Röntgenmikroskopie

Röntgenmikroskopie

Soft X-ray microscopy (SXM)

… uses electromagnetic radiation of 240 to 1.800 eV with wave lengths in the order of  7 nm to 0.7 nm, defining the physical limit of resolution.

… was developed in the 70ties  by the group of Prof.  Schmahl in Göttingen and is a powerful tool  to image  structures of solids by using high brilliant synchrotron  radiation. In combination with spectroscopic methods it provides element specific information on the density distribution  of local chemical, electronic and magnetic structure of solids.

… offers new routes to address a variety of static and dynamic issues in modern solid state science including material science, physics, chemistry and biology due to the unique simultaneous combination of important properties:

  • High penetration  depths  of 100 nm to some µm
  • High resolution down to 9 nm (best case)
  • High temporal resolution down to 10 psec
  • Significant chemical sensitivity Large magnetic cross sections

In the soft x-ray range  nearly any element exhibit a  characteric absorption edge.

Principle of Scanning X-ray Microscopy

X-rays emitted from an undulator as insertion device in an electron (or positron) storage ring is monochromatized and focused onto the sample by a zone plate, whereby the selected order of diffraction is selected by the OSA ring. The sample is raster scanned and the transmitted intensity monitored by a x-ray sensitive detector (i.e. avalanche diode or x-ray sensitive  CCD camera)

About 50 storage rings world wide provide beam time
7 centers (BESSY, Berlin, NSLS, Brookhaven, ALS, Berkeley; CLS, Kanada;   PSI, Schweiz; Elletra, Italien; SSL, Shanghai) run a scanning x-ray microscopy beamline



Polarization and time structure of synchrotron radiation

With an undulator the x-ray polarization can be easily varied by adjusting the magnet structure.

The time structure of the synchrotron radiation (typically flashes of 35 ps pulse widths and 2 ns distance) can be easily be used for pump probe experiments.

MAXYMUS, the new SXM at BESSY

Since 2011 the new endstation MAXYMUS ( MAgnetic X-raY Microscope with UHV Spectroscopy) operated by the MPI-IS is open for users

A view into the microscope MAXYMUS

Properties:

  • UHV conditions 10-9 mbar
  • Zone plate scanning mode
  • Goniometer for tomography 360°
  • Variable magnetic fields up to 0.4 T
  • Wide temperature range 80 – 500 K
  • Photocurrent measurements for nontransparent samples and surfaces

MAXYMUS in operating conditions

  1. Zone Plate Piezo Stage
  2. Zone Plate Holder
  3. Order Separating Aperture
  4. Sample
  5. Photomultiplier
  6. Sample Piezo Stage

Typical Results

Imaging of the magnetic vortex core:

As fundamental spin structure of micro- to nanosized magnetic platelets the magnetic vortex with an in-plane curling magnetization and a perpendicular magnetized core has gained a lot of attraction in the last years From the theoretical micro magnetic point of view as well as a basic structure for spintronics applications.

Taking the advantage of the element-specificity the magnetization of layer stacks with different elemental composition can be visualized independently. This allows to measure the change in perpendicular magnetization of a Gd/Fe layer on top of a PY vortex structure separated by a Al layer by flipping the vortex core in a rotating B-field (see AG Stoll).

Comparison of transmission and TEY mode

Due to the UHV condition the absorption of the x-rays can be also monitored via detection of the total electron yield (TEY). This method, which provides a quasi-3D image similar to SEM pictures, is inherently strongly surface sensitive probing a depth of 1 to 3 nm as demonstrated by a study of agglomerated FexOy/PtFe hybrid particles.

By combining TEY and transmission mode differences of the chemical characteristics of the surface and the bulk can be separated.

In the transmission mode the profile of the absorption spectra are more likely Fe3O4 like. The corresponding TEY absorption spectra prove , that the surface chemical
characteristic is y-Fe2O3 like

 
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