Spin Nd Nano

8/6/2019 Spin Nd Nano

1/5

Comparison of Spintronics and Nanoelectronics forInformation Processing

Kang L. Wang, Igor V. Ovchinnikov, Alex Khitun and Ming BaoDevice Research Laboratory (DRL)University of California, Los AngelesLos Angeles, CA, 90095-1594, [email protected]

AbstractTo date, electronics uses electron charge as a statevariable which is often represented as voltage or current.In this representation of state variable in today'selectronics, carriers in electronics devices workindependently even to a few and single electron cases.As the scaling continues to reduce the feature size,

power dissipation and variability become two majorchallenges among others as identified in ITRS. Thispaper presents the exposition that spintronics as acollective effect may be favorably used as state variablesin the near future information processing beyondconventional electronics for room temperature. Anexample is presented to compare electronics andspintronics in terms of variability, quantum and thermalfluctuations. This example shows the benefit of scalingto smaller sizes in the case of spintronics(nanomagnetics), which will have a much reducedvariability problem as compared with today's electronics.Finally, spin wave bus is used to illustrate the potentialuse as a state variable for logic application. Prototypelogic devices using the spin wave bus concept have beendemonstrated. The requirements and benchmarks forchoosing a state variable are also discussed in terms ofits interaction strength for the energy efficiency.

1. Spintronics for Information ProcessingThere has been increasing interest in consideringspintronics as an alternate to today scaled CMOS fornext generations of information processing beyondtoday's electronics. In the past, scaled CMOS, electroncharge and its representations as e.g., voltage and currentare used as a state variable for device to perform logicfunctions. Upon further scaling to the nanometer scale,

the use of electron charge and its long-range growingstrong Coulomb interactions resulted in two majorproblems: power dissipation per unit area and variabilityof the device. The first comes from the strong Coulombinteraction. It is shown that the minimal energy of aswitch is kT r; for n electrons, the energy requiredwill be nkT r if electrons work independently,

where n is the number of electronics and r is thereliability factor, or reciprocal of the probability offailure. The variability comes from the result ofindependent electrons in the device; the scaling to smallfeature sizes makes the quantum fluctuation of electronsimportant. However, in spintronics, there are twoscenarios: one is single spin case and the other thecollection of interaction spins (with exchange interaction,which is the consequence of many-electron effect ofCoulomb interaction). For the former, it will be similarto that of electronics. In this paper, we will focus onthe case of correlated electron systems, e.g.,ferromagnetism. We will show that there are advantagesin energy and in variability when using correlatedelectrons, in particular, in the case that the correlatedenergy is higher than kT for room temperatureapplications.It may be this collective variable, which may becomethe Information carriers or the representation of theInformation for beyond independent electron devices(CMOS) in the near future. From the point of view ofthe concept of Information, the physical representationthe collective variable is what it matters. From theinformation processing point of view, however, aminimum set is needed for the unique identification ofthe information processing device, and this minimum setincludes: the state variable, outer interaction and inner

interaction. The Outer interaction is the interaction, withwhich different units of the computational device interactamong each other and with which the computationaldevice is connected with the outside world includinginput and output. The Inner interaction is the interaction,which is responsible for the inner physics of the statevariable itself. There are three technological limits(connected to each entity to the three elements), whichput constraints for us in making a choice among differentinformation processing bases.The first limit is in the sensitivity of our devices(outer Interaction). We, humans, live and think on amacroscopic level. Even if we gethold of asub-nuclei-size computer, somehow, which worksperfectly from information processing point ofview, we will never know what this computer

978-1-4244-2186-2/08/$25.00 2008 IEEE


2/5

calculates until we perform a read function; norwill we be able to ask this computer to calculatesomething for us unless we provide input. Thus,there is a problem of bringing the microscopicallyrepresented information to the interface that we cancomprehend and thus control. The stronger theouter interaction of the State Variable with ourevery-day world, the easier it is for us to controlthe information processing and read-out. The firstinformation processing device was mechanical one.(For those mechanical devices one had to spin alever to multiply numbers.) The lever of themechanical multiplying-numbers machine was oneof the easiest use perform control or computing.With the advent of semiconductor industry morethan a half a century ago, the informationprocessing changed the physical basis from themechanical one to the semiconductor electronics

(ii) The second limit is the heat dissipation orpower dissipation (in Inner and outer Interactions).The inner Interaction, unlike Outer Interaction, isthe Interaction responsible for the physical state ofan (information) State Variable. also describeshow much energy goes in and out of a StateVariable for controlling the process of computing.The stronger the inner interaction is, the morepower will probably be dissipated into theenvironment during computing. Therefore, theweaker the inner interaction, the better it is fromthe heating problem point of view. The outerinteraction is the interaction of the State Variablewith the input and the output. This will also giverise to power dissipation. Consequently, how wellone can take the heating energy away from aninformation processing device is anothertechnology limit.(iii) The third limit is in the size of the StateVariable that we can manufacture and operate at.We want to store and operate as many operations aspossible, so that the smallest-size of the statevariable is preferred for increasing the informationdensity and information throughout. At the sametime, we have to be crafty enough to assess amongdifferent small-size information units so that thefluctuations will not be the problem for roomtemperature operation. This is anothertechnological issue.

Using these three limitations as the benchmarks, wecan compare the State Variable-Interactions and choosethe candidate for information processing.Today's electronics is ideal for accommodating limits(i) and (iii). The electronic (semiconductor) circuits arewell developed by now and the charge state variable canbe scaled down to nanometers. The form of the Coulombinteraction (Ifr) produces a common bound for the limits

(ii) and (iii). The attempts to satisfy each requirement(controlled by the Inner Interaction and the Size) becomecontroversial. In this case, our objective will be tominimize the canonical numbers as the result of thelimits (ii) and (iii), which is defined as (voltage percharge i.e. capacitance) size. The bound from the limitsof (ii) and (iii) together approaches a commonfundamental limit.Magneto-electronic circuits: The information isrepresented by the collective spin degree of freedom asmagnetic polarization, but the connection betweendifferent units of the computational circuit is realizedjust as in the conventional electronics (MTJ devices).The inner interaction (Exchange) unlike that of Coulombis local. This means that the performance is not boundedby limits (ii) and (iii). Rather, it is bounded by limits (i)and (iii). That is, the sensibility as the result ofinteraction of spintronics devices does not allow thescaling of a collective spin state variable down to asingle spin. The progress in lowering the limit (i) istechnologically behind for spin. Thusmagneto-electronic-circuits use Coulomb interaction asouter interaction can be only a temporary solution for (i).3) Multi-Ferroics: Again, we see the use ofmultiferroi c materials as a temporary solution inresolving limit (i) on the sensibility of spins, by turningto an electric force (via ferroelectric polarization thistime).

- - - r - - - -' - -10 Ie' 10 10' 10' 10' ~ o ' 10'

Fig.l Domains of quantum fluctuations for threequantum numbers: N, S, and Sz. The curves, TN,TS and Tsz separate the areas of quantum (to the leftand below) and thermal (to the right and above)behaviors of the corresponding quantum number. At300 K, the fluctuations in magnetic quantum numberS give the limiting number of 10 atoms vs. theelectronics counterpart of 10 atoms,4) Single Spin: Today's Spintronics using a single

electron spin level is not realizable for room temperatureoperation because of limit (i) for spins as their


3/5

interactions are too weak. Thermal noise or fluctuationseems to overwhelm the interaction (Zeeman) energy forthe single spin case.5) Molecular shuttle- or rotor like device: Limit (i)also does not allow an easy observation and control ofthe state collective variable describing the electronconfiguration in a single molecule. Though the limitation(ii) and (iii) are well satisfied.From the above analysis, we see that there is a highprobability that collective variables will be the new statevariables for new generations of logic devices.Nevertheless, the control of the collective variables andinterconnects between them at present must still beaccomplished by the conventional electronics tools, eventhough it is possible in the future that some moreappropriate methods (without implication of theCoulomb interaction) will be developed.

2. Variability of Electronics and SpintronicsAs we discussed before, in the continuing scaling ofthe feature size of semiconductor devices, there are twomajor issues for further scaling: power dissipation andvariability [1]. In search of low power and variabilitydevices, spintronics [2] has emerged as a potentialcandidate in addition to others. In this section, weaddress the variability issue of nanoscaled electronic andspintronics devices. There are different sources ofvariability due to thermal and quantum fluctuations,resulted from fluctuations of the number of electrons, forexample, and process control such as pattern size anddopant variations in semiconductor FET. We will,however, in this paper look at the fundamental of

quantum fluctuations [3].In order to assess electronic and spin based circuits fortheir variability, we address the problems of thefluctuations in the elemental units - FET and withoutloss of generality Spin-FETe The input-outputfluctuations in a generic transistor can be understoodfrom rather general thermodynamics reasoning. Whenthe retardation (delay) effects are neglected, the outputvariable can be assumed to be in an "on-time"correspondence with the state variable of the gate (thecharge Q or the magnetization M), so that Vout f(Q). Intum, the state variable is controlled by the inputparameter (Vin or Sin), which is effectively its Legendreconjugate in a thermodynamic potential (e.g. the freeenergy F), governing the behavior of the system, whichis thermally open to heat bathes). The effective couplingsin the thermodynamical potential are QVin and SinM,Depending on which variable is fixed, the correspondingpotential must be used. Thus, just as for any Legendreconjugate pair (e.g., pressure-volume, temperatureentropy, chemical potential-number of particles), fixingone of the variable (the input) we leave the other variable

(output) loose and let it fluctuate - due to thermalfluctuations at high temperatures and/or due to thequantum discreteness of the state variable's eigenvaluesat low temperatures. In Fig.1, we summarized the resultsof theoretical analysis showing the comparison of chargeand spin based minimal feature sizes due to fluctuations,and illustrating the feature size on top and the number ofatoms for a given devices using a spherical gate. Theroom temperature line is also shown. Clearly the exactgeometry of the gate electrode will change the capacityand also affect the number. In addition, for realisticdevices, the gate capacitance will also depend on thedielectric layer. But the number obtained should be stillclose to the same order of magnitude. These numberscan be plotted together with the limitation based onenergy consideration discussed above. Differentmaterials can be used and there will be different sizes asa result of using atomic sizes.The fact that the spin degree of freedom is moreadvantageous than the charge degree of freedom fromthe quantum fluctuations point of view has its origin inthe locality of the interaction responsible for themagnetism (exchange interaction). On the energy scalethe energy per "quantum unit" scales as llr forelectronics (Coulomb repulsion), whereas it is sizeindependent for spins (exchange). In the abovediscussion, we only limit ourselves to electron and spinbased devices. For other cases, as in molecules andferroelectric materials, there will, more or less, givesimilar geometric dependences of the scaling rules ofcorrelated effects or many-body (collective) effects innanoscale systems.3. Logic Devices with SpinWave BusFinally, we would like to present as example of usingcollective phenomena- spin wave as a state variable forlogic application. Spin wave is a collective oscillation ofspins in an ordered spin lattice studied for decades in avariety of magnetic materials and nanostructures [6-10].The near-neighbor spins in the lattice are connected viathe exchange interaction. Potentially, it is possible to useferromagnetic films as a conduit for spin wavepropagation, referred to as "Spin Wave Bus" [11], wherethe information can be coded into a phase or theamplitude of the spin wave as a state variable. Spin wave,as a highly correlated system of spins, possesses quite

long coherence length. As it was experimentallydemonstrated in spin wave Mach-Zehnder-typeinterferometer, spin wave signals remain coherent afterpropagation of millimeters distance in yttrium irongarnet film [12]. In the permalloy films [8], thecoherence length is of the order of tens of microns inroom temperature.Earlier, spin-wave based logic circuit has been


4/5

substrate, a C030Fe70 IOOnm thick film, and a 300nmthick silicon dioxide layer. There are three asymmetriccoplanar strip (ACPS) transmission lines on the top ofthe structure. The edge ACPS lines are the transducers toexcite spin waves, and the line in the center is the

AcknowledgmentsThe authors acknowledge the research support ofFCRP Center on Functional Engineered NanoArchitectonics - FENA and Western Institute ofNanoelectronics WIN under the support ofNanoelectronics Research Initiative (NRI).References:[1] G. E. Moore, "Gramming more componentsonto integrated circuits," Electronics, vol. 38, pp.114-117, 1965.

[2] S. A. Wolf, D. D. Awschalom, R. A. Buhrman, J. M.Daughton, S. von Molnar, M. L. Roukes, A. Y.Chtchelkanova, and D. M. Treger, "Spintronics: aspin-based electronics vision for the future," Science, vol.294,pp.1488-95,2001.[3] V. Ovchinnikov and K. L. Wang,

receiver to detect the inductive voltage produced by twospin wave signals. The distance between the microstripsis In Figure 2(b), it is shown the experimental dataon the output inductive voltage measured at the centralACPS line at different values of the external magneticfield (excitation frequency 3GHz). The red and blackcurves depict the output power for the in-phase ( L 1 ~ = O )and the out-of-phase ( L 1 ~ = Jr) cases, respectively. Theinitial phase difference between two input signals isdefined by the direction of the current flow in theexcitation lines. The phase difference is zero if thedirection of current flow is the same (clockwise orcounter-clockwise wise) in both lines. If the directions inthe excitation lines are such as one loop is clockwise andthe other is counter-clockwise, the spin wave signalsreceive a relative phase difference. These experimentaldata show a prominent (about 4 times) output powerdifference. All measurements are carried out at roomtemperature.4. SummaryIn summary, we showed that collective effects may beused as state variables for scaled room temperatureinformation processing. The collective state variablesseem to have the benefi ts of lower variability and highefficiency in energy needed for performing computing.

For room temperature operation, the collective effectstate variables are the choice, in particular, the use ofcollections of spins as nanomagnets. For single chargeelectronics, it was shown that the quantum fluctuationwill dictate and result in the variabili ty l imit due to thelarge Coulomb interaction energy. On the other hand,for single spin, thermal energy is much higher than theenergy of single particle systems at room temperature,i.e., Zeeman splitting, rendering room temperatureoperation unlikely. We also introduced the spin wavebus concept to illustrate a potential of using collectivespins for constructing logic devices and circuits.

=

Q)co+- '(5

0

Fig.2 (a) Genera l view of the three-tenninal spin wave logicdevice. The core of the s truc tu re from the bo ttom to the topconsists of a si licon subst ra te, a 100nm thick CoFe film, and a300nm thick silicon dioxide layer. Three ACPS lines on the topare used as the two input (edge) and one output (in the middle)ports . (b) Experimental data. Output signal amplitude for twospin waves coming in-phase (red curve) and out of phase (blackcurve).

experimentally demonstrated by M. Kostylev et al. [12].The authors built a spin wave Mach-Zehnderinterferometer, which consists of a spin wave generator,a splitter that divides the input spin wave pulses into twoferrite channels made of Yttrium-Iron-Garnet (YIG),two controllable phase shifters attached to the branches,and a mixer where the signals modified by the phaseshifter. The phases of the propagating spin waves arecontrolled by the magnetic fields produced by electriccurrents in the conducting wires under the waveguides.Depending on the phase shift, the device can operate asNOT or XOR logic gate. The same group has alsodemonstrated exclusive-not-OR and not-AND gatesbased on a similar Mach-Zehnder-type spin-waveinterferometer structure [14].Another prototype spin wave logic device has beenrecently realized by our group [13]. In Figure 2 (a), weshow the general view of the device. The core of thestructure from the bottom to the top consists of a silicon


5/5

"Variability of electronics and spintronics nanoscaledevices," Appl ied Physics Letters, vol. 92, pp.093503-1-3, 2008.[4] L. Michalak, C. M. Canali, and V. G. Benza,"Electron-magnon coupling and nonlinear tunneling

transport in magnetic nanoparticles," Physical ReviewLetters, vol. 97, pp. 096804/1-4, 2006.[5] E. Abe, K. M. Itoh, 1. Isoya, and S. Yamasaki,"Electron-spin phase relaxation of phosphorus donors in

nuclear-spin-enriched silicon," Physical Review B, vol.70, Jul 2004.[6] T. 1. Silva, C. S. Lee, T. M. Crawford, and C.

T. Rogers, "Inductive measurement of ultrafastmagnetization dynamics in thin-film Permalloy," JournalofApplied Physics, vol. 85, pp. 7849-62, 1999.

[7] B. A. Kalinikos, N. G. Kovshikov, M. P.Kostylev, P. Kabos, and C. E. Patton, "Observation of theamplification of spin-wave envelope solitons inferromagnetic films by parallel magnetic pumping,"JETP Letters, vol. 66, pp. 371-5,1997.[8] M. Covington, T. M. Crawford, and G. J.Parker, "Time-resolved measurement of propagating spinwaves in ferromagnetic thin films," Physical ReviewLetters, vol. 89, pp. 237202-1-4, 2002.

[9] M. Bailleul, D. Olligs, C. Fermon, and S.Demokritov, "Spin waves propagation and confinementin conducting films at the micrometer scale,"Europhysics Letters, vol. 56, pp. 741-7,2001.[10] M. Bailleul, R. Hollinger, and C. Fermon,"Microwave spectrum of square permalloy dots:quasisaturated state," Physical Review B (CondensedMatter andMaterials Physics), vol. 73, pp. 104424-1-14,2006.[11 ] A. Khitun and K. Wang, "Nano scalecomputational architectures with Spin Wave Bus,"

Superlattices Microstructures, vol. 38, pp. 184-200,2005.[12] M. P. Kostylev, A. A. Serga, T. Schneider, B.Leven, and B. Hillebrands, "Spin-wave logical gates,"AppliedPhysics Letters, vol. 87, pp. 153501-1-3,2005.[13] A. Khitun, B. M., Y. Wu, 1.-Y. Kim, A. Hong,A. Jacob, K. Galatsis, and K. L. Wang, "Logic Deviceswith Spin Wave Buses - an Approach to ScalableMagneto-Electric Circuitry," Procedings of the MaterialResearch Society (in press), vol. spring meeting, 2008,2008.[14] T. Schneider, A. A. Serga, B. Leven, B.Hillebrands, R. L. Stamps, and M. P. Kostylev,"Realization of spin-wave logic gates," Appl. Phys. Lett.,vol. 92, pp. 022505-3, 2008.

Spin Nd Nano

Documents

Transcript of Spin Nd Nano