national laboratory for advanced Tecnologies and nAnoSCience

1. 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 national laboratory for advanced Tecnologies and nAnoSCience Trieste, 20.10.06

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

3. 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

4. GMR based Spin Valves and Magnetic tunnel junction AF layer (A) or AF/FM/Ru/ trilayer (B) to pin the magnetization of the top FM layer Prinz, Science 282, 1660 (98) Wolf et al, Science 294, 1488 (01) national laboratory for advanced Tecnologies and nAnoSCience

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

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

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

8. How can we measure the magnetic state of a thin epilayer: SQUID measurements but also Anomalous Hall effect R0=1/pe Ordinary Hall effect contribution, negligible. RHall is proportional to M. national laboratory for advanced Tecnologies and nAnoSCience

9. Two main issues in semiconductor spintronics: • Avaiability of suitable materials • Ideal material should be • Easily integrable with ‘‘electronic’’ materials • Able to incorporate both n- and p-type dopants • With a TC above room T • 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 • ................. national laboratory for advanced Tecnologies and nAnoSCience

10. Magnetic semiconductor, constituted by a periodic array of magnetic ions • 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 integrate • Crystal growth very slow and difficult • Low TC national laboratory for advanced Tecnologies and nAnoSCience

11. 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

12. 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

13. 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

14. 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, 165204(05) and reference therein national laboratory for advanced Tecnologies and nAnoSCience

15. 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 TC is expected for Ga0.9Mn0.1As. See PRB 72, 165204(05) and reference therein national laboratory for advanced Tecnologies and nAnoSCience

16. Know-how learning curve for GaMnAs MBE growth Why it’s so difficult to rise TC??? Recipe determined by the Nottingham Univ. group (TC=173 K, world record) national laboratory for advanced Tecnologies and nAnoSCience

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

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

19. 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 ofAs4 national laboratory for advanced Tecnologies and nAnoSCience

20. As antisite cannot be eliminated by post-growth treatments !! C.T.Foxon, private comm. national laboratory for advanced Tecnologies and nAnoSCience

21. Interstitial Mn • 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 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 TC and p Yu et al,PRB 65,201303R (02) Edmonds et al,PRL 92, 037201 (04) national laboratory for advanced Tecnologies and nAnoSCience

22. Long annealing at T=180C. TC increases with annealing p increases with annealing, no compensation in annealed samples TC increase nearly linearly with xeff RT TC expected at xeff = 0.10. national laboratory for advanced Tecnologies and nAnoSCience Jungwirth et al, PRB72, 165204 (05)

23. Nanoengineered TC by lateral patterning Lateral patterning Tc + 50K with annealing! Free surface is important for interstitials passivation Eid et al, APL86, 152505 (05) 50 nm Ga0.94Mn0.06As + 10 nm GaAs cap annealing is uneffective! national laboratory for advanced Tecnologies and nAnoSCience

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

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

26. 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, 165328 (03) Fermi Energy effect? national laboratory for advanced Tecnologies and nAnoSCience

27. Alternative 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 parameter no FM for ds≥ 5nm national laboratory for advanced Tecnologies and nAnoSCience Nazmul et al, PRB 67, 241308R(03)

28. 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 nAnoSCience Nazmul et al, PRL 95, 017201 (05)

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

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

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

32. First observation of spin-dependent MR in all-semiconductor heterostructure • 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 Akiba et al. JAP 87, 6436 (00)

33. Large TMR in semiconductor magnetic tunnel junction • In plane magnetic field • Optimal barrier thickness 1.6 nm • Antiparallel configuration is stable • ΔR/R=70% national laboratory for advanced Tecnologies and nAnoSCience Tanaka et al, PRL 87, 026602 (01)

34. Large Magnetoresistance in GaMnAs nanoconstriction • Large MR expected in transport trough domain wall • Constrictions pin domain walls Rüster et al, PRL 91, 216602 (03) national laboratory for advanced Tecnologies and nAnoSCience

35. GMR: • 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 Rüster et al, PRL 91, 216602 (03)

36. 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 national laboratory for advanced Tecnologies and nAnoSCience Gould et al, PRL 93, 117203 (04)

37. 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 national laboratory for advanced Tecnologies and nAnoSCience Rüster et al, PRL 94, 27203 (05)

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

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