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A High Efficiency Electron Polarimeter Based on Exchange Scattering from a Magnetic Target

A High Efficiency Electron Polarimeter Based on Exchange Scattering from a Magnetic Target Lamberto Duò R. Bertacco, M. Marcon, M. Portalupi, F. Ciccacci Dipartimento di Fisica – Politecnico di Milano. Polarimetry - Spin-Polarization Analysis. Polarimeter Performances. F ~ 3 x 10 -3.

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A High Efficiency Electron Polarimeter Based on Exchange Scattering from a Magnetic Target

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  1. A High Efficiency Electron Polarimeter Based on Exchange Scattering from a Magnetic Target Lamberto Duò R. Bertacco, M. Marcon, M. Portalupi, F. Ciccacci Dipartimento di Fisica – Politecnico di Milano Lamberto Duò - PESP-2002, MIT-Bates Linear Accelerator Center

  2. Polarimetry - Spin-Polarization Analysis Lamberto Duò - PESP-2002, MIT-Bates Linear Accelerator Center

  3. Polarimeter Performances F ~ 3 x 10-3 Lamberto Duò - PESP-2002, MIT-Bates Linear Accelerator Center

  4. Spin Polarized Electron Absorption and Reflection from Fe(001)-p(1x1)O Absorption: Target Current Reflection: Faraday Cup Lamberto Duò - PESP-2002, MIT-Bates Linear Accelerator Center

  5. Spin Polarized Electron Absorption and Reflection from Fe(001)-p(1x1)O Faraday Cup entrance Holder Absorption: Target Current Fe [001] ns q Reflection: Faraday Cup n q Coil P0 (s=+1) M (m= -1) q = 15º Lamberto Duò - PESP-2002, MIT-Bates Linear Accelerator Center

  6. Oxygen-induced Enhancement of Spin Dependent Effects: Fe(001)-p(1x1)O Fe bandstructure (*) maj. Z min. Fe(001) C 20 0 -20 Fe(001)-p(1x1)O B2 C B1 60 S 40 20 0 -20 16.0 20.0 0.0 4.0 8.0 12.0 (c) Fe bandstructure (*) kz maj. k min. H12 H12 H15 H15 H H (b) Fe(001) C B2 B1 S B1 B2 S (x10) RC AC 1.0 0.5 Intensity (arb. un.) Intensity (arb.un.) 0.0 (a) Fe(001)-p(1x1)O (x10) X Asymmetry (%) D C B2 B1 S (x10) RC 1.0 AC 0.5 0.0 0.0 4.0 8.0 12.0 16.0 20.0 E-EF (eV) E-EF (eV) AC: absorbed current RC: reflected current R. Bertacco and F. Ciccacci, Phys. Rev. B 59, 4207 (1999) (*) E. Tamura and R. Feder, Phys. Rev. Lett. 57, 759 (1986) Lamberto Duò - PESP-2002, MIT-Bates Linear Accelerator Center

  7. 1.0 0.8 0.6 0.4 I/I0 0.2 4 -M M 0.0 Am (%) 0 -4 + strained GaAs GaAs -8 0.0 4.0 8.0 12.0 Kinetic Energy (eV) Absorption from Fe(001)-p(1x1)O A(M) = - A(-M) Negligible spurious effects (experimental asymmetries) PGaAs = 25 %; Pstrained GaAs = 65 % A scales with P (not normalized to P!) Lamberto Duò - PESP-2002, MIT-Bates Linear Accelerator Center

  8. Fe(001)-p(1x1)O Relevant Characteristics for Polarimetry not annealed annealed Freshly Prepared 14 14 12 12 10 10 A peak (%) 8 8 Heat-cleaning A peak (%) 6 6 4 4 2 2 0 1 2 3 4 0 Surface Treatment 10 100 1000 1 freshly prepared 2 after 4 days in UHV 3 after 2 weeks at 2•10-9 Torr 4 after exposure to atmosphere Exposure (L) R. Bertacco, et al., Appl. Phys. Lett. 72, 2050 (1998) Lamberto Duò - PESP-2002, MIT-Bates Linear Accelerator Center

  9. Fe(001)-p(1x1)O Relevant Characteristics for Polarimetry Reflection Absorption F > 5 x 10-3 15° 0° Fe(001) 6 35° 2 0 Fe(001)-p(1x1)O A0 (%) -2 12 F x 103 -6 8 Apeak (%) 4 -10 25° 0 -20 0 20 40 Incidence angle -14 2 6 10 14 Kinetic Energy (eV) Kinetic Energy (eV) R. Bertacco, F.Ciccacci, Surf. Sci. 419, 265 (1999) Lamberto Duò - PESP-2002, MIT-Bates Linear Accelerator Center

  10. Operating Device for Energy and Spin Analysis Requirements 1) Reliable procedures for target preparation and magnetization (in remanence) 2) Coupling the polarimeter to an energy analyzer 3) Single electron counting reflection geometry Lamberto Duò - PESP-2002, MIT-Bates Linear Accelerator Center

  11. Target Preparation - thin film epitaxy in UHV (MBE): 2000 Å Fe/MgO(001) - exposure to O2: few minutes at 10-6 Torr heating at 900 K - check via XPS, LEED, SPIPE dedicated UHV system - removal (without exposure to air) Hemispherical Analyzer (VSW HA-150) -insertion in the operating position close to the exit slit of an energy analyzer + heat-cleaning in O2 - coupling to the HA: => electron optics Lamberto Duò - PESP-2002, MIT-Bates Linear Accelerator Center

  12. Analyzer Pass Energy (eV) A  8 mm diameter Ek= 4 eV Wq = ± 15º Fpol = 40 mm2 sr eV 4.4 44 Fan = 20 ÷ 200 mm2 sr eV Target To HA Coupling: Electron-optics } Small volume accepted Fpol = A•Ek•W Channeltron Ftot = Fpol(No/Noo) = S 2·(N/Noo) Ftot < Fpol N Noo Analyzer exit slit Transfer optics Target No Lamberto Duò - PESP-2002, MIT-Bates Linear Accelerator Center

  13. K 2 n s K 1 30° D=30 mm 1 2 3 4 6 7 5 Target To HA Coupling: Electron-optics q q n P M Taget holder Analizer exit flange (CF 100) Channeltron measured signal NooorN Exit slit 1 – 5 Heddle configuration 5 –7 Zoom lens R. Bertacco et al., Rev. Sci. Instrum., in press (2002) Lamberto Duò - PESP-2002, MIT-Bates Linear Accelerator Center

  14. Ray Tracing • SIMION 3D Exit Slit • At the Analyzer Exit Slit: Pass Energy = 22 eV Beam Diameter = 6 mm Angular Divergence = ± 7º Heddle Lens Zoom Lens • At the Target: Kinetic Energy = 4 eV Beam Diameter = 8 mm Angular Divergence = ± 15º • Transmission = 32 % Target Lamberto Duò - PESP-2002, MIT-Bates Linear Accelerator Center

  15. Experimental Setup CF 100 flange Lamberto Duò - PESP-2002, MIT-Bates Linear Accelerator Center

  16. Test & Calibration P = 25 % Lamberto Duò - PESP-2002, MIT-Bates Linear Accelerator Center

  17. Results & Performance • Stot < Spol = 40x10-2 Very large spot size and angular divergence (up to 35x10-2 with different voltages but small Ftot) • Spurious asymmetry ~ 10÷15 % Lever effect (total length ~ 1.5 m) • F values still larger than best results for energy and spin analysis Ftot = 1.4x10-4(*) (*) G. Ghiringhelli, K. Larsson, N.B. Brookes, Rev. Sci. Instrum. 70, 4225 (1999) Lamberto Duò - PESP-2002, MIT-Bates Linear Accelerator Center

  18. Valence Band Photoemission on Fe(001) UPS HeI (hn = 21.2 eV) Intensity (arb. units) majority spin minority spin Binding Energy (eV) Total acquisition time ~ 1 hour (PE = 9 eV) Lamberto Duò - PESP-2002, MIT-Bates Linear Accelerator Center

  19. Conclusions Lamberto Duò - PESP-2002, MIT-Bates Linear Accelerator Center

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