Spintronics: A New Nanoelectronics Adventure
International Journal of Advanced Computer Research (ISSN (print): 2249-7277 ISSN (online): 2277-7970)Volume-3 Number-1 Issue-8 March-2013Spintronics: A New Nanoelectronics AdventureRohit Apurva1, Sonia Chandan2, Abhilash Katkar3, Prashant Shinde4Dept. of E&TC Engineering, Dr. Babasaheb Ambedkar Technological University, Raigad, (MS) 1, 2, 3, 4integrated circuits will double every 18 months. Therapid improvement in performance and reduction incost of computers and communications devices wasfuelled by Moore‘s Law. Quantum effects show theirpresence in atomic levels whose predominance hasbeen seen lately. It‘s now MORE the Moore‘s Law,which will lead to the end of the silicon era. For thisreason and also to enhance the multi-functionality ofdevices (for example, carrying out processing anddata storage on the same chip) investigators havebeen eager to exploit the electronic property of spin.In fact, the spintronics dream is a seamlessintegration of electronic, optoelectronic and magnetoelectronic multi-functionality on a single device thatcan perform much more than is possible with today‘smicroelectronic devices [1].AbstractSpintronics is a new paradigm for electronics whichutilizes the electron's spin in addition to its chargefor device functionality. It is a rapidly emergingfield of science and technology that will most likelyhave a significant impact on the future of all aspectsof electronics as we continue to move into the 21stcentury. The primary areas for potentialapplications are information storage, computing,and quantum information. The main objective ofthis paper is to present a current status offundamentals of Spintronics including the recentadvantages and well established results like MRAM,Quantum computer – only one step remain to comeon Earth etc. The primary focus is on basicsphysical and quantum properties of electronunderlying Spin mechanics, Spin polarization, Spintransport through metal and semiconductor, Spininjection etc., principal of GMR with their types andapplications is also discussed in details in thispaper.A. History and BackgroundSpintronics emerged from discoveries in the 1980sconcerning spin-dependent electron transportphenomena in solid-state devices. this includes theobservation of spin-polarized electron injection from aferromagnetic metal to a normal metal by Johnson andSilsbee (1985) and the discovery of giant magnetoresistance independently by Albert Fert and PeterGrünberg (1988)[2]. The origins of spintronics can ductor tunnelling experimentspioneered by Meservey and Tedrow and initialexperiments on magnetic tunnel junctions by Jullierein the 1970s [3]. the use of semiconductors forspintronics can be traced back at least as far as thetheoretical proposal of a spin field-effect-transistor byDatta and Das in 1990[4].KeywordsSpintronics, GMR, Spin , Quantum mechanics.1. IntroductionThe new generation of Microelectronics will professthe newly discovered devices based on electron spin,rather than electronic charge. The last half of the 20thcentury, it has been argued with considerablejustification, could be called the microelectronics era.From the earliest transistor to the remarkablypowerful microprocessor in your desktop computer,most electronic devices have employed circuits thatexpress data as binary digits, or bits—ones andzeroes represented by the existence or absence ofelectric charge. Moreover, it is known that thecommunication in microelectronics devices takesplace by flow of electric charges in binary form. Thismere logic has led to multi trillion dollar industriesper year whose products are omnipresent. Indeed, therelentless growth of microelectronics is oftenpopularly summarized in Moore‘s Law, which holdsthat the numbers of transistors per square inch on2. Electron Spin: TheoryThe fundamental property of electrons is the spinangular momentum. The direction of spin can havetwo possible experimental outcomes, spin up andspin down. A Stern-Gerlach apparatus is used tomeasure the direction of spin. Electrons withdifferent spins experience different resistance in amagnetized conductor. Giant Magneto Resistanceeffect in Ferromagnetic layers is then seen in thisphenomenon. Magnetic read head and magneticrandom access memory are two examples of devices295
International Journal of Advanced Computer Research (ISSN (print): 2249-7277 ISSN (online): 2277-7970)Volume-3 Number-1 Issue-8 March-2013that are based on the giant magneto resistance effect.By manipulating information qubits that spinsencode, we may build super fast quantum computers.In order to understand what spin is, we can image anelectron as a charged sphere rotating with respectiveto itself. According to classical mechanics, such asphere would have an angular momentum (S)associated with its rotational motion about its rotationaxis. In addition, since the sphere is charged, therotating charge will give rise to a current loop andclassical magnetism tells us that there exists amagnetic moment ( ) associated with such a currentloop. However, the above picture of a spinningsphere is an understanding aid rather than the reality,for the electron‘s size is so small that it would need tospin faster than the speed of light in order to have thecorrect values of S and[4]. Nonetheless,experiments reveal that spin angular momentum (S)and spin magnetic moment ( ) are real phenomena,and what we call spin is actually spin angularmomentum. Like other quantities in the quantum,atomic world, spin angular momentum (S) can takeonly certain direction and magnitude (angularmomentum is a vector with both direction andmagnitude). The magnitude of spin angularmomentum S is given byFigure 1: Spin DirectionThe binary directional state of spin seems to makeelectron spin a perfect quantity for computerinformation storage and processing. One bit ofinformation can store either one or zero. If we let spinup represent one and spin down zero (or the otherway around), then one electron can carry one bit ofinformation. However, the quantum world isprobabilistic instead of deterministic. Before wemake any measurements, we cannot know for surewhether an electron is spin up or down. Unless wepolarized the electron, the electron is in an indefinitestate, having both possibility of becoming spin upand down. Such an electron carries one quantum bit,or qubit (represents one, zero, and the superpositionof both), of information [5].(1)Where ћ (h bar) is Planck‘s constant (h) over 2p and sis the spin quantum number, which equals to 1/2. Thedirection of spin is specified by the component of Salong a particular axis, usually denoted as the z-axis(Fig. 1). The magnitude of this component Sz is givenby,,(2)A. The Logic of Spin:1) Long Coherence: All the spin of electroncan stay in one direction (up or down).Thisproperty is called ‗Coherence‘.2) Long Relaxation time: Once the spin iscreated, then how long it is remainsunchanged is given by ‗Relaxation Time‘(how spins are created and disappear).3) Low Power Consumption: Unlike chargestates, which are easily destroyed byscattering or collision with defects andimpurities, spin-based-electronic systemsconsume less power to change spin statesand therefore is more efficient than chargebased electronic systems.4) Easy Manipulation: Spin over charge is thatspin can be easily manipulated by externallyapplied magnetic fields, a property alreadyin use in magnetic storage technology.Where the spin magnetic number ms can be either 1/2or -1/2. When ms 1/2, the electron is said to ―spinup‖, and ―spin down‖ otherwise [5].296
International Journal of Advanced Computer Research (ISSN (print): 2249-7277 ISSN (online): 2277-7970)Volume-3 Number-1 Issue-8 March-20135) Information Carrier: The movement ofspin, like the flow of charge, can also carryinformation among devices [6].can be used as magnetic field sensors. The imposedmagnetic field changes the magnetic orientation ofone of the two layers, disrupting their relativeorientation and thus changing the resistivity [7].3. Giant Magneto-resistanceGMR was discovered in layers iron, chrome andferrite by Peter Grünbergs research team of theJülich Research Centre (Germany) in 1988. PeterGrünberg owns a patent for the technology.Figure 3: Resistance Ratio in Fe/Cr GMR WithDifferent Layers Of Fe And Cr.B. Applications:The discovery of GMR has heavily contributed inHDD‘s read heads technology. From a long timeAMR (Anisotropic Magneto Resistance) had beenused but that resulted in lesser magneto resistance.AMR read heads was approaching its sensitivity limitwith the reduction of the head and the bits dimension.Nevertheless, the introduction of the spin-valve based(GMR) read head by IBM in 1997 immediatelyincreased growth rate for storage areal density up to100 percent per year (Fig. 6) [8].Figure 2: Simple GMR with Resistance PossessedBy Electron through GMRIt was also discovered by Albert Ferts research groupof the University of Paris-Sud (France) in layersferrite and chrome. The Fert group were first to seewhat they thought of as a large effect which is whythey gave the name "Giant". The Fert group was alsofirst to explain the physics behind GMR.A. Working:Giant magneto resistive effect: The first practicalapplication of this phenomenon is in the GiantMagneto Resistive effect (GMR). The GMR isobserved in thin-film materials composed of alternateferro-anti-ferro magnet magnetic and nonmagneticlayers as shown in figure [4]. The resistance of thematerial is the least when the magnetic moments inferromagnetic layers are aligned in the same directionand highest when they are anti aligned this is becausethe spin-aligned currents from one layer are scatteredmore powerfully when they confront a layer that ismagnetically arrayed in the inverse direction, creatingadditional resistance. But when the magnetic fieldsare tailored in the same direction, the spin-alignedcurrents pass through comfortably. Current GMRmaterials operate at room temperature and exhibitsignificant changes in resistivity when subjected torelatively small external magnetic fields. Thus theyIn more details, the spin-valve sensor is just a trilayer film in which one layer has its magnetizationpinned along on orientation. The rotation of the freelayer magnetization then controls the flow ofelectrons by giant magneto resistance effect. Thestandard spin valve shows about 5 6 % magnetoresistance. Therefore, the sequential introduction ofthe magneto resistance and spin-valve head, byproviding a sensitive and scalable read technique,contributed to increase the raw HDD areal recordingdensity by three orders of magnitude around 10 years.GMR has motivated people to develop solid statemagnetic storage. The free layer magnetization of thespin valve is constrained to take only the twoopposite orientation of an easy magnetization axis,arrays of patterned spin-valve elements can be usedto store binary information with resistive readout. Byreplacing the non-magnetic metallic spacer layer of297
International Journal of Advanced Computer Research (ISSN (print): 2249-7277 ISSN (online): 2277-7970)Volume-3 Number-1 Issue-8 March-2013the pin valve by a thin non-magnetic insulating layer,so creating a magnetic tunnel junction (MTJ).4. Advantages of SpintronicsThe various other advantages of Spintronics are asfollows:1) Common metals such as iron, copper, silverand aluminium can be used forimplementation of Spintronics. No specialsemiconductor is needed.2) To change the spin of an electron, lessenergy is needed. Therefore, Spintronics ismore power efficient.3) Spins don‘t change when power is cut off.This explains non-volatility of the memory.A. Spin Based Quantum ComputersOne of the most ambitious spintronics devices is thespin-based quantum Computer (QC) in solid-statestructures. Using electron (or nuclear) spin for QCpurposes is a manifestly obvious idea since a fermionwith spin 1/2 is a natural and intrinsic qubit [9].Quantum computation requires both long quantumcoherence time and precise external control. Becauseof the requirement of very long coherence time for aQC, both nuclear spin and electron spin have beenproposed as qubit in a QC. Quantum computers leadthe race by their totally different and innovativealgorithm harnessing the power of quantum effect inthe nature. The following are the fundamental factorsthat throw a light on Quantum Computers:Figure 4: GMR Reading and Writing HeadIn this structure, the electrons travel from oneferromagnetic layer to the other by a tunnel effect,which conserves the spin. Since the discovery of TMJin 1994, a research of developing magnetic randomaccess memories (MRAM) has started. The principleof MRAM is shown in Fig. 7. The binary information0 and 1 is recorded on the two opposite orientationsof the magnetization of the free layer along its easymagnetization axis. From the two cross points of theperpendicular arrays of the parallel conducting lines,MTJs are connected. Current pulses are sent throughone line of each array for writing operation, and thegenerated magnetic field, at the crossing point ofthese lines, is high enough to orient the magnetizationof the free layer. The addressed cell is measured formeasuring the resistance between the connectinglines.1. Effectively More StorageAccording to Quantum mechanics, an atom whichhas two distinct electronic states can also be preparedin coherence with the two states, as in, superpositionof the two states, only if an atom is considered as aphysical bit. Now if we can push the idea ofsuperposition of numbers a bit further, then we caneasily understand that as the number of quantum bitsincreases, power of storage grows exponentially. Anyclassical register of 3 bit can store in a given momentof time only one out of eight different numbers whilea quantum register composed of three qubits canstore in a given moment all eight numbers in a singlequantum superposition. If we keep adding qubits tothe register, we increase its storage capacityexponentially i.e. three qubits has the capability tostore 8 different numbers at once, four qubits has itfor 16 different numbers at once, and so on; thereforewe can state that L qubits has the capability to store2L numbers at once. And thus storage power keeps onincreasing massively, with each successive additionof atom [10].Figure 5: MRAM Reading and Writing.298
International Journal of Advanced Computer Research (ISSN (print): 2249-7277 ISSN (online): 2277-7970)Volume-3 Number-1 Issue-8 March-2013opens fascinating new fields and is also verypromising of multiple applications. Anotherperspective, out of the scope, should be theexploitation of the truly quantum-mechanical natureof spin and the long spin coherence time in confinedgeometry for quantum computing in an even morerevolutionary application. ―Spin much like mass andcharge is an intrinsic property of electron which canbe exploited in future.2. Extremely Faster ProcessingOnce a quantum register is prepared in asuperposition of different numbers we can at onceperform operations on all of them. For example, ifqubits are atoms then suitably tuned laser pulsesaffect atomic electronic states and evolve initialsuperposition of encoded numbers into differentsuperposition. We generate monumental amount ofparallel computation even though there is one pieceof quantum hardware because during such evolutions,each superposition is affected. This means that aquantum computer in only one computational stepcan perform the same mathematical operation on 2Ldifferent input numbers encoded in coherentsuperposition of L qubits. In other words a quantumcomputer offers an enormous gain in the use ofcomputational resources such as time and memory.Quantum computers however, will be able toexamine data using spins, which has can have manydifferent states. Next generation Quantumcomputers‖ will be able to process information muchfaster than the conventional microchip machines andthe capacity can be increased by factor of manythousands. An inherent advantage of spintronics overelectronics- the fact that magnets tend to stymagnetized – is sparking industry in replacingcomputer‘s semiconductor based components withmagnetic ones, starting with the RAM. Spintronicsshould take an important place in the science andtechnology of our century.3. Highly Efficient AlgorithmsQuantum algorithms have the potential to bedramatically faster than their conventionalcounterparts. A good example is an algorithm forsearching through lists. The problem is to find aperson's name in a telephone directory, given his orher phone number. If the directory contains N entries,then on average, you would have to search throughN/2 entries before you find it. Grover's quantumalgorithm does much better. It finds the name aftersearching through only N entries, on average. Sofor a directory of 10,000 names, the task wouldrequire (10,000) 100 steps, rather than 5000.[11]The algorithm works by first creating a superpositionof all 10,000 entries in which each entry has the samelikelihood of appearing in response to a measurementmade on the system. Then, to increase the probabilityof a measurement producing the required entry, thesuperposition is subjected to a series of quantumoperations that recognize the required entry andincrease its chances of appearing.AcknowledgmentWe take immense pleasure in thanking Dr. L. N.Singh and Prof. Amit Naik to permit us to carry outthis review letter. We would like to greatlyacknowledge the enthusiastic supervision of themand thank them for the general advice. We are alsoindebted to the entire teaching and non- teaching staffand laboratory assistants of the University for theirvaluable Co-operation.References[1] S. Das Sharma. ―Spintronics,‖ AmericanScientist, Vol. 89,pp518, Nov.-Dec. 2001.[2] Phys. Rev. Lett. 61 (1988): M. N. Baibich, J. M.Broto, A. Fert, F. Nguyen Van Dau, F. Petroff, P.Eitenne, G. Creuzet, A. Friederich, and J.Chazelas - Giant Magneto resistance.[3] Beiser. Concepts of Modern Physics. SanFrancisco, CA: McGraw-Hill, 2003, pp.229-231.[4] S. Datta and B. Das (1990). "Electronic analog ofthe electro optic modulator". Applied D.[5] D. Awschalom, M. Flatté, N. Samarth.―Spintronics,‖ Scientific American, Vol. 286, no.6, pp. 66-73. June 2002.[6] Ziese, M. & Th ornton, M. J. (eds) SpinElectronics (Lecture Notes in Physics series, Vol.569, Springer-Verlag, Heidelberg, 2001).5. ConclusionIn less than 20 years, we have seen spintronicsincreasing considerably the capacity of our harddisks, extending the hard disk technology to mobileappliances like cameras or portable multimediaplayers, entering the automotive industry andbiomedical technology and, with TMR and spintransfer, getting ready to enter the RAM of ourcomputers or the microwave emitters of our cellphones. The research of today on the spin transferphenomena, on multi ferric materials, on spintronicswith semiconductors, and molecular spintronics,299
International Journal of Advanced Computer Research (ISSN (print): 2249-7277 ISSN (online): 2277-7970)Volume-3 Number-1 Issue-8 March-2013[7] M. N. Baibich , J. M. Broto, A. Fert, F. NguyenVan Dau, F. Petroff, P. Eitenne, G. Creuzet, A.Friederich, and J. Chazelas (1988). "GiantMagnetoresistance of (001)Fe/(001)Cr MagneticSuper lattices". Physical Review Letters 61 (21):2472–2475. doi:10.1103/PhysRevLett.61.2472.[8] Claude Chapter, Albert Fert, and Federic NguyenVan Dau. The emergence of spin electronics inData storage. Nat mater, 6(11):813-823,112007/11/print.[9] Claude Chapter, Albert Fert, and Federic NguyenVan Dau. The emergence of spin electronics inData storage. Nat mater, 6(11):813-823,112007/11/print.[10] Hu, X., and S. Das Sarma. 2000. Hilbert spacestructure of a solid-state quantum computer.Physical Review A 61:062301.[11] Bennett, C. H., and D. P. DiVincenzo. 2000.Quantum information and computation. Nature(404):267.U.G. Student at Dr.Ambedkar TechnologicalRaigad (M.S
Spintronics: A New Nanoelectronics Adventure Rohit Apurva 1 , 3Sonia Chandan 2 , Abhilash Katkar , Prashant Shinde 4 Dept. of E&TC Engineering, Dr. Babasaheb Ambedkar Technological University, Raigad, (MS) 1, 2, 3, 4
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