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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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**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**Polarimetry - Spin-Polarization Analysis**Lamberto Duò - PESP-2002, MIT-Bates Linear Accelerator Center**Polarimeter Performances**F ~ 3 x 10-3 Lamberto Duò - PESP-2002, MIT-Bates Linear Accelerator Center**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**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**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**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**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**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**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**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**Analyzer Pass Energy (eV)**A 8 mm diameter Ek= 4 eV Wq = ± 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**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**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**Experimental Setup**CF 100 flange Lamberto Duò - PESP-2002, MIT-Bates Linear Accelerator Center**Test & Calibration**P = 25 % Lamberto Duò - PESP-2002, MIT-Bates Linear Accelerator Center**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**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**Conclusions**Lamberto Duò - PESP-2002, MIT-Bates Linear Accelerator Center

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