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Polarized Soft X-ray Science at the APS

Polarized Soft X-ray Science at the APS. David Keavney X-ray Operations and Research Advanced Photon Source. Workshop on Soft X-ray Beamlines at the NSLS-II Brookhaven, NY, February 4, 2008. Outline. Beamline 4-ID-C at the APS Recent science using XMCD at 4-ID-C

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Polarized Soft X-ray Science at the APS

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  1. Polarized Soft X-ray Science at the APS David Keavney X-ray Operations and Research Advanced Photon Source Workshop on Soft X-ray Beamlines at the NSLS-II Brookhaven, NY, February 4, 2008

  2. Outline • Beamline 4-ID-C at the APS • Recent science using XMCD at 4-ID-C • Proposed Upgrades (higher sensitivity and throughput) • Existing beamline • New soft x-ray magnetism sector • Estimated Performance and Expected Issues

  3. 4-ID-C beamline: 7 T 0.1 T PEEM Station High-Field Station XMCD/Scattering Station Beamline Optics Magnetic Studies at the APS – Sector 4 XMCD Magnetic Imaging (PEEM) Reflectivity XPS 4-ID-C 0.5-3.0 keV XMCD High Field Scattering www.aps.anl.gov 4-ID-D 2.8-40 keV Helical undulator e-

  4. 4-ID-C Helical undulator M3-Sphere M2-Sphere M1-Plane K-B pair Sample • Source: • Fully Electromagnetic CPU • 12.8cm period, 2.47m • Linear H and V, Circular R and L modes • RCP/LCP Switching Rate: 1Hz • Optics: • Spherical Grating Monochromator • 500-3000eV Energy Range • E/∆E~5000 typical • ~100x100µm spot (variable focus) • 700 and 1200 l/mm gratings • Rh coatings Exit Slit Ent. Slit SG

  5. III GaAs V Metal (Ga,Mn)As Mn:GaAs TC~120K Magnetic Semiconductors Controversies: • Extrinsic vs. intrinsic magnetism • Are conduction electrons involved? • Second phase formation • Contamination • Source material, substrate, metal tools H. Ohno, et al. Appl. Phys. Lett. 69, 363 (1996). Mn:ZnO TC>300K Mi et al., J. Appl. Phys. 101, 023904 (2007).

  6. Spin Configuration in Mn:GaAs Consistent with Density Functional Theory Mn As Ga D.J. Keavney, D. Wu, J.W. Freeland, E. Johnston-Halperin, D.D. Awschalom, J. Shi, Phys. Rev. Lett. 91 187203 (2003).

  7. Doped oxides - correlation with carriers • Activation of FM order via carrier doping Co:TiO2 Slow-Grown Fast-Grown S.A. Chambers, Surf. Sci. Rep. 61, 345 (2006). • Correlation with extrinsic defects

  8. Co-doped ZnO • MOCVD-grown films, Zn-diffused, S. Chambers et al. Bulk Remanent Magnetization: ~0.04 µB/Co Fluorescence Yield data: XAS: Co2+ XMCD: 0.8% signal SQUID XMCD is consistent with bulk magnetization But, S/N~3, still too low to rule out Co(0) 100 nm film 10% Co XMCD

  9. Ferromagnetism in Cu-doped ZnO D.B. Buchholz, et al., APL 87, 082504 (2005). Ye et al., PRB 73, 033203 (2006).

  10. Fluorescence Yield Data: • Two Cu2+ spectral components • A: substitutions, paramagnetic • B: CuO inclusions, non-magnetic • Noise level ~0.6% (spectra) • Upper bound of <0.1 mB/Cu from XMCD • ~0.3-0.4 mB/Cu from SQUID Cu-doped ZnO • PLD-grown with O2 or N2O atmosphere 20 mTorr O2: n-type, non-FM 50 mTorr N2O: p-type, FM Similar results seen in Mn:ZnO, Co:ZnO, Cr:TiO2, V:SnO2, Mn:GaN, Cr:AlN, Co:TiO2, etc. Magnetism in doped semiconductors may be unrelated to the dopant ions. Except for Mn:GaAs Flux limits ability to say for sure. D. J. Keavney, D. B. Buchholz, Q. Ma, R. P. H Chang, Appl. Phys. Lett. 91 012501 (2007).

  11. Mixing Ferromagnetism and Superconductivity at the Atomic Scale YBa2Cu3O7-δ Cu (x10) La2/3Ca1/3MnO3 Mn J. Chakhalian, J.W. Freeland, et. al. Nature Physics 2, 244 (2006).

  12. B 100ps magnetization dynamics via time-resolved PEEM 6 µm Ni80Fe20 disk

  13. √N Fe K-edge XMCD @ 40Hz D. Haskel More Photons Lock-in detection Rationale for Upgrades Current detection limit: 5x1015 µB/cm2 (100nm film 10% doped with 0.05µB/dopant ion) • Long Undulators & Efficient Optics • Enhanced Detectors • Fast Polarization Switching Proposed detection goal: 5x1012 µB/cm2 (100nm film 1% doped with 0.0005µB/dopant ion)

  14. M3-Plane M1-Plane M2-Sphere Source (CPU) K-B pair Exit Slit Sample VLS-PG Proposed Upgrade to 4-ID-C 10% grating efficiency 95% of beamtime requests Proposed: varied line-spacing plane grating design Energy range: 400-2000eV Peak efficiency @ 1000eV Resolution: 5000-10000 Improved flux by 1-2 orders of magnitude between 500 and 1000 eV.

  15. High-sensitivity magnetism beamline on long straight section Fast-switching helical undulator 9+ T New high sensitivity beamline 0.8 T e- High-Field, sub-K Station Multipole XMCD/Scattering Station Beamline Optics • 2-3 orders of magnitude more flux • Lock-in detection • Enhanced Instrumentation

  16. Fast Polarization Switching Approaches 1. Long CPU LCP/RCP beam to user • Uses entire straight section • Max switching rate not clear (CPU: 1Hz) • Orbit corrections difficult • Power Supply (CPU: H:1600A, V:±400A) 2. Dual Apple Devices with Kickers RCP LCP LCP beam to user RCP beam to user • Possibly faster switching • Orbit corrections easier • Arbitrary polarization • Switching mode throws away ½ flux • Matching and aligning probably difficult

  17. Expected Performance • Higher Flux: x30 • Lock-in: x10 @ 40Hz • Multi-element Si-drift detectors: x10

  18. Orbit Corrections CPU creates significant orbit perturbations and beam size fluctuations Peaks represent about 250 µm of orbit error Corrected to 7 µm after some effort

  19. Summary • Sensitivity limits in soft x-rays • Small moments • Very dilute limits • Inefficient detection (PEEM) • Upgrade Existing beamline x~100 • New fast polarization switching beamline x~1000

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