X-Ray Microscopy Techniques L. J. Heyderman The Swiss Light Source, - PowerPoint PPT Presentation

1 X-Ray Microscopy Techniques L. J. Heyderman The Swiss Light Source, Paul Scherrer Institut

3 • The electrons are accelerated close to the speed of light in a linear accelerator and injected into the storage ring • Bending magnets or insertion devices (wigglers or undulators) cause electrons to bend or wobble through the section and emit light. Reference energy: 2.4 GeV L. J. Heyderman Circumference: 288 m Current: 350 mA ( 400 mA) 18 Beamlines

X-ray Absorption Spectroscopy • X-rays excite an electron from the core to the valence band E Photo • Relaxation of the electron from h ν electron the valence to the core level gives: Auger electron Soft x-rays: more Auger es Hard x-rays: more fluorescence h ν • Therefore different interactions: E vac more than just imaging E F Valence band Fluorescence L. J. Heyderman Core levels

X-ray Absorption Spectroscopy core level valence band 2p 1/2 2p 3/2 d states Spectrum: change energy & s,p states observe absorption: ~ ~ energy E F Absorption • Peak corresponds to (set of) transition(s) from core level to valence band • Density of unoccupied states above Fermi level L. J. Heyderman • Each element: own characteristic peaks photon energy

7 PEEM & TXM Photoemission secondary electrons h ν Electron vacuum (indirect measure of Microscopy absorption) (PEEM) to probe sample surface Electron surface / interfaces escape depth 5 nm ~ Photon penetration sample depth 50 nm ~ Transmission X-rays L. J. Heyderman X-ray Microscopy (direct measure of (TXM) absorption)

8 Photoemission Electron Microscopy Slow electrons: mean free path is submono to several monolayers (few nm’s) Surfaces, thin films and interfaces ….consequences for electron optics. L. J. Heyderman Frithjof Nolting, Swiss Light Source

9 The Surface and Interface Microscopy (SIM) Beamline SIM Beamline, Swiss Light Source Close-up of the PEEM L. J. Heyderman The Photoemission Electron Microscope (PEEM)

10 • Channel plate: amplifies electrons Spectromicroscope • Phosophor screen: converts to light • Image with CCD camera LEED/LEEM L. J. Heyderman L.H. Veneklasen: Ultramicroscopy 36 (1991), 76 Image courtesy of S. Heun (ELETTRA) Elmitec Elektronenmikroskopie GmbH Clausthal-Zellerfeld, Germany

12 Slow Electrons Probe : slow electrons Imaging : high energy electrons (more stable and maintain spatial information) Lens Equivalent has two functions: accelerating field due to potential & focussing function High voltage: 0 eV • reduced sensitivity 20 keV 20 keV to external magnetic fields • reduced energy spread and smaller electron beam Sample High Voltage / diameters “integral part of lens” Ojective Lens Immersion lens: electrons have before and after the lens different L. J. Heyderman velocity (different wavelength) Cathode lens: Sample is cathode electron microscope is anode

13 Two Kinds of Aberrations Spherical Chromatic Beams parallel away from the Different wavelengths of lens axis are focused in a slightly light are focused to different place than beams close different positions. to the axis and therefore a Light  electrons L. J. Heyderman blurring of the image. Glass  electrostatic/magnetic lenses http://en.wikipedia.org/wiki/Lens_(optics)#Spherical_aberration

14 Energy Filter To remove chromatic aberations: Electron energy E Electron energy E+ ∆ E Aperture cuts off transmission of electrons with higher energy 400 1250 350 1200 300 Energy distribution is 1150 500 250 1100 narrowed but 200 200 1050 100 transmission 75 150 1000 50 (intensity) is reduced. 25 100 950 10 L. J. Heyderman Therefore need to find 50 900 compromise. 0 850 0 5 10 15 energy (eV)

15 Effect of aperture size on resolution • Spatial resolution depends on aperture size - limits pencil angle of transmitted electrons and transmission Highest resolution is achieved with 12 µ m aperture for PEEM2 • Aperture 50 µ m 20 µ m 12 µ m 2 mm diameter 5 µ m L. J. Heyderman 1s 4.2 s 0.4 s 10 s Exposure time 39 % 9 % Transmission 100 % 4 %

16 Spatial Resolution for Magnetic Imaging PEEM with X-rays: 50-20 nm spatial resolution Aberration-corrected instruments using an electron mirror: SMART (spectromicroscope for all relevant techniques) • at BESSY II, Berlin, Germany • collaboration of seven Universities in Germany PEEM III • at ALS, Berkeley, USA • mainly ALS L. J. Heyderman down to a few nm spatial resolution

17 Photoemission Electron Microscope Armin Kleibert SIM beamline (SLS) Carlos Vaz analyzer • elemental composition • chemistry • structural parameters • electronic structure • magnetic properties • topography Magnetic Co lenses e - Fe La Ti L. J. Heyderman 20 kV 16° Elemental Contrast

18 Photoemission Electron Microscope Armin Kleibert SIM beamline (SLS) Carlos Vaz analyzer • elemental composition • chemistry • structural parameters • electronic structure • magnetic properties • topography Magnetic Topographical Contrast lenses e - L. J. Heyderman 20 kV 16° Microfocussing due to distortion of the local electric fields

19 Photoemission Electron Microscope Armin Kleibert SIM beamline (SLS) Carlos Vaz analyzer • elemental composition • chemistry • structural parameters • electronic structure • magnetic properties • topography Magnetic lenses Element Specific e - CCD Antiferromagnet Interfaces L. J. Heyderman 20 kV Time Resolved 16°

20 X-Ray Magnetic Circular Dichroism (XMCD) • L-edge absorption in d band transition metal • Magnetic metal: d valence band E Fermi split into spin-up and spin-down with different occupation ∆ E ~ 1eV • Absorption of right/left circular ∆ l = ±1 polarisation: light mainly excites ∆ s =0 spin-up/down photoelectrons • Spin flips forbidden: measured resonance intensity reflects 2p 3/2 number of empty d-band states of 2p 1/2 a given spin L. J. Heyderman Spin - up Spin - down • Can determine sizes and directions of atomic magnetic moment G. Schütz et al. PRL (1987)

21 Time for a game…. L. J. Heyderman

22 Magnetostatic or Stray Field Energy L. J. Heyderman

23 Rectangle, Square, Disk M H D L. J. Heyderman

24 Rectangle, Square, Disk M H D L. J. Heyderman

25 Rectangle, Square, Disk M H D L. J. Heyderman

26 Rectangle, Square, Disk M H D L. J. Heyderman

27 X-ray Magnetic Circular Dichroism (XMCD) XMCD ~ M cos( M , S ) TEY (a.u.) S L 3 L 2 Square Ferromagnetic Element: Landau Domain Pattern 775 780 785 790 795 800 805 Photon energy (eV) Polarisation: Polarisation: circular plus circular minus L. J. Heyderman 4 µm Magnetic contrast reverses

28 Rectangle, Square, Disk M H D L. J. Heyderman

29 Ring Vortex Onion L. J. Heyderman

31 Interacting Magnets….. ….with the help of some frogs….! L. J. Heyderman

32 Interacting Magnetic Frogs L. J. Heyderman

34 Ring of Nanomagnets L. J. Heyderman

35 Rings of Nanomagnets ? L. J. Heyderman

36 Artificial Spin Ice in PEEM Two rings Three rings One ring 500 nm L. J. Heyderman X-ray direction

37 X-ray Magnetic Linear Dichroism (XMLD) XMLD ~ < M 2 > cos 2 ( M , E ) E B TEY (a.u.) E A 720 722 724 726 L 2 L 3 705 710 715 720 725 730 Photon energy (eV) L. J. Heyderman 2µm L 3a lin. vert L 3b lin. vert

38 Undulator Magnetic Structure: changing phase, changes polarisation symmetric circular left or right shift π /2 asymmetric shift 0 linear 0 - 90 o L. J. Heyderman horizontal

39 Brief Examples L. J. Heyderman

40 Element specific contrast Cobalt lines Co L 3 Ni Fe Co 5 µ m 700 800 900 Photon Energy (eV) Permalloy film Fe L 3 Cobalt lines Permalloy film Substrate L. J. Heyderman Coupling of hard and soft magnetic layer: L. Heyderman, A. Fraile-Rodriguez, A. Hoffmann

42 Strain and Magnetic Domains M 010 → Light Stripes M 100 5 µm Fe X-ray Linear Dichroism Fe X-ray Magnetic Circular Dichroism Fe XLD: strain domains CoFe 2 O 4 → Dark Stripes Fe XMCD: magnetic domains BaTiO 3 Substrate-induced strain L. J. Heyderman M 001 ~ 0 strongly modifies magnetic anisotropy RV Chopdekar et al. PRB (2012)

43 Antidot Arrays – Basic Domain Configuration w d A A A A A A A A A A A A p H A L. J. Heyderman

44 Remanent Hysteresis Loop in Antidot Arrays MOKE cobalt Observe magnetisation reversal in applied magnetic field: PEEM Spins “Magnetising Holder” to Spins 400 Oe S L. J. Heyderman 2µm L. J. Heyderman, F. Nolting, D. Backes, S. Czekaj, L. López-Díaz , M. Kläui, U. Rüdiger et al

45 Cobalt Antidot Arrays a c 2 µ m MSD b 200 nm MSD L. J. Heyderman L. J. Heyderman et al., APL (2003), JAP (2004), PRB (2006), JMMM (2007) Mengotti et al., JAP (2007)

47 Switching with a Heat Pulse Only  200 nm domains in 400 nm GdFeCo nanostructure Switching Experiments ¤ ⊗ a L. J. Heyderman L. Le Guyader et al., APL (2012) T. A. Ostler et al., Nature Communications (2012)

48 Thermally Active Switching a L. J. Heyderman A. Farhan et al. Nature Physics (2013), PRL (2013) & PRB (2014)