Soft X-ray Physics DELNOR-WIGGINS PASS STATE PARK

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1 Soft X-ray Physics Overview of research in Prof. Tonner s group Introduction to synchrotron radiation physics Photoemission spectroscopy: band-mapping and photoelectron diffraction Magnetic spectroscopy X-ray microscopy and spectro-microscopy DELNOR-WIGGINS PASS STATE PARK

2 The Three Principle Soft X-ray Spectroscopies X-ray Absorption Fine Structure (XAFS) X-ray Fluorescence Spectroscopy (XRF) X-ray Photoemission Spectroscopy (XPS)

3 X-ray Absorption Edges 2s,2p 1s Note the increases in absorption at characteristic energies. 3p 2p (2s) 1s

4 X-ray Absorption Spectroscopy Absorption Rate VALENCE Photon Energy CORE

5 Three methods of XAS measurement (X-ray Absorption Spectroscopy) (a) Transmission bulk properties I o I t I t = I e 1 µ ( hω )d t ( ) = ln d I µ hω I (b) Total electron yield surface properties I o e - I (c) Fluorescence dilute species; Buried interfaces

6 Electron yield X-ray Absorption Spectroscopy i yield hωµ hω ( )

7 Information in X-ray Absorption Spectroscopy Absorption Intensity Near-edge region -5 ev Valence transitions Multiple-scattering theory Chemical bonds (NEXAFS or XANES) White lines Photon Energy Extended X-ray Absorption Fine Structure (EXAFS) >5 ev to several 1 s of ev Near-neighbor geometry

8 Background Removal of Continuum Contributions Mn L 2,3 Absorption Intensity (arb. units) ML Mn/Cu(1) 21 ML Mn/Cu(1) Photon Energy (ev)

9 Mn L-edge XAFS of Bio-inorganic compounds MnO 2, MR4 reducing bacteria Mn 4+ L-edge structure from 2p to 3d dipole allowed transitions Absorption Intensity Mn nodule, oxidizing bacteria Mn 3+ Mn 2+ Structure in near-edge can often be explained by atomic theory of crystal field effects Many materials do not agree with atomic theory, because they have more de-localized orbitals need a many-body theory Photon Energy (ev)

10 Above plane, + XAS with Circular Polarization 6 Fe L-edge absorption Below plane, - Photons carry angular momentum (spin), which is parallel or antiparallel to the direction of propagation for circularly polarized light. I XMCD r Σ r M Absorption Intensity I XMCD The effect is an atomic physics effect: Spin-orbit splitting of d-shell electrons Photon Energy (ev)

11 The technique of X-ray Magnetic Circular Dichroism (XMCD) LEED Electromagnet I XMCD Σ M Ar ion gun UHV e-beam growth chamber X-ray beam Polarized X-rays electrons Electrometer +/- H app Sample in Parallel and Perp positions

12 XMCD: Element specific magnetometry 12 6 Absorption Intensity ML Fe 8 ML fcc Co Cu(1) Fe Co XMCD Intensity Photon Energy (ev) -4

13 Spin and Orbital Contributions to Magnetic moment Total moment is M=(L + 2S)µ B. For (Fe, Co, Ni), L/2S is about 1/1, so the orbital moment is small, but. Orbital moments contribute signficantly to the magnetic anisotropy (spin-orbit) Surface moment Interface moment Orbital moments may be enhanced at surfaces or interfaces.

14 Dichroism sum rules M o = 4 h 3 M + 7M = 2h S D L 2, 3 M dω ( σ + σ ) dω + L L 2, 3 σ σ dω 2 σ dω M L 3 2 M ( σ + σ ) dω + L 2, 3 M orbit = L 3 + L 2 M spin = L L 2 Absorption Intensity L 3 L 2 M S Mo + 7M D σmdω 2 L2, 3 = 3 σ dω 2 σ dω L M L 3 2 M Photon Energy (ev)

15 XMCD of Magnetic properties of ultrathin films 4 2 Normalized XMCD Intensity at L 3 (%) Cu Ni Cu(1) Ni Cu(1) Schulz and Baberschke * have already determined K I +K S (Interface plus surface anisotropy) to be -.38 ergs/cm 2 Note this favor in-plane M Using this value and the results presented here we determine that K I =-.16 ergs/cm 2 K S =-.22 ergs/cm 2 This means: both interface and surface anisotropy are negative Ni thickness (ML) * PRB (1994).

16 XMCD Magnetometry of Ultrathin films Main features are film independent Coercivities rise sharply near the critical thickness Co L XMCD (%) Co/Cu(1) 18.4 ML 13.9 ML 1. ML 5. ML 1.8 ML Applied Magnetic Field (Oe) Coercive field (Oe) 8 4 Cu(1) Co d =12 ML c Coercive Field (Oe) Fe Pd(1) Coercive Field (Oe) ML Co thickness (ML) Fe Thickness (ML) Ni film thickness (ML)

17 XMCD Microscopy DETECTOR PROJECTIVE (A) (B) Stigmator/ Deflector 5µ INTERMEDIATE Pinhole X-RAYS OBJECTIVE Linear polarization: Topographical information Circular polarization difference image: Magnetic bit information SAMPLE

18 Step bunches can explain anomalous uniaxial anisotropy Two different sites for atoms near steps - strong uniaxial anisotropy terrace - weaker uniaxial anisotropy (non-zero) A - fully magnetized B Magnetization (arb. units) A B C D C Magnetic Field H (Oe) D - rotate the step moments full magnetization Magnetization Reversal along anomalous axis: Schematic explaining the magnetization reversal along the anomalous easy axis of magnetization for miscut fcc Co films. There are two spin sites, a terrace site which has a weak uniaxial anisotropy which may or may not be zero and a step site which has a strong uniaxial anisotropy.

19 XMCD microscopy of step bunch domains Co/Cu ultrathin films 11 Easy axis 2 µ 2 µ Ordinary easy axis Spontaneous domain formation 1µ 1 µ Anomalous easy axis Hard-axis magnetization