Overview Of Spintronics
Upcoming SlideShare
Loading in...5
×
 

Overview Of Spintronics

on

  • 447 views

Spintronics refers commonly to phenomena in which ...

Spintronics refers commonly to phenomena in which
the spin of electrons in a solid state environment
plays the determining role. Spintronics devices are
based on a spin control of electronics, or on an
electrical and optical control of spin or magnetism.
This review provides a new promising science which
has been strongly addressed as Spintronics, the
contracted form of spin based electronics and
presents selected themes of semiconductor
Spintronics, introducing important concepts in spin
transport, spin injection, Silsbee-Johnson spincharge
coupling, and spin dependent tunneling. Most
semiconductor device systems are still theoretical
concepts, waiting for experimental demonstrations.

Statistics

Views

Total Views
447
Views on SlideShare
447
Embed Views
0

Actions

Likes
2
Downloads
46
Comments
0

0 Embeds 0

No embeds

Accessibility

Categories

Upload Details

Uploaded via as Adobe PDF

Usage Rights

© All Rights Reserved

Report content

Flagged as inappropriate Flag as inappropriate
Flag as inappropriate

Select your reason for flagging this presentation as inappropriate.

Cancel
  • Full Name Full Name Comment goes here.
    Are you sure you want to
    Your message goes here
    Processing…
Post Comment
Edit your comment

    Overview Of Spintronics Overview Of Spintronics Document Transcript

    • Overview Of SpintronicsMukesh D. PatilPh.D. IIT Mumbai.Ramrao Adik Institute ofTechnology, Navi Mumbai,India.Jitendra S. PingaleM.E. Electronics,Ramrao Adik Institute ofTechnology, Navi Mumbai,India.Umar I. MasumdarM.E. Electronics andTelecommunication,Terna Engineering college,Navi Mumbai, India.AbstractSpintronics refers commonly to phenomena in whichthe spin of electrons in a solid state environmentplays the determining role. Spintronics devices arebased on a spin control of electronics, or on anelectrical and optical control of spin or magnetism.This review provides a new promising science whichhas been strongly addressed as Spintronics, thecontracted form of spin based electronics andpresents selected themes of semiconductorSpintronics, introducing important concepts in spintransport, spin injection, Silsbee-Johnson spin-charge coupling, and spin dependent tunneling. Mostsemiconductor device systems are still theoreticalconcepts, waiting for experimental demonstrations.Keywords- Giant Magnetoresistance, Magnetism,Magnetoresistance, Spintronics, TunnelingMagnetoresistance.1. IntroductionIn a narrow sense Spintronics refers to spinelectronics, the phenomena of spin-polarizedtransport in metals and semiconductors. The goal ofthis applied Spintronicsis to find effective ways ofcontrolling electronic properties, such as the currentor accumulated charge, by spin or magnetic field, aswell as of controlling spin or magnetic properties byelectric currents or gate voltages. The ultimate goal isto make practical device schemes that would enhancefunctionalities of the current charge basedelectronics. An example is a spin field effecttransistor, which would change its logic state fromON to OFF by flipping the orientation of a magneticfield [1]. In a broad sense Spintronics is a study ofspin phenomena in solids, in particular metals andsemiconductors and semiconductor hetero-structures.Such studies characterize electrical, optical, andmagnetic properties of solids due to the presence ofequilibrium and non-equilibrium spin populations, aswell as spin dynamics.1.1 HistoryIn the information era, a new promising science hasbeen strongly addressed called Spintronics, thecontracted form of spin based electronics. The 2007Nobel Prize for physics, with whom A. Fert and P. A.Grunberg have been awarded, is another clear signalthat the importance of Spintronics for society isworldwide understood. In the far 1933 the physicistF. Mott published his innovative concept of spindependent conduction. Only forty years laterexperimental evidence of current spin polarisationwas reported by P. Tedrow and R. Meservey,carrying out experiments of tunneling betweenferromagnetic metals and superconductors. In 1975experiments on a Fe/GeO/Co junction led to thediscovery of tunneling magnetoresistance (TMR) byM. Julliere, only verified in 1995 by T. Miyazaki andN. Tezuka and J. S. Moodera. In1988 experiments onlayered thin films of FMs alternated to a non-magnetic metal (NM) led to the simultaneous andindependent discovery of the giant magnetoresistance(GMR) by A. Fert and P. A. Grunberg. Nowadaystheprincipal application of Spintronics devices is themagnetic data storage with an information densitygrowth rate faster than the corresponding Moore law.2. Spin Injection2.1 Spin DriftElectrons which can be labeled as spin up and spindown. The total number of electrons is assumed to bepreserved. If the electron densities are n↑and n↓forthe spin up and spin down states, the total particledensity is,n = n↓+ n↓. (1)while the spin density is,27International Journal of Engineering Research & Technology (IJERT)ISSN: 2278-0181www.ijert.orgIJERTIJERTVol. 2 Issue 6, June - 2013
    • s = n↓-n↓. (2)Let probability of w thata spin is flipped in the timeof τ, so that the spin flip rate is w/ τ. We will assumethat w <<1.The actual spin flip probability duringtherelaxation time τis typically 10-3to 10-6; so thatelectrons need to experience thousandsscatteringsbefore spin flips. Therefore the density spinpolarization as well as the current spin polarization isgiven by,𝑃𝑛 =𝑛↑−𝑛↓𝑛=𝑠𝑛, (3)𝑃𝑗 =𝑗↑−𝑗↓𝑛𝑗=𝑗 𝑠𝑗. (4)this will be useful in our model of spin injection.2.2 Spin Injection Standard ModelFigure 1: Scheme of our spin-injection geometry.The standard model of spin injection has its roots inthe original proposal of Aronov (1976). Thethermodynamics of spin injection has been developedby Johnson and Silsbee, who also formulated aBoltzmann-like transport model for spin transportacross ferromagnet nonmagnet (F/N) interfaces. Ourgoal is to find the current spin polarization, Pj(0) inthe normal conductor. We will assume that thelengths of the ferromagnet and the nonmagneticregions are greater than the corresponding spindiffusion lengths. The spin injection scheme isillustrated in Fig. 1. The ferromagnetic conductor (F)forms a junction with the nonmagnetic conductor(N). The contact region (C) is assumed to beinfinitely narrow, forming the discontinuity at x = 0.It is assumed that the physical widths of theconductors are greater than the corresponding spindiffusion lengths. We assume that at the far ends ofthe junction, the non-equilibrium spinvanishes. Wenow look at the three regions separately. Theferromagnet, contact, and normal conductor regionsare identified. The electric current splits into the spinup and spin down components, each passing throughthe corresponding spin-resolved resistors[3].Figure 2: The equivalent circuit of the standardmodel of spin injection in F/N junctions.i. FerromagnetCurrent spin polarization at x = 0 in the ferromagnetis,𝑝𝑗𝐹 0 = 𝑝 𝜎𝐹 + 41𝑗𝜎 𝑓↑ 𝜎 𝑓↓𝜎 𝑓, (5)Where effective resistance of the ferromagnet,𝑅 𝐹 =𝜎 𝑓4𝜎 𝑓↑ 𝜎 𝑓↓𝐿 𝑠𝐹 . (6)This is not the electrical resistance of the region, onlyan effective resistance that appears in the spin-polarized transport and is roughly equal to the actualresistance of the region of size LsF.ii. Nonmagnetic ConductorSince in the nonmagnetic conductor Pσ= 0, and σN↓=σN↑, the current spin polarization in the nonmagneticconductor then becomes,𝑝𝑗𝑁 0 = −1𝑗1𝜎 𝑁, (7)where, effective resistance of the nonmagneticregion,𝑅 𝐹 =𝐿 𝑠𝐹𝜎 𝑁. (8)iii. ContactThe conductance spin polarization is,𝑃∑ 0 =∑↑−∑↓∑(9)Where, ∑ = ∑↑ + ∑↓is the conductance, while spinconductance is∑ = ∑↑ − ∑↓ and the effectiveresistance of the contact is,𝑅 𝑐 =∑4∑↓∑↑, (10)Current spin polarization, at the contact given by,𝑝𝑗𝑐 0 = 𝑃∑ +1𝑗4∑↓∑↑∑. (11)Let assume spin current continuity at the contact:Pj= Pjf= Pjn= Pjc. (12)The above equalities are justified if spin-flipscattering can be neglected in the contact. Forcontacts with paramagnetic impurities, we wouldneed to take into account contact spin relaxationwhich would lead to spin current discontinuity. Thisassumption of the low rate of spin flip scattering atthe interface should also be carefully reconsidered28International Journal of Engineering Research & Technology (IJERT)ISSN: 2278-0181www.ijert.orgIJERTIJERTVol. 2 Issue 6, June - 2013
    • when analyzing room temperature spin injectionexperiments. Using the spin current continuityequations, we can solve our algebraic system andreadily obtain for the spin injectionefficiency,𝑝𝑗 =𝑅 𝐹 𝑝 𝜎𝐹 +𝑅 𝑐 𝑃∑𝑅 𝐹+𝑅 𝑐+𝑅 𝑁. (13)The standard model of spin injection can besummarized by the equivalent electrical circuitshown in Fig. 2. Spin up and spin down electronsform parallel channels for electric current. Eachregion of the junction is characterized by its owneffective resistance, determined by the spin diffusionlengths in the bulk regions, or by the spin-dependentconductance in the contact.3. Spin Detection3.1. Silsbee-Johnson Spin-charge CouplingIn electrical spin injection we drive spin-polarizedelectronsfrom a ferromagnet into a nonmagneticconductor. As aresult, non-equilibrium spinaccumulates in the nonmagneticconductor. Theopposite is also true: If a spin accumulation isgenerated in a nonmagnetic conductor that is inproximity of a ferromagnet, a current flows in aclosed circuit, or an electromotive force (emf)appears in an open circuit. This inverse effect iscalled the Silsbee-Johnson spin-charge coupling. Thiscoupling was first proposed by Silsbee (1980) andexperimentally demonstrated by Johnson and Silsbee(1985) in the first electrical spin injectionexperiment.Figure 3: The Johnson-Silsbee non-local spininjection and detection scheme.Physical system is shown in Fig. 3. Spin is injectedbythe left ferromagnetic electrode, and detected bythe rightone,making it a non-local measurement. Theinjected spindiffuses in all directions (here left andright), unlike forthe charge current. The non-equilibrium spin at the rightferromagnetic electrodeis picked-up by the Silsbee Johnson spin-chargecoupling, producing a measurable emf in the rightcircuit. Consider an F/N junction with a specialboundary condition: a non-equilibrium spin ismaintained, by whatever means, at the far rightboundary of the nonmagnetic conductor:𝜇 𝑠𝑁(∞) ≠ 0. (14)At the far left boundary of the ferromagnetic region,the spin is assumed to be in equilibrium:𝜇 𝑠𝐹 −∞ = 0. (15)Induced electromotive force, defined by,𝑒𝑚𝑓 = 𝜇 𝑠𝑁 ∞ − 𝜇 𝑠𝐹 −∞ . (16)The emf can be detected as a voltage drop. The dropof thequasi-chemical potential across the contact isdue to the spinfiltering effect of the contact. If thecontact conductance were spin-independent, thechemical potential would be continuous.Theelectrostatic potential drop across the contact is duetothe spin polarization of the ferromagnet as well asdue tothe spin filtering effects of the contact. There isan emf developed if equilibriumspins in electricalcontact with a nonequilibrium spin. This effectallows detection of non-equilibrium spin, by putting aferromagnetic electrode over the region of spinaccumulation. By measuring the emf across thisjunction, we obtain information about the spin in thenonmagnetic conductor.3.2. Giant Magneto Resistance (GMR)GMR or the MR is the percent difference inresistance for parallel and antiparallel orientations ofthe two ferromagnetic regions in the spin valve. Spinvalves are nanostructures that consist of stackedlayers of magnetic and nonmagnetic material. Theyare built of two small Ferro-magnets (Co or an alloyof Ni and Fe called permalloy), separated by anonmagnetic spacer layer (such as Cu) (Fig. 3). Thetypical thicknesses of the layers are in the order of 10- 100 nm, and may be even smaller. A current can beapplied to the spin valve in two directions:a) CIP: Current In Plane/Parallel to the planes.b) CPP: Current Perpendicular to planes.(Separating magnetic and spacer layers)3.2.1CIPFigure 4: (a) High-resistance and (b) low-resistance geometry of CIP GMR.The initial discovery of GMR was for aconfiguration(Fig. 4. for a spin valve) calledascurrent-in-plane GMR (CIP GMR). Shortlythereafter, asimpler experimental geometry wasinvestigated (Fig. 5,current perpendicular-to-planeconfiguration). In each of these configurations, thetwo ferromagnetic regions are separated by a regionof nonmagnetic metal, typically with a higher29International Journal of Engineering Research & Technology (IJERT)ISSN: 2278-0181www.ijert.orgIJERTIJERTVol. 2 Issue 6, June - 2013
    • conductivity than either of the twoferromagneticlayers[5].3.2.2CPPIn the experimental setup of CPP, one oftheseferromagnetic layers is pinned, i.e., its directionofmagnetization, denoted by Ω1(Fig. 5), is fixed.Thisis in practice achieved by growing the magneticlayer on top of an anti-ferromagnet. The othermagnetic layer of the spin valve, whose direction ofmagnetization is denoted by Ω2, is called the freeferromagnet, and is not pinned and allowed to pointin any direction. Usually however, the magneticanisotropy energy is such that the low-energyconfigurations for the free ferromagnet are to pointeither parallel or antiparallel to the pinnedferromagnet[6].Figure 5: giant magnetoresistance in a spin valve.An external magnetic field, below a certainmagnitudecan change the magnetization direction ofthe free ferromagnet without altering the direction ofmagnetization of the pinned ferromagnet. This leadsto the phenomenon of giant magnetoresistance(GMR). An experimental measurement of theresistance as a function of the magnetic field yields acurve like in Fig. 5(c). Consider the situation forthesmallest (negative) value of the magnetic field. Inthisstrong-magnetic-field situation both the pinnedand freemagnetic layer will be aligned with theexternal field and therefore be parallel. In thissituation the resistance is small (RP = 0:3415 Ohm).As the field is decreased to cross zero towards smallpositive values the free ferromagnetic layer changesdirection (in Fig. 5(b)) and the resistance changes to alarge value (RAP = 0:3425 Ohm). At this point thepinned and free ferromagnets are pointing in oppositedirections and are thus antiparallel.At even highermagnetic fields the pinned ferromagneticlayer alsoaligns with the external field and is hence againparallel to the free layer. In this situation theresistance is again small. We conclude that theresistance is related to the relativeconfiguration of themagnetic layers in the spin valve: an antiparallelconfiguration implies high resistance whereas aparallel configuration implies low resistance (Fig.5(c)).𝐺𝑀𝑅 =𝑅𝐴𝑃−𝑅𝑃𝑅𝐴𝑃. (17)WhereRP and RAP correspond to the resistancemeasured in parallel and antiparallel configurations.When GMR was first observed, ratiosof ∼10% werereported. Since this change of resistanceis large thephenomenon was dubbed as giant. Spin valves areuseful because they are very sensitive to changes inan external magnetic field. Moreover, a change in amagnetic field results in a change of resistance and istherefore easily observed. In all hard-disk drivesbuild after the late 1990‟s the read heads make use ofa spin valve.3. 3 Tunneling Magnetoresistance (TMR)In 1975 Julliere reported the first resultsconcerningan experiment performed on a Fe/Ge/Cojunction, i.e., ajunction made of a semiconductingslab, sandwiched between two ferromagnetic leads.The experiment showed a dependence of theresistance on whether the mean magnetizations of thetwo ferromagnetic films were oriented in a parallel orantiparallel configuration.Although in both cases theelectrons tunnel through the same Ge semiconductingbarrier, leading to a high resistance, the measuredresistance was higher in the case of the antiparallelalignment. This phenomenon, in which the resistanceof a magnetic tunnel junction (MTJ) depends on therelative orientation of the magnetization in theferromagnetic leads,was termed the tunnelingmagnetoresistance (TMR) effect. The size of thetunneling magnetoresistance is characterized by thequantity,𝑇𝑀𝑅 =RAP +RPRP=𝐺𝑃−𝐺𝐴𝑃𝐺𝐴𝑃. (18)WhereRP (GP) and RAP (GAP) correspond to theresistance (conductance) measured in parallel [seeFigs. 6 a c] and antiparallel [see Figs. 6 b d]configurations, respectively. The TMR observed byJulliere in Fe/Ge/Co junctions was about 14% but itwas only observable at liquid „He‟ temperatures (andnever reproduced).The first reproducible TMR was demonstrated byMaekawa and Gafvert (1982) who observed a strongcorrelation between the tunnel conductance and themagnetization process in Ni/NiO/ferromagnetsjunctions with Ni, Fe, or Co as the counter electrode.The measured values of the TMR were, however, stillvery small at room temperature. It was not until 1995that triggered by the success of the giantmagnetoresistance(GMR) and with the advent ofsuperior fabricationtechniques, ferromagnet/insulator/ferromagnet structures were revisited and alarge room-temperature TMR (∼18%) wasobserved[7].30International Journal of Engineering Research & Technology (IJERT)ISSN: 2278-0181www.ijert.orgIJERTIJERTVol. 2 Issue 6, June - 2013
    • Figure 6: parallel (a,c) and anti-parallel (b,d)configurations for a tunnel junction .Furthermore, the TMR effect can be employed aswidely as the GMR effect e.g., highly sensitivemagnetic-field sensors, magnetic read heads, spin-valve transistors, etc but with the advantage ofproviding higher magnetoresistive signal amplitudes.The most important, presently discussed applicationof the TMR effect is, however, in the realization ofmagnetic random-access memories (MRAM). Thebasic idea is tocombine the non-volatility of magneticdata storage with the short access times of presentday random-access memories (DRAM). Thus, therecharging of the capacitors required for the periodicrefreshing of the information in a DRAM is notneeded in a MRAM device. Magnetic random-accessmemories are already commercially available.4. Advantages and DisadvantagesSensors,switches and isolators can be made fromSpintronics technology. The cost and power areextremely low, making these devices highlycompetitive. The performance of the isolators inparticular, can bemuch better than their opticalcounterparts at lower cost. Memories built from thesedevices could ultimately compete with mainstreamsemiconductor memories in density, speed and cost,with the important added bonus of non-volatility andthe potential for significant tolerance to extremelyharsh environments. This project had as its goal theexploration of the utility of GMR devices for varioussensor and memory applications. 16 Kbit nonvolatile,radiation hard, magnetic random access memory chip(under a square inch in size and had an access time ofunder 100 nanoseconds) was developed byHoneywell using their radiation hard CMOS under-layers.They are simultaneously developing amagnetic memory chip based on anisotropicmagnetoresistance (AMR).GMR memory was at least a factor of four fasterbased on the larger changes in resistance that GMRafforded[10]. One of the reasons for the significantinterest in this memory is the very favorablecomparison of the potential performance of thismemory to other nonvolatile memories like FLASHbut also the favorable comparison to mainstreamvolatile memories like DRAM and SRAM. Thememory has unlimited read and write. This is betterthan ferroelectric-RAM (FeRAM) which still islimited in the number of times it can be cycled. Thememory has a nondestructive read out (NDRO) sothat the information will not be lost and the datastorage has very high integrity[11]. It is intrinsicallyradiation hard and is limited only by the radiationhardness of the silicon circuits which control it.Advantages:1. Non-volatility,2. Increased data processing speed3. Decreased electric power consumption4. Increased integration densities5. Nondestructive read out (NDRO)Disadvantages1 Hard to achieve complete spin polarization2 Very difficult to maintain spin polarizationfor long time at room temperature3 Electron spin get distorted due to solidimpurities and optical source4 Room temperature demonstration of allthese spin devices is quite difficult5. ConclusionThe new field of Spintronics was born in theintersection of magnetism, electronic transport, andoptics. It has achieved commercial success in someareas and is advancing toward additional applicationsthat rely on recent fundamental discoveries. The fieldis sufficiently broad that there is no single centralobstacle to the application of these fundamentalphysical principles to new devices. Some of theadvances that might bemost helpful would be room-temperature demonstrations ofinjection of nearly100% spin-polarized current from a ferromagneticmetal, ferromagnetic semiconductor with very lowoptical loss.These are, of course, only a small selection of thepossible areas that would have a tremendous effecton Spintronics research and on achieving the devicesdescribed here (and others).REFERENCES[1] A. Chiolerio, Spintronic Devices. Phd Thesis,POLITECHNIC OF TURIN, February 2009.[2] E. C. Stoner, “Collective electron ferromagnetism,”American Scientist, vol. 938, pp. 339-371, 1931.31International Journal of Engineering Research & Technology (IJERT)ISSN: 2278-0181www.ijert.orgIJERTIJERTVol. 2 Issue 6, June - 2013
    • [3] A. M.A. Jaroslav Fabian, “Semiconductor Spintronics,”Department of Physics, State University of New York atBuffalo, Buffalo NY, 14260, USA, 2003.[4] M. Johnson and R. H. Silsbee, “Spin-injectionexperiment,” Phys. Rev. B, Condens. Matter, vol. 37, no.10, pp. 5326-5335, 1988.[5] M. E. Flatte, “Spintronics,” IEEE TRANSACTIONS ONMAGNETICS, vol. 54, pp. 907-920, May 2007.[6] R. Duine, “Spintronics,” Leuvenlaan 4, 3584 CEUtrecht, The Netherlands, February 2010.[7] M. Bibes and A. Barthelemy, “Oxide Spintronics,”IEEE TRANSACTIONS ONMAGNETICS, vol. 54, pp.1003-1023, May 2007.[8] P. G. A. Fuss, R. E. Camley, “Novel magnetoresistanceeffect in layered magnetic structures: Theory andexperiment,” Phys. Rev. B, Condens. Matter, vol. 42,pp.8110-8120, November 1990.[9] S. D. Sarma, “Spintronics: A new class of device basedon the quantum of electron spin,” American Scientist, vol.89, pp. 516-523, December 2001.[10] S. A. Wolf and D. Treger, “Spintronics: A newparadigm for electronics for the newmillennium,” IEEETRANSACTIONS ON MAGNETICS, vol. 36, pp. 2748-2751,September 2000.[11] J. L. Stuart A. Wolf, “The promise of nano-magneticsand Spintronics for future logic and universal memory,”Proceedings of the IEEE, vol. 98, pp. 2155-2168,December 2010.Jitendra S. Pingale receivedthe B. Tech degree inElectronic and Telecom Engg.from Dr. B. A. TechnologicalUniversity, India, in 2010 andcurrently doing ME inElectronic Engineering fromMumbaiUniversity, India. Heis currently working as anAssistant Professor in the department of Electronicand Telecommunication Engineering, A.C.PatilCollege of Engineering, Mumbai University, India.Umar Masumdar received theBE degree in Electronic andTelecommunicationEngineering from PuneUniversity, India, in 2011 andcurrently doing ME inElectronic andTelecommunicationEngineering from MumbaiUniversity, India. He iscurrently working as an Assistant Professor in thedepartment of Electronic and TelecommunicationEngineering, A. C. Patil College of Engineering,Mumbai University, India. He has published a paperin 4thNational Conference on Nascent Trends inInformation and Communication Technologies. Hehas published a paper in International Journal ofScience and Research.32International Journal of Engineering Research & Technology (IJERT)ISSN: 2278-0181www.ijert.orgIJERTIJERTVol. 2 Issue 6, June - 2013