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XAS end station

X-ray absorption spectroscopy method

XAS in the soft X-ray energy range is often termed NEXAFS (Near Edge X-Ray Absorption Fine Structure). This spectroscopy provides elemental and chemical specificity, as well as sensitivity to polarization effects related to magnetic and crystal structure of the materials by means of XMCD and XMLD effects.

Equipment

The XAS end station is a UHV system equipped with two-chambers (one for spectroscopy and other for preparation).

The preparation chamber, directly attached to the spectroscopy chamber, is as a standard equipped with:

  • Vacuum load-lock;
  • LEED/AES Spectrometer;
  • Metal vapour sources (Fe, Co, Au, Cr) controlled with a quartz crystal balance;
  • Broad beam ion gun (Ar);
  • O2 dosing.

Sample requirements

  • UHV compatible;
  • Samples (powder and bulk) fitting a standard flag-style (an Omicron type plate) sample holder;
  • Should not charge under X-ray illumination.

Measurement conditions

  • Total electron yield (TEY) detection mode, giving an information depth of several nm;
  • Spectrum acquisition time from minutes to hours;
  • Available external magnetic field of 0-0.2 T parallel to the beam;
  • Sample environment from UHV to high vacuum. Special conditions like He exchange gas (maximum 10 mbar) can be also available but rarely and it depends on actual beamline status. Due to that it is not offered as standard sample environment. Such a need has to be explicitly discussed with beamline scientists well in advance;
  • Sample temperature 20 K - 420 K for experiments in high vacuum or UHV conditions. For exchange gas environment limiting heating and cooling is possible. In the preparation chamber 150 K - 2000 K.

Measurement geometry

The magnetic field up to 0.2 Tesla along the beam direction can be applied. The sample can be rotated about the vertical axis +45/-20 degree in the centre of the electromagnet. The electromagnet coils are retractable, what increases available rotation range to ± 90 degree.

Sample holder

The drawings below show the omicron-type plates (flag style) used as sample holder The marked area determines the maximum possible size of the sample. Minimum size is limited by the beam spot size at XAS station which is 2 mm x 2 mm.