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Magnetism Studies with Microfocused X-rays

Magnetism Studies with Microfocused X-rays. George Srajer, Sector 4 (Polarization Studies Group). Three Examples. ~ 10 µm resolution Imaging of Spiral Magnetic Domains (2) ~ 1.0 µm resolution Biquadratic Exchange in SmCo/Fe Bilayer (3) ~ 0.1 µm resolution Imaging of Co Nanodot Arrays.

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Magnetism Studies with Microfocused X-rays

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  1. Magnetism Studies with Microfocused X-rays George Srajer, Sector 4 (Polarization Studies Group)

  2. Three Examples • ~ 10 µm resolution Imaging of Spiral Magnetic Domains (2) ~ 1.0 µm resolution Biquadratic Exchange in SmCo/Fe Bilayer (3) ~ 0.1 µm resolution Imaging of Co Nanodot Arrays Scanning microscopy - diffraction contrast Scanning microscopy - absorption contrast Full field microscopy - absorption contrast

  3. Common Features  In Sector 4 J.W. Freeland, J.C. Lang, G. Srajer, et al., Rev. Sci. Instrum. 73, 1408 (2002)  All experiments performed with circularly polarized x-rays 4-ID-C: Circularly polarized undulator 4-ID-D: Phase retarder

  4. 1. Imaging of Spiral Domains (J. Lang, D. McWhan) Charge peak Holmium Technique: Reverse helicity of circularly polarized beam to measure chirality of domains at a particular t • : turn angle between atomic layers Left handed or right handed spiral

  5. Experimental Setup on 4-ID-D diamond (111) phase retarder 1.0 0.3 0.2 0.5 0.2 degree of circ. polarization 0.0 transmitted intensity 0.1 -0.5 0.1 -1.0 0 100 -150 -100 -50 0 50 150

  6. Spiral Domains in Holmium Image taken at (0,0,4+t) reflection 2-dimensional scan in 500 x 640 mm Color scheme reflects the asymmetry ratio: a • Domains ~100µm • Domain walls uncorrelated with the crystal lattice

  7. Temperature Dependence of Ho Domains Domains freeze in; not many changes

  8. Imaging of Dysprosium Spiral Domains T=90K warming T=160K warming T=90K cooling Warming from ferro phase yields many smaller domains which merge with larger domain upon heating. J.C. Lang, D. McWhan, D. Haskel, D. Lee and G. Srajer, in preparation

  9. 2. Biquadratic Exchange in Sm/Co Bilayer XMCD M • Micro beam • Element-specificity (G. Srajer, D. Haskel) > 10 mm Easy axis H direction Fe g SmCo ~ 50 nm Fe (200Å)/SmCo (200Å) Vlasko-Vlasov et al., PRL 86, 4386 (2001)

  10. Microfocusing on 4-ID-D 1.0 0.3 0.2 0.5 0.2 0.0 0.1 -0.5 0.1 -1.0 0 -150 -100 -50 0 50 100 150 Circularly Polarized Beam Detect fluorescence signal using Ge SSD magnetic field up to 0.8 T diamond (111) phase retarder K-B Mirror Properties: focal length ~30 cm focal size 1.0  1.0 mm2 flux ~ 1010 ph/s (100mA) Pc ~ 0.99 degree of circ. polarization transmitted intensity

  11. Experimental Setup Sample Detectors KB Mirrors Electromagnet J.C. Lang et al. SPIE Proc. 4499, 1 (2001) J. Pollmann et al. J. Appl.Phys. 89, 7165 (2001)

  12. Sm Remanent Hysteresis Loop > 10 mm Fe ~ 50 nm Goal is to determine the field at which SmCo layer is demagnetized H Easy axis SmCo If Fe domains large, Fe magnetization direction frustrated! H = -3.4 kOe

  13. Fe Domains for Demagnetized SmCo Layer 1 Two large domains! 2 G. Srajer, D. Haskel, J.C. Lang, C. Kmety-Stevenson, S. Jiang and S. Bader, in preparation

  14. 3. Imaging of Co Nanodot Arrays Screen Projector lenses Soft X-ray beamline (4-ID-C) 500-3000 eV Aperture Objective Mirror Entrance slit Exit slit Circular Polarized Undulator Spherical Grating Monochromator (D. J. Keavney)

  15. Chemical and Magnetic Image 1 mm • Chemical contrast: • Self-assembled systems • Segregation • Local electronic structure • Buried layers (~5 nm) • Magnetic contrast: • Domain imaging • Ground states on nanoscales • Interactions in particle arrays • Finite size effects Chemical map (I++I-) 1 mm 1 mm Magnetic map (I+-I-) Vortex domains 1x1x0.015mm Co

  16. Rectangular Cobalt Arrays 5 mm X=1000 nm 500 nm 250 nm 10 mm Y X Length scale of interaction Magnetostatic energy • multi-domains Exchange interaction • single domains 150 nm

  17. Micromagnetics Calculations 500 x 1000 nm dots Two configurations: Ms=1350 emu/cm3; A=3.05 merg/cm 50 nm cell size ~ 250 nm D.J. Keavney, W.C. Uhlig and J. Shi, in preparation

  18. Summary • Chirality and temperature dependence of spiral and conical domains in single crystals • Direct evidence of biquadratic coupling in a Fe/SmCo bilayer • Length scales of magnetic interactions in Co patterned arrays

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