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βNMR: A Depth-Resolved Probe of Nanoscale Materials

This article discusses the use of βNMR as a method to study materials at the nanoscale. It explores various applications, such as magnetic proximity effects in metals, spin injection in dilute magnetic semiconductors, and interface properties of high Tc superconductors. The article also explains the βNMR method and its implementation at the TRIUMF facility in Vancouver, Canada.

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βNMR: A Depth-Resolved Probe of Nanoscale Materials

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  1. HFI/NQI 2010, Sep 15, 2010 beta detected NMR: a new depth-resolved probe of materials at the nanoscale W.A. MacFarlane Chemistry Department University of British Columbia, Vancouver, Canada

  2. Downtown Vancouver Whistler 123 km UBC 2 km TRIUMF 12 km http://www.triumf.ca/

  3. Outline • TRIUMF βNMR/βNQR facility • Why use βNMR to study materials ? • A few examples: a. magnetic proximity effects in metals b. spin injection, dilute magnetic semiconductors c. interface properties of high Tc superconductors

  4. 1. The βNMR Method

  5. Parity Violation ß emission is correlated with the spin direction of the decaying nucleus, violating mirror symmetry Lee and Yang 1957

  6. Asymmetric Nuclear b-decay of 8Li 8Li 8Be + e- + ne Spin=2, Q=+31 mb g =6.3 MHz/T <A>=-1/3 t= 1.2s q N(E) E 13 MeV Polar plot representing the beta emission probability as a function of angle

  7. Isotopes for bNMR at ISAC Isotope Spin τ1/2g b-Decay Estimated (MHz/T) Asymmetry Rate (s-1) 8Li 2 0.8 6.3 0.33 108 11Be 1/2 13.8 22 ~0.3 107 15O 1/2 122 10.8 0.66 108 19O 5/2 26.9 4.6 0.71 108 17Ne 1/2 0.1 0.33 106 require: light, short-lived, high asymmetry

  8. βNMResonance Backward H0 Bloc w0 = gH0 Forward w0 H1cos(ωt) two polarizations

  9. βNMR Measurement of the Spin Lattice Relaxation Rate Backward H0 time 1 / T1 Forward Pd Foil T = 293 K B0 ~ 140 G ELi = 30 keV Beam on For 0.5s nb: error bars grow as exp(t/τ) data for the two polarizations T1 ~ 1.65 s

  10. TRIUMF Implementation see βNMR: Morris et al., Phys. Rev. Lett. 93, 157601 (2004). βNQR: Salman et al., Phys. Rev. B 70, 104404 (2004). facility: Kiefl et al., Physica B 326, 189 (2003). polarizer: Levy et al., NIMB 204, 689 (2003).

  11. ISAC Production Target Foils: Ta SiC ZrC Extracted ion beam ~30 keV 500 MeV Proton Beam × To Isotope Separator M. Dombsky/TRIUMF for Li+: surface ionization

  12. ISAC Low Energy Area Titan βNQR βNMR Osaka

  13. Optical Polarizer Li+ ion beam Circularly Polarized Laser D1 in Li: 671 nm

  14. Optical Polarized Value Polarization of 8Li Nuclei in 4.1 T Thermal Equilibrium Value

  15. Spin Polarized 8Li+ βNMR Spectrometer βNQR Spectrometer Polarizer Fast Kicker (2005) allows semi-simultaneous operation

  16. βNMR Spectrometer 9 Tesla NMR Magnet

  17. Loading a sample into the high-field βNMR spectrometer Load Lock Cryostat drives into solenoid bore (9 Tesla) 10-9 torr Gold Foil Hapke/TRIUMF

  18. βNQR Spectrometer

  19. 8Li at 5 keV 8 mm beam stopped in scintillator, imaged with CCD Typical rate: ~106 spin polarized 8Li+ per second

  20. βNQR sample ladder

  21. βNMR Cleanroom Uses: - Sample handling - UHV cryostat maintenance R. Abasalti

  22. 2. Why use βNMRto study materials?

  23. atomic resolution B A B B A B A B B B A B A B A B B A B A B A B B A B A B A B B A B A B A B B A B A B A B B A B A B B B A B A B B B Solid Interfaces A B

  24. Deceleration of the Ion Beam Beam Energy 30.4 keV range in the probe ions: depth resolution!

  25. Relaxation: Relation to Fundamental Properties Magnetic Susceptibility Shift: can be multicomponent and/or inhomogeneous Moriya Expression In the RF (μeV to zero) also: quadrupolar effects, diamagnetic shielding, ...

  26. 3. Examples

  27. M ? Magnetic Proximity Effect Metallic Ferromagnet Nonmagnetic Metal Fe Ag

  28. 4nm Gold (20 monolayers) 80nm Silver 2nm Iron GaAs Depth Resolved βNMR in Magnetic Multilayers • Ag/Fe epitaxial heterostructures • T.A. Keeler et al., Phys. Rev. B77, 144429 (2008) 2 μg!

  29. M e.g. Magnetic Proximity Effect Metallic Ferromagnet Nonmagnetic Metal Using implanted 8Li ßNMR, aymptotic behaviour of the envelope ~ r -2 Keeler et al. Phys. Rev. B 77, 144429 (2008) Fe Ag

  30. M ? Metallic Ferromagnet Semiconductor Fe GaAs

  31. Epitaxial Relation Fe/GaAs

  32. Samples from B. Heinrich Lab, SFU Schottky Barrier - + - + - + magnetized 14ML 20ML Fe Au n-GaAs

  33. Access to the Schottky Barrier Region electrostatic potential within GaAs implantation profiles from SRIM Q. Song

  34. Systematic Depth Dependence (Unbiased Junction) T = 150 K

  35. Resonance Spectrum in Fe

  36. Towards Spin Injection

  37. V spin injection Schottky Barrier - + - + - + magnetized 14ML 20ML Fe Au n-GaAs

  38. Avoiding the Schottky Barrier withDilute Magnetic Semiconductors: GaAs:Mn Q. Song

  39. Dilute Magnetic Semiconductors a background of free carriers (holes) dilute local (atomic) magnetic moments Mn2+ S = 5/2

  40. Mn doped GaAs Mn acceptor (STM) Yakunin et al. PRL 92, 216806 (04) Substitutional (Ga): Acceptor Interstitial: Double Donor Ga1-xMnxAs is not stable in bulk

  41. 180 nm Ga0.95Mn0.05As / GaAs Sharp Substrate Line

  42. Depth Dependence at 50 K (< TC) Q. Song et al., Physica B (2009) broad, negatively shifted line, fast spin relaxation from the Mn doped layer

  43. Temperature Dependence

  44. Temperature Dependence amplitude linewidth resonance frequency A measure of the hole susceptibility!

  45. SQUID Magnetization in 1.3 T

  46. Contributions to the Local Field

  47. Clogston Jaccarino Analysis Corrected for Demagnetization Raw shift of resonance linewidth

  48. Time Reversal SymmetryBreaking Superconductivity? H. Saadaoui

  49. CuO2 Planes High Tc SuperconductorPhase Diagram strange metal AFM sc holes into CuO2 Planes

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