Spintronics (1).ppt

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spintronics in detail

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  • national laboratory for advanced Tecnologies and nAnoSCienceMaterial and devices for spintronics

    What is spintronics?Ferromagnetic semiconductorsPhysical basisMaterial issuesExamples of spintronic devicesElectric field control of magnetismSpin injectorsSpin valvesTrieste, 20.10.06

  • national laboratory for advanced Tecnologies and nAnoSCienceSpintronics = spin-based electronics

    Information is carried by the electron spin,not (only) by the electron charge.Ferromagnetic metallic alloys- based devices

    silvia

  • national laboratory for advanced Tecnologies and nAnoSCience1988: discovery of GMR (Giant Magnetoresistive effect):

    In alternateFM/nonmagnetic layered system, R is low when the magnetic moments in the FM layers are aligned,R is high when the magnetic moments in the FM layers are antialigned.

    (Baibich et al, PRL61, 2472 (88) Binach et al, PRB39, 4828 (89))

  • national laboratory for advanced Tecnologies and nAnoSCienceGMR based Spin Valves and Magnetic tunnel junctionPrinz, Science 282, 1660 (98)Wolf et al, Science 294, 1488 (01)

    AF layer (A) or AF/FM/Ru/ trilayer (B)to pin the magnetization of the top FM layer

  • national laboratory for advanced Tecnologies and nAnoSCienceGMR based Spin Valves for read head in hard drivesPrinz, Science 282, 1660 (98)Wolf, Science 294, 1488 (01)Standard geometry for GMR based Spin ValvesBut also MRAM

  • national laboratory for advanced Tecnologies and nAnoSCienceSpintronics = spin-based electronicsFerromagnetic metal - based devices

    Semiconductor based spin electronicsCourtesy C.T. Foxon

  • national laboratory for advanced Tecnologies and nAnoSCienceSpintronics = spin-based electronicsFerromagnetic metal - based devices

    Devices for the manipulation of single spin (quantum computing).The idea:Electron spins could be used as qubits.They can be up or down, but also incoherent superpositions of up and down statesSemiconductor based spin electronics devices

  • national laboratory for advanced Tecnologies and nAnoSCienceHow can we measure the magnetic state of a thin epilayer:SQUID measurements but also Anomalous Hall effectR0=1/peOrdinary Hall effect contribution, negligible.RHall is proportional to M.

  • national laboratory for advanced Tecnologies and nAnoSCienceTwo main issues in semiconductor spintronics:

    Avaiability of suitable materialsIdeal material should beEasily integrable with electronic materialsAble to incorporate both n- and p-type dopantsWith a TC above room T

    Understandig and controlling the physical phenomena:Spin injectionTransport of spin polarized carriers across interfacesSpin interactions in solids: role of defects, dimensionality, semiconductor band structure.................

  • Examples: Eu dichalcogenides (EuS, GdS,EuSe) and spinels CdCr2Se4.

    Extensively studied in 60-70.Exchange interaction between electrons in the semiconducting band and localized electrons at the magnetic ions.

    Interesting properties, but Crystal structure quite different from Si and GaAs, difficult to integrateCrystal growth very slow and difficultLow TCnational laboratory for advanced Tecnologies and nAnoSCienceMagnetic semiconductor, constituted bya periodic array of magnetic ions

  • national laboratory for advanced Tecnologies and nAnoSCienceAs one can obtain n- o p-type semiconductors by doping, one can syntetize new magnetic materials by introducing magnetic impurities in non magnetic semiconductors.Alloys of a nonmagnetic semiconductor and magnetic elements:Diluted Magnetic Semiconductors (DMS)

  • national laboratory for advanced Tecnologies and nAnoSCienceII-VI DMS

    ZnSe, CdSe and related alloys + Mn

    Mn (group II) substitute the cation.Isoelectronic incorporation, no solubility limit.

    Easy to prepare both as bulk material and epitaxial layers and etherostructuresButMagnetic interaction dominated by antiferromagnetic direct exchange among Mn spins. In undoped material paramagnetic, antiferromagnetic and spinglass behavior, no FMInteresting: Giant Zeeman splitting !!

  • national laboratory for advanced Tecnologies and nAnoSCienceIII-V DMSGaAs, InAs and their alloy + Mn. Mn substitute the cation and introduce a hole.

    Low solubility of the magnetic element, max 0.1 at % under normal growth condition.

    Non-equilibrium epitaxial growth methods (MBE)to overcome the thermodynamic solubility limit.Standard MBE growth condition not sufficiently far from equilibrium

    Low temperature MBE1992 FM InMnAs1996 FM GaMnAs

  • national laboratory for advanced Tecnologies and nAnoSCienceThe mechanism of FM in Mn based Zincblend DMS

    Antiferromagnetic direct coupling between Mn ions. Dominate in undoped materials.

    Ferromagnetic coupling in p-type materials as a result of exchange interaction between substitutional Mn S=5/2 and hole spins.The exchange interaction follows from hybridization between Mn d orbital and valence band p orbital. Hole mediated FM

    See PRB 72, 165204(05)and reference therein

  • national laboratory for advanced Tecnologies and nAnoSCienceHole mediated FM

    In a mean field virtual crystal approximation

    x = substitutional Mnp = hole density

    In III-V DMS the holes comes from Mn !!!x and p are intimately related

    Room temperature TC is expected for Ga0.9Mn0.1As.

    See PRB 72, 165204(05)and reference therein

  • national laboratory for advanced Tecnologies and nAnoSCienceKnow-how learning curve for GaMnAs MBE growthWhy its so difficultto rise TC??? Recipe determined by the Nottingham Univ. group (TC=173 K, world record)

  • national laboratory for advanced Tecnologies and nAnoSCienceGaMnAs structureTo increase TC one has toMinimize As antisite defectsMinimize interstitial MnGet sufficiently high Mn content

  • national laboratory for advanced Tecnologies and nAnoSCienceTo increase Mn content and minimize surface segregation,low growth temperatureR.P. Campion et al, JCG 251, 311 (03)Ideal temperature vs Mn content identified by monitoring the RHEED : the highest T giving 2D RHEEDMn incorporation

  • national laboratory for advanced Tecnologies and nAnoSCience

    As antisite

    As flux reduced to the minimum necessary in order to maintain a 2D RHEED pattern at the selected temperature.

    2 Ga cell to maintain the exact stoichiometry during both GaAs and GaMnAs growth.

    Use of As2 instead of As4

  • national laboratory for advanced Tecnologies and nAnoSCience

    As antisite cannot be eliminated by post-growth treatments !!C.T.Foxon, private comm.

  • national laboratory for advanced Tecnologies and nAnoSCienceInterstitial MnEvidences (by RBS and PIXE) of the presence of interstitial Mn in as grown GaMnAs.

    Low T annealing reduce the interstitials density that diffuse toward the surface, rise TC and p

    Yu et al, PRB 65,201303R (02)Edmonds et al, PRL 92, 037201 (04) Interstitial Mn are detrimental for FM: are double donor are attracted by substitutional Mn and coupled with them antiferromagnetically reduce the effective Mn moments concentration xeff

  • national laboratory for advanced Tecnologies and nAnoSCienceLong annealing at T=180C.

    TC increases with annealingp increases with annealing,no compensation in annealed samplesTC increase nearly linearly with xeffRT TC expected at xeff = 0.10.Jungwirth et al, PRB72, 165204 (05)

  • national laboratory for advanced Tecnologies and nAnoSCience

    Eid et al, APL86, 152505 (05)Nanoengineered TC by lateral patterning50 nm Ga0.94Mn0.06As+ 10 nm GaAs capannealing is uneffective!

  • national laboratory for advanced Tecnologies and nAnoSCienceEnergy formation of interstitials depend on the Fermi energy of the material !!!Yu et al, APL84, 4325 (04)Magnetization data in three p-typeAlGaAs/GaMnAs/AlGaAsmodulation dopedheterostructures (MDH):N-MDH: Be above GaMnAsI-MDH: Be below GaMnAs.

    Lower TC and more interstitials in GaMnAs grown on p-type semicondctor!! This may be a limit for TC

  • national laboratory for advanced Tecnologies and nAnoSCienceAlternative to bulk GaMnAs growth: Digital ferromagnetic heterostructure (DFH)Kawakami et al, APL 77, 2379 (00)Alternate deposition of GaAs and MnAsMax TC = 50 Kbut also a single MnAs layer is FM!

  • national laboratory for advanced Tecnologies and nAnoSCiencen- and p-type doping of DFH by doping the GaAs spacers!! independent control of magnetism and free carriersJohnston-Halperin et al,PRB 68, 165328 (03)Fermi Energy effect?

  • national laboratory for advanced Tecnologies and nAnoSCienceAlternative to bulk GaMnAs growth: Mn d-doping = d-like doping profile along the growth direction.Holes/Mn not enough to get FM.

    + p selectively doped heterostructure (p-SDHS) FM!!!ds is the critical parameterno FM for ds 5nmNazmul et al, PRB 67, 241308R(03)

  • national laboratory for advanced Tecnologies and nAnoSCienceNazmul et al, PRL 95, 017201 (05)Mn d-doping and heterostructue design Record TC = 190 K after annealing Record TC = 250 K after annealing !EF effect on Mn interstitial density?

  • national laboratory for advanced Tecnologies and nAnoSCienceElectric field control of ferromagnetismOhno et al, Nature 408, 944 (00)The idea: in hole mediated FMDecrease/increase of hole densityDecraese/increase exchange interaction between Mn Metal insulator FETInMnAs with TC above 20KIsothermal and reversible change of the magnetic state

  • national laboratory for advanced Tecnologies and nAnoSCienceII-VI Spin injectorsGiant Zeeman splitting in II-VISpin polarization detected from light polarizationFiederling et al, Nature 402, 787 (99)B0, low TPopt= (I(+)-I(- ))/ (I(+)