were tiny spinning balls.2. Associated with the spin is magnetic field like that of a tiny bar magnet lined up with the spin axis. . Fig.1. Electron spinning2. Scientists represent the spin with a vector. For a sphere spinning ―west to east‖, the vector points ―north‖ or ―up‖. It points ―south‖ or ―down‖ for the spin from ―east to west‖.4. In a magnetic field, electrons with ―spin up‖ and ―spin down‖ have different energies.5. In an ordinary electronic circuit the spins are oriented at random and have no effect on current flow. Page 8
    • 6. Spintronic devices create spin-polarized currents and use the spin to control current flow.Imagine a small electronically charged sphere spinning rapidly. The circulating charges in thesphere amount to tiny loops of electric current which creates a magnetic field. A spinning spherein an external magnetic field changes its total energy according to how its spin vector is alignedwith the spin. In some ways, an electron is just like a spinning sphere of charge, an electron has aquantity of angular momentum (spin) an associated magnetism. In an ambient magnetic field andthe spin changing this magnetic field can change orientation. Its energy is dependent on how itsspin vector is oriented. The bottom line is that the spin along with mass and charge is definingcharacteristics of an electron. In an ordinary electric current, the spin points at random and playsno role in determining the resistance of a wire or the amplification of a transistor circuit.Spintronic devices in contrast rely on the differences in the transport of spin-up and spin-downelectrons.Fig 2. Magnetic Orientation of electrons Page 9
    • Chapter 3 Giant Magnetoresistance Electrons like all fundamental particles have a property called spin which canbe orientated in one direction or the other – called „spin-up‟ or „spin-down‟ – like a topspinning anticlockwise or clockwise. When electron spins are aligned (i.e. all spin-up or allspin-down) they create a large-scale net magnetic moment as seen in magnetic materialslike iron and cobalt. Magnetism is an intrinsic physical property associated with the spinsof electrons in a material. Magnetism is already exploited in recording devices such as computer harddisks Data are recorded and stored as tiny areas of magnetised iron or chromium oxide.To access the information, a read head detects the minute changes in magnetic field asthe disk spins underneath it. This induces corresponding changes in the head‟s electricalresistance – an effect called magnetoresistance. Spintronics burst on the scene in 1988 when French and German physicistsdiscovered a much more powerful effect called „giant magnetoresistance‟ (GMR). It resultsfrom subtle electron-spin effects in ultra-thin „multilayers‟ of magnetic materials, whichcause huge changes in their electrical resistance when a magnetic field is applied. GMR is200 times stronger than ordinary magnetoresistance. IBM soon realised that read headsincorporating GMR materials would be able to sense much smaller magnetic fields,allowing the storage capacity of a hard disk to increase from 1 to 20 gigabits. In 1997 IBMlaunched GMR read heads, into a market worth about a billion dollars a year. The basic GMR device consistsmof a three-layer sandwich of a magnetic metalsuch as cobalt with a nonmagnetic metal filling such as silver (see diagram).A current passes through the layers consisting of spin-up and spin-down electrons. Thoseoriented in the same direction as the electron spins in a magneticlayer pass through quite easily while those oriented in the opposite direction arescattered. If the orientation of one of the magnetic layers can easily be changed by thepresence of a magnetic field then the device will act as a filter, or „spin valve‟, lettingthrough more electrons when the spin orientations in the two layers are the same andfewer when orientations are oppositely aligned. The electrical resistance of the device cantherefore be changed dramatically. Fig 3. A GMR read head Page 10
    • Magnetism is the integral part of the present day‘s data storage techniques. Right from theGramophone disks to the hard disks of the super computer magnetism plays an important role.Data is recorded and stored as tiny areas of magnetized iron or chromium oxide. To access theinformation, a read head detects the minute changes in magnetic field as the disk spinsunderneath it. In this way the read heads detect the data and send it to the various succeedingcircuits.The effect is observed as a significant change in the electrical resistance depending on whetherthe magnetization of adjacent ferromagnetic layers are in a parallel or anantiparallel alignment.The overall resistance is relatively low for parallel alignment and relatively high for antiparallelalignment.The magneto resistant devices can sense the changes in the magnetic field only to a small extent,which is appropriate to the existing memory devices. When we reduce the size and increase datastorage density, we reduce the bits, so our sensor also has to be small and maintain very, veryhigh sensitivity. The thought gave rise to the powerful effect called ―Giant Magnetoresistance‖(GMR). GMR is a quantum mechanical magnetoresistance effect observed in thin film structurescomposed of alternating ferromagnetic and non magnetic layers. The 2007 Nobel Prize inphysics was awarded to Albert Fert and Peter Grünberg for the discovery of GMR.Giant magnetoresistance (GMR) came into picture in 1988, which lead the rise of spintronics. Itresults from subtle electron-spin effects in ultra-thin ‗multilayer‘ of magnetic materials, whichcause huge changes in their electrical resistance when a magnetic field is applied. GMR is 200times stronger than ordinary magnetoresistance. It was soon realized that read headsincorporating GMR materials would be able to sense much smaller magnetic fields, allowing thestorage capacity of a hard disk to increase from 1 to 20 gigabits. Page 11
    • Chapter 4 Construction of GMRThe basic GMR device consists of a three-layer sandwich of a magnetic metal such as cobaltwith a nonmagnetic metal filling such as silver. Current passes through the layers consisting ofspin-up and spin-down electrons. Those oriented in the same direction as the electron spins in amagnetic layer pass through quite easily while those oriented in the opposite direction arescattered. If the orientation of one of the magnetic layers can easily be changed by the presenceof a magnetic field then the device will act as a filter, or ‗spin valve‘, letting through moreelectrons when the spin orientations in the two layers are the same and fewer when orientationsare oppositely aligned. The electrical resistance of the device can therefore be changeddramatically. In an ordinary electric current, the spin points at random and plays no role indetermining the resistance of a wire or the amplification of a transistor circuit. Spintronic devicesin contrast, rely on differences in the transport of ―spin up‖ and ―spin down‖ electrons. When acurrent passes through the Ferro magnet, electrons of one spin direction tend to be obstructed.A ferromagnet can even affect the flow of a current in a nearby nonmagnetic metal. For example,in the present-day read heads in computer hard drives, wherein a layer of a nonmagnetic metal issandwiched between two ferromagnetic metallic layers, the magnetization of the first layer isfixed, or pinned, but the second ferromagnetic layer is not. As the read head travels along a trackof data on a computer disk, the small magnetic fields of the recorded 1‘s and 0`s change thesecond layer‘s magnetization back and forth parallel or antiparallel to the magnetization of thepinned layer. In the parallel case, only electrons that are oriented in the favored direction flowthrough the conductor easily. In the antiparallel case, all electrons are impeded. The resultingchanges in the current allow GMR read heads to detect weaker fields than their predecessors; sothat data can be stored using more tightly packaged magnetized spots on a disk. GMR has triggered the rise of a new field of electronics called spintronics which has been used extensively in the read heads of modern hard drives and magnetic sensors. A hard disk storing binary information can use the difference in resistance between parallel and antiparallel layer alignments as a method of storing 1s and 0s. A high GMR is preferred for optimal data storage density. Current perpendicular-to-plane (CPP) Spin valve GMR currently yields the highest GMR. Research continues with older current-in-plane configuration and in the tunnelling magnetoresistance (TMR) spin valves which enable disk drive densities exceeding 1 Terabyte per square inch. Page 12
    • Hard disk drive manufacturers have investigated magnetic sensors based on the colossalmagnetoresistance effect (CMR) and the giant planar Hall effect. In the lab, such sensors havedemonstrated sensitivity which is orders of magnitude stronger than GMR. In principle, thiscould lead to orders of magnitude improvement in hard drive data density. As of 2003, onlyGMR has been exploited in commercial disk read-and-write heads because researchers have notdemonstrated the CMR or giant planar hall effects at temperatures above 150K.Magnetocoupler is a device that uses giant magnetoresistance (GMR) to couple two electricalcircuits galvanicly isolated and works from AC down to DC.Vibration measurement in MEMS systems.Detecting DNA or protein binding to capture molecules in a surface layer by measuring the strayfield from superparamagnetic label particles. Fig 4. A GMR Device Page 13
    • Chapter 5 Memory Chips Physicists have been quick to see the further possibilities of spin valves. Notonly are they highly sensitive magnetic sensors (see Box), they can also be made to actas switches by flipping the magnetisation in one of the layers. This allows information tobe stored as 0s and 1s (magnetisations of the layers parallel or antiparallel) as in aconventional transistor memory device. An obvious application is a magnetic version of arandom access memory (RAM) device of the kind used in your computer. The advantageof magnetic random access memory (MRAM) is that it is „non-volatile‟ – information isn‟tlost when the system is switched off. MRAM devices would be smaller, faster, cheaper,use less power and would be much more robust in extreme conditions such as hightemperature, or highlevel radiation or interference. The US electronics companyHoneywell has already shown that arrays of linked MRAMS could be made to work. Thepotential market for MRAMS is worth 100 billion dollars annually. Over the past three years or so, researchers around the world have beenworking hard on a whole range of MRAM devices. A particularly promising device is themagnetic tunnel junction, which has two magnetic layers separated by an insulating metal-oxide layer. Electrons can „tunnel‟ through from one layer to the other only whenmagnetisations of the layers point in the same direction, otherwise the resistance is high –in fact, 1000 times higher than in the standard spin valve.Even more interesting are devices that combine the magnetic layers with semiconductorslike silicon. The advantage is that silicon is still the favourite material of the electronicsindustry and likely to remain so. Such hybrid devices could be made to behave more likeconventional transistors. They could be used as non-volatile logic elements which couldbe reprogrammed using software during actual processing to create an entirely new typeof very fast computing. The field of spintronics is extremely young and it‟s difficult topredict how it will evolve. New physics is still being discovered and new materials beingdeveloped, such as magnetic semiconductors, and exotic oxides that manifest an evenmore extreme effect called colossal magnetoresistance. What is certain is that the time-span from a breakthrough in fundamental physics to first commercial exploitation hasbeen less than 10 years. The business opportunities for spintronics are still wide open.European research collaborations, some involving the UK, have a strong lead indeveloping the underlying physics and technology for this lucrative fledgling industry. Fig 5. A general magnetic field sensor made of GMR multilayers ( iron-nickel with silver ) Page 14
    • Chapter 6 GMR SENSORS GMR sensors are already being developed in UK universities. They have a wide rangeof applications and the market is worth 8 billion dollars a year. Applications include:• Fast accurate position and motion sensing of mechanical components in precision engineeringand in robotics• All kinds of automotive sensors for fuel handling systems, electronic engine control, antiskidsystems, speed control and navigation• Missile guidance• Position and motion sensing in computer video games• Key-hole surgery and post-operative care Fig 6. Spintronic sensor technology being tested on a Mercedes V8 engine at Oxford Page 15
    • Chapter 7 Spin Valve GMR If the orientation of one of the magnetic layers can easily be changed by the presence of amagnetic field then the device will act as a filter, or ‗spin valve‘, letting through more electronswhen the spin orientations in the two layers are the same and fewer when orientations areoppositely aligned. The electrical resistance of the device can therefore be changed dramatically. Fig 7. Standard geometry for GMR based Spin ValvesAn electron passing through the spin-valve will be scattered more if the spin of the electron isopposite to the direction of the magnetisation in the FM layer. Fig 8. GMR based Spin Valves for read head in hard drives Page 16
    • Chapter 8 Spintronic Devices Spintronic devices are those devices which use the Spintronic technology. Spintronic-devices combine the advantages of magnetic materials and semiconductors. They are expected to be non- volatile, versatile, fast and capable of simultaneous data storage and processing, while at the same time consuming less energy. Spintronic-devices are playing an increasingly significant role in high-density data storage, microelectronics, sensors, quantum computing and bio-medical applications, etc. Electronic Devices Spintronic devices1. Based on properties of charge of the 1. Based on intrinsic property spin of electron. electron2. Classical property 2. Quantum property4. Materials: conductors and semiconductors 4. Materials: ferromagnetic materials5. Based on the number of charges and their 5. Two basic spin states; spin-up and spindown. energy6. Speed is limited and power dissipation is 6. Based on direction of spin and spin and spin high coupling, high speed.Some of the Spintronic devices are:  Magnetoresistive Random Access Memory(MRAM)  Spin Transistor  Quantum Computer  Spintronic Scanner Page 17
    • Chapter 9 MRAM (Magnetoresistive Random Access Memory) An important spintronic device, which is supposed to be one of the firstspintronic devices that have been invented, is MRAM. Unlike conventional random-access, MRAMs do not lose stored informationonce the power is turned off...A MRAM computer uses power, the four page e mail will beright there for you. Today pc use SRAM and DRAM both known as volatile memory. Theycan store information only if we have power. DRAM is a series ofcapacitors, a charged capacitor represents 1 where as an uncharged capacitor represents 0. Toretain 1 you must constantly feed the capacitor with power because the charge you put into thecapacitor is constantly leaking out.MRAM is based on integration of magnetic tunnel junction (MJT). Magnetic tunnel junction is athree-layered device having a thin insulating layer between two metallic ferromagnets. Currentflows through the device by the process of quantum tunneling; a small number of electronsmanage to jump through the barrier even though they are forbidden to be in the insulator. Thetunneling current is obstructed when the two ferromagnetic layers have opposite orientations andis allowed when their orientations are the same. MRAM stores bits as magnetic polarities ratherthan electric charges. When a big polarity points in one direction it holds1, when its polaritypoints in other direction it holds 0. These bits need electricity to change the direction but not tomaintain them. MRAM is non volatile so, when you turn your computer off all the bits retaintheir 1`s and 0`s. Fig 9. 256 K MRAM Page 18
    • Chapter 10 SPIN TRANSISTORS Traditional transistors use on-and-off charge currents to create bits- the binary zeroes and ones ofcomputer information. “Quantum spin field effect” transistor will use up-and-down spin states to generatethe same binary data. One can think of electron spin as an arrow; it can point upward or downward; “spinupand spin-down can be thought of as a digital system, representing the binary 0 and 1. The quantumtransistor employs also called “spin-flip” mechanism to flip an up-spin to a downspin, or change the binarystate from 0 to 1.One proposed design of a spin FET (spintronic field-effect transistor) has a source and a drain, separatedby a narrow semi conducting channel, the same as in a conventional FET. In the spin FET, both the sourceand the drain are ferromagnetic. The source sends spin-polarized electrons in to the channel, and this spincurrent flow easily if it reaches the drain unaltered (top). A voltage applied to the gate electrode produces anelectric field in the channel, which causes the spins of fastmoving electrons to process, or rotate (bottom). Thedrain impedes the spin current according to how far the spins have been rotated. Flipping spins in this waytakes much less energy and is much faster than the conventional FET process of pushing charges out of thechannel with a larger electric filed. Fig 10.In these devices a non magnetic layer which is used for transmitting and controlling the spinpolarized electrons from source to drain plays a crucial role. For functioning of this device firstthe spins have to be injected from source into this non-magnetic layer and then transmitted to thecollector. These non-magnetic layers are also called as semimetals, because they have very largespin diffusion lengths. The injected spins which are transmitted through this layer startprecessing as illustrated in Figure before they reach the collector due to the spin-orbit couplingeffect. Page 19
    • Vgg Collector gate Source InAlAs InGaAs Fig.11 Spin polarized field effect transistor.Vg is the gate voltage. When Vg is zero the injected spins which are transmitted through the2DEG layer starts processing before they reach the collector, thereby reducing the net spinpolarization. Vg is the gate voltage. When Vg >> 0 the precession of the electrons is controlledwith electric filed thereby allowing the spins to reach at the collector with the same polarization.Hence the net spin polarization is reduced. In order to solve this problem an electric field isapplied perpendicularly to the plane of the film by depositing a gate electrode on the top toreduce the spin-orbit coupling effect as illustrated in Figure 4. By controlling the gate voltageand polarity can the current in the collector can be modulated there by mimicking the MOSFETof the conventional electronics. Here again the problem of conductivity mismatch between thesource and the transmitting layer is an important issue. The interesting thing would be if aHeusler alloy is used as the spin source and a semimetallic Heusler alloy as the transmittinglayer, the problem of conductivity mismatch may be solved. For example from the Slater-Paulingcurve Mt = Zt - 24, Heusler alloys with Mt >>0 can act as spin sources and alloys with Mt ~ 0can act as semimetals. Since both the constituents are of same structure the possibility ofconductivity mismatch may be less.Traditional transistors use on-and-off charge currents to create bits—the binary zeroes and onesof computer information. ―Quantum spin field effect‖ transistor will use up-and-down spin statesto generate the same binary data. One can think of electron spin as an arrow; it can point upwardor downward; ―spin-up and spin-down can be thought of as a digital system, representing the Page 20
    • binary 0 and 1. The quantum transistor employs also called ―spin-flip‖ mechanism to flip an up-spin to a downspin, or change the binary state from 0 to 1.One proposed design of a spin FET (spintronic field-effect transistor) has a source and a drain,separated by a narrow semi conducting channel, the same as in a conventional FET.In the spin FET, both the source and the drain are ferromagnetic. The source sends spin-polarized electrons in to the channel, and this spin current flow easily if it reaches the drainunaltered (top). A voltage applied to the gate electrode produces an electric field in the channel,which causes the spins of fast-moving electrons to process, or rotate (bottom). The drain impedesthe spin current according to how far the spins have been rotated. Flipping spins in this way takesmuch less energy and is much faster than the conventional FET process of pushing charges outof the channel with a larger electric filed.One advantage over regular transistors is that these spin states can be detected and alteredwithout necessarily requiring the application of an electric current. This allows for detectionhardware that are much smaller but even more sensitive than todays devices, which rely onnoisy amplifiers to detect the minute charges used on todays data storage devices. The potentialend result is devices that can store more data in less space and consume less power, using lesscostly materials. The increased sensitivity of spin transistors is also being researched in creatingmore sensitive automotive sensors, a move being encouraged by a push for moreenvironmentally-friendly vehiclesA second advantage of a spin transistor is that the spin of an electron is semi-permanent and canbe used as means of creating cost-effective non volatile solid state storage that does not requirethe constant application of current to sustain. It is one of the technologies being explored forMagnetic Random Access Memory (MRAM)Spin transistors are often used in computers for data processing. They can also be used toproduce a computers random access memory and are being tested for use in magnetic RAM.This memory is superfast and information stored on it is held in place after the computer ispowered off, much like a hard disk. Page 21
    • Chapter 11 Spintronic Scanner Cancer cells are the somatic cells which are grown into abnormal size.The Cancer cells have different electromagnetic sample when compared to normal cells. Formany types of Cancer, it is easier to treat and cure the Cancer if it is found early. There aremany different types of Cancer, but most Cancers begin with abnormal cells growing out ofcontrol, forming a lump thats called a tumor. The tumor can continue to grow until the Cancerbegins to spread to other parts of the body. If the tumor is found when it is still very small,curing the Cancer can be easy. However, the longer the tumor goes unnoticed, the greater thechance that the Cancer has spread. This makes treatment more difficult. Tumor developed inhuman body, is removed by performing a surgery. Even if a single cell is present after thesurgery, it would again develop into a tumor. In order to prevent this, an efficient routefor detecting the Cancer cells is required. Here, in this paper, we introduce a new route fordetecting the Cancer cells after a surgery. This accurate detection of the existence of Cancercells at the beginning stage itself entertains the prevention of further development of the tumor.This spintronic scanning technique is an efficient technique to detect cancer cells even when theyare less in number.An innovative approach to detect the cancer cells with the help of Spintronics:The following setup is used for the detection of cancer cells in a human body:(a) Polarized electron source(b) Spin detector(c) Magnetic FieldPolarized electron source:A beam of electrons is said to be polarized if their spins point, on average, in a specific direction.There are several ways to employ spin on electrons and to control them. The requirement for thispaper is an electron beam with all its electrons polarized in a specific direction. The followingare the ways to meet the above said requirement: Photoemission from negative electron affinityGaAs Chemi-ionization of optically pumped meta stable Helium An opticallypumped electron spin filter A Wein style injector in the electron source A spin filter is moreefficient electron polarizer which uses an ordinary electron source along with a gaseous layer of Page 22
    • Rb. Free electrons diffuse under the action of an electric field through Rb vapour that has beenspin polarized in optical pumping. Through spin exchange collisions with the Rb, the freeelectrons become polarized and are extracted to form a beam. To reduce the emission ofdepolarizing radiation, N2 is used to quench the excited Rb atoms during the optical pumpingcycle.Spin detectors:There are many ways by which the spin of the electrons can be detected efficiently. The spinpolarization of the electron beam can be analyzed by using: (a)Mott polarimeter (b)Compton polarimeter (c)Moller type polarimeterTypical Mott polarimeters require electron energies of ~100 kV. But Mini Mott polarimeter usesenergies of ~25 keV, requiring a smaller overall design. The Mini Mott polarimeterhas three major sections: the electron transport system, the target chamber, and the detectors. Thefirst section the electrons enter is the transport system. An Einsel lens configuration was usedhere. Two sets of four deflectors were used as the first and last lens. The electrons next enter thetarget chamber. The chamber consists of a cylindrical target within a polished stainless steelhemisphere. A common material used for the high-Z nuclei target is gold. Low-Z nuclei helpminimize unwanted scattering, so aluminum was chosen. Scattered electrons then exit the targetchamber and are collected in the detectors. Thus there are many methods for detecting the spinpolarization of electrons.External Magnetic Field:An external magnetic field is required during this experiment. The magnetic field is applied afterthe surgery has undergone. First, it is applied to an unaffected part of the body and then to thesurgery undergone part of the body. It is already mentioned that the magnetic field could easilyalter the polarization of electrons. Page 23
    • This technique using spintronics is suggested by us to identify tumor cells after surgery.The procedure for doing this experiment is as follows:Optical Spin Filter:After surgery and the removal of the tumor, the patient is exposed to a strong magnetic field.Now the polarized electron beam is applied over the unaffected part and spin orientation ofelectrons are determined using polarimeter. Then the same polarized beam is targeted over theaffected part of the body and from the reflected beam, change in spin is determined. Based onthese two values of spin orientation, the presence of tumor cells can be detected even if they arevery few in number. Hence, we suggest this method for the detection purpose. A detailed view ofthis innovative approach is given as follows.Spin Orientation of the unaffected part of the body:Applying Magnetic Field:When the magnetic field is applied to the unaffected part of the human body, the normal somaticcells absorbs the magnetic energy and retains it.Determinig the Spin orientation:When the electrons get incident on the cells the magnetic energy absorbed by the cells alters thespin orientation of the electrons. These electrons get reflected and it is detected by the Mottpolarimeter. Then the change in spin orientation of the electrons is measured as Sx.Spin Orientation of the surgery undergone part of the body:Applying Magnetic Filter:In the surgery undergone part of the body an external magnetic field is applied. The cancer cellswhich are present, if any, will absorb more magnetic energy than the normal cells since theydiffer in their electromagnetic pattern.Determinig the spin Orientation:Now an electron beam which is polarized is incident on the surgery undergone part of the body.The magnetic energy absorbed by the cancer cell alter the spin orientation of the electron beam.Since cancer cells absorb more magnetic energy, the change in orientation caused by them is alsomore. If no cancer cells are present the amount of change is equal to the previous case. Thechange in spin is measured by the polarimeter as Sy. Page 24
    • Inference:If the change in the spin in the unaffected part of the body is same as that of the surgeryundergone part, i.e.If Sx=SyThen,There are no cancer cells in the surgery undergone part of the body and all the cells havebeen removed by the surgery.If the change in spin in the unaffected part is not equal to the change caused by the surgeryundergone part of the body, i.e.If Sx not equals SyThen,There are some cancer cells in the surgery undergone part of the body and the cancer cells arenot completely removed by the surgery.The steps involved are:1) The patient is exposed to a strong magnetic field so that his body cell gets magnetized.2) A beam of electrons with polarized spin is introduced on the unaffected part of the body and the change in spin is detected by a polarimeter. Let it be X3) A beam of electrons with polarized spin is introduced on the part which had undergone surgery. And the corresponding change in spin be Y4) If X - Y = 0, it indicates that 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 thathas undergone surgery to prevent any further development. Page 25
    • CONCLUSIONSpintronics is one of the most exciting and challenging areas in nanotechnology, important to bothfundamental scientific research and industrial applications. These spintronic-devices, combiningthe advantages of magnetic materials and semiconductors, are expected to be non-volatile,versatile, fast and capable of simultaneous data storage and processing, while at the same timeconsuming less energy. They are playing an increasingly significant role in highdensitydata storage, microelectronics, sensors, quantum computing and bio-medical applications, etc.It is expected that the impact of spintronics to the microelectronics industry might be comparable tothe development of the transistor 50 years ago.Today everyone already has a spintronic device on their desktop, as all modern computers use thespin valve in order to read and write data on their hard drive. It was followed immediately by thediscovery of Tunneling Magnetoresistance (TMR) leading to the magnetic tunnel junction that hasbeen utilized for the next generation computer memory known as Magnetic Random AccessMemory (MRAM), another spintronic device for computers. Therefore, the initial driving force forspintronics has been the improvement of computer technology. At present the research has beenconcentrating on the fabrication of spin transistors and spin logics devices integrating magnetic andsemiconductors, with the aim of improving the existing capabilities of electronic transistors andlogics devices so that the future computation and thus the future computer could become faster andconsume less energy.There are four main areas in spintronics:. 1) Understanding the fundamental physics, such as spin-dependant transports across the magnetic/ semiconductor interfaces and spin coherence length in semiconductors. 2) Synthesising suitable spintronic materials with Curie temperatures above room temperature, large spin polarisation at the Fermi level and matching conductivity between the magnetic and semiconductor materials. 3) Fabricating devices with nanometre feature sizes and developing new techniques for mass production. 4) Integrating spin-devices with current microelectronics and computing. Page 26
    • REFERENCES1. IEEE Digital Explore Library2. School of Physics & Astronomy, University of Nottingham3. Department of Physics and MARTECH , Florida State University4. Department of Physics and Center for Advanced Photonic and Electronic Materials University at Buffalo ,The State University of New York5. Research Councils UK www.rcuk.ac.uk6. Engineering and Physical Sciences Research Council (EPSRC) www.epsrc.ac.uk7. Particle Physics and Astronomy Research Council (PPARC) www.pparc.ac.uk8. Council for the Central Laboratory of the Research Councils www.cclrc.ac.uk Page 27