national laboratory for advanced Tecnologies and nAnoSCience Material and devices for spintronics What is spintronics? Ferromagnetic semiconductors Physical basis Material issues Examples of spintronic devices Electric field control of magnetism Spin injectors Spin valves Trieste,

national laboratory for advanced Tecnologies and nAnoSCience Spintronics = spin-based electronics Information is carried by the electron spin, not (only) by the electron charge. 1.Ferromagnetic metallic alloys- based devices Transport in FM metals is naturally spin-polarized Ideal, fully polarized case, only spin down states are available

national laboratory for advanced Tecnologies and nAnoSCience 1988: 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 nAnoSCience GMR based Spin Valves and Magnetic tunnel junction Prinz, 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 nAnoSCience GMR based Spin Valves for read head in hard drives Prinz, Science 282, 1660 (98) Wolf, Science 294, 1488 (01) Standard geometry for GMR based Spin Valves But also MRAM

national laboratory for advanced Tecnologies and nAnoSCience Spintronics = spin-based electronics 1.Ferromagnetic metal - based devices 2.Semiconductor based spin electronics Courtesy C.T. Foxon

national laboratory for advanced Tecnologies and nAnoSCience Spintronics = spin-based electronics 1.Ferromagnetic metal - based devices 3.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 in coherent superpositions of up and down states 2.Semiconductor based spin electronics devices

national laboratory for advanced Tecnologies and nAnoSCience How can we measure the magnetic state of a thin epilayer: SQUID measurements but also Anomalous Hall effect R 0 =1/pe Ordinary Hall effect contribution, negligible. R Hall is proportional to M.

national laboratory for advanced Tecnologies and nAnoSCience Two main issues in semiconductor spintronics: 1.Avaiability of suitable materials Ideal material should be Easily integrable with ‘‘electronic’’ materials Able to incorporate both n- and p-type dopants With a T C above room T 2.Understandig and controlling the physical phenomena: Spin injection Transport of spin polarized carriers across interfaces Spin interactions in solids: role of defects, dimensionality, semiconductor band structure

Examples: Eu– dichalcogenides (EuS, GdS, EuSe) and spinels CdCr 2 Se 4. 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 integrate Crystal growth very slow and difficult Low T C national laboratory for advanced Tecnologies and nAnoSCience Magnetic semiconductor, constituted by a periodic array of magnetic ions

national laboratory for advanced Tecnologies and nAnoSCience As 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 nAnoSCience II-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 etherostructures But Magnetic interaction dominated by antiferromagnetic direct exchange among Mn spins. In undoped material paramagnetic, antiferromagnetic and spinglass behavior, no FM Interesting: ‘‘Giant’’ Zeeman splitting !!

national laboratory for advanced Tecnologies and nAnoSCience III-V DMS GaAs, 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 MBE 1992 FM InMnAs 1996 FM GaMnAs

national laboratory for advanced Tecnologies and nAnoSCience The 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, (05) and reference therein

national laboratory for advanced Tecnologies and nAnoSCience Hole mediated FM In a mean field virtual crystal approximation x = substitutional Mn p = hole density In III-V DMS the holes comes from Mn !!! x and p are intimately related Room temperature T C is expected for Ga 0.9 Mn 0.1 As. See PRB 72, (05) and reference therein

national laboratory for advanced Tecnologies and nAnoSCience Know-how learning curve for GaMnAs MBE growth Why it’s so difficult to rise T C ??? Recipe determined by the Nottingham Univ. group (T C =173 K, world record)

national laboratory for advanced Tecnologies and nAnoSCience GaMnAs structure To increase T C one has to Minimize As antisite defects Minimize interstitial Mn Get sufficiently high Mn content

national laboratory for advanced Tecnologies and nAnoSCience To increase Mn content and minimize surface segregation, low growth temperature R.P. Campion et al, JCG 251, 311 (03) Ideal temperature vs Mn content identified by monitoring the RHEED : the highest T giving 2D RHEED Mn 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 As 2 instead of As 4

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 nAnoSCience Interstitial Mn Evidences (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 T C and p Yu et al, PRB 65,201303R (02) Edmonds et al, PRL 92, (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 x eff

national laboratory for advanced Tecnologies and nAnoSCience Long annealing at T=180C. T C increases with annealing p increases with annealing, no compensation in annealed samples T C increase nearly linearly with x eff RT T C expected at x eff = Jungwirth et al, PRB72, (05)

national laboratory for advanced Tecnologies and nAnoSCience Eid et al, APL86, (05) Nanoengineered T C by lateral patterning 50 nm Ga 0.94 Mn 0.06 As + 10 nm GaAs cap annealing is uneffective! Lateral patterning Tc + 50K with annealing! Free surface is important for interstitials passivation

national laboratory for advanced Tecnologies and nAnoSCience Energy formation of interstitials depend on the Fermi energy of the material !!! Yu et al, APL84, 4325 (04) Magnetization data in three p-type AlGaAs/GaMnAs/AlGaAs modulation doped heterostructures (MDH): N-MDH: Be above GaMnAs I-MDH: Be below GaMnAs. Lower T C and more interstitials in GaMnAs grown on p-type semicondctor!! This may be a limit for T C

national laboratory for advanced Tecnologies and nAnoSCience Alternative to bulk GaMnAs growth: Digital ferromagnetic heterostructure (DFH) Kawakami et al, APL 77, 2379 (00) Alternate deposition of GaAs and MnAs Max T C = 50 K but also a single MnAs layer is FM!

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

national laboratory for advanced Tecnologies and nAnoSCience Alternative to bulk GaMnAs growth: Mn  doping =  - like doping profile along the growth direction. Holes/Mn not enough to get FM. + p selectively doped heterostructure (p-SDHS) FM!!! d s is the critical parameter no FM for d s ≥ 5nm Nazmul et al, PRB 67, R(03)

national laboratory for advanced Tecnologies and nAnoSCience Nazmul et al, PRL 95, (05) Mn  -doping and heterostructue design Record T C = 190 K after annealing Record T C = 250 K after annealing ! E F effect on Mn interstitial density?

national laboratory for advanced Tecnologies and nAnoSCience Electric field control of ferromagnetism Ohno et al, Nature 408, 944 (00) The idea: in hole mediated FM Decrease/increase of hole density Decraese/increase exchange interaction between Mn Metal insulator FET InMnAs with T C above 20K Isothermal and reversible change of the magnetic state

national laboratory for advanced Tecnologies and nAnoSCience II-VI Spin injectors Giant Zeeman splitting in II-VI Spin polarization detected from light polarization Fiederling et al, Nature 402, 787 (99) B≠0, low T P opt = (I(σ + )-I(σ - ))/ (I(σ + )+I(σ - )) =1/2 P spin

national laboratory for advanced Tecnologies and nAnoSCience III-V Spin injectors Ohno et al, Nature 402, 790 (99) Below T C polarization survive also at H=0 FM GaMnAs as spin aligner Spin-polarization measured from el-emission polarization

national laboratory for advanced Tecnologies and nAnoSCience First observation of spin-dependent MR in all-semiconductor heterostructure Akiba et al. JAP 87, 6436 (00) InGaAs buffer to get tensile strain and out of plane easy axis Two different Mn x to get different coercitive field ΔR/R=0.2%

national laboratory for advanced Tecnologies and nAnoSCience Large TMR in semiconductor magnetic tunnel junction Tanaka et al, PRL 87, (01) In plane magnetic field Optimal barrier thickness 1.6 nm Antiparallel configuration is stable ΔR/R=70%

national laboratory for advanced Tecnologies and nAnoSCience Large Magnetoresistance in GaMnAs nanoconstriction Rüster et al, PRL 91, (03) Large MR expected in transport trough domain wall Constrictions pin domain walls

national laboratory for advanced Tecnologies and nAnoSCience Rüster et al, PRL 91, (03) GMR: (a)1.5% when R=48kΩ further etching, (b) 8% when R=78kΩ further etching, 2000% when R=4MΩ!!! TMR!

national laboratory for advanced Tecnologies and nAnoSCience Tunneling anisotropic magnetoresistance -TAMR New physics! Single GaMnAs magnetic layer AlOx tunnel barrier Two resistance states Position and sign of the switch depend on Φ Interplay of anisotropic DOS with Φ and a two step magnetization reversal process Gould et al, PRL 93, (04)

national laboratory for advanced Tecnologies and nAnoSCience Tunneling anisotropic magnetoresistance -TAMR Huge effects and new physics H perpendicular to the film (hard axis) No histeresis!! Related to the absolute and not relative orientation Rüster et al, PRL 94, (05)

national laboratory for advanced Tecnologies and nAnoSCience In plane Field Angular dependence! Sensor of B orientation? Φ = 95° T=1.7K and low bias

national laboratory for advanced Tecnologies and nAnoSCience