X-ray absorption spectroscopy (XAS):
The absorption of X-rays is an element-specific analysis method. Soft X-rays between 50eV and 2000eV photon energy are typically used for spectroscopic studies. To obtain surface-sensitive and element-specific information on thick samples one can measure the total electron yield (TEY) which is the current necessary to keep the sample at fixed potential. The use of polarized X-rays allows in addition to assess the magnetic properties of the sample surface. The polarization dependent cross sections of magnetic samples are exploited in X-ray magnetic linear dichroism (XMLD) and X-ray magnetic linear dichroism (XMCD).

Principles of XMCD:
XMCD combines sensitivity to magnetic polarization with element specificity and surface sensitivity. Fig.1 shows X-ray absorption spectra measured as total electron yield for two circular polarizations (blue and red trace). The spectra cover the L₂ and L₃ adsorption edges of Fe. Both edges exhibit circular dichroism as is evident from the dependence of the spectrum on beam polarization (sigma⁺ and sigma^- ). The difference between the two traces provides the XMCD signal shown in Fig.1 as the black trace below. The peak areas A₃ and A₂ (Fig.1) are a measure for the magnetic polarization of the sample projected on the direction of the incoming light. Sum-rules [B.T. Thole et al.; Phys.Rev.Lett. 68 (1992) 1943] allow a separation of orbit and spin magnetic moments. When the number of holes in the electronic state is known, even absolute magnetic moments per atom can be determined. In experiment, an external magnetic field parallel to the incident light is used to magnetize the sample. Hysteresis curves are measured by ramping this field while recording the magnetisation through the XMCD signal. The method can be used to monitor also the magnetic splitting of electronic states on elements which do not exhibit ferromagnetism in their bulk. As an example, we succeded to find magnetic polarization on Pt in a FePt alloy containing only 0.5 monolayers (ML) of Fe.

Figure 1: XAS spectra (top) of the Fe L₃ and L₂ absorption edges and the corresponding XMCD spectrum (bottom). The sample is an ultra-thin Fe film evaporated on Pt(997).

We did measurements in collaboration with the groups of C.Carbone (ISM-CNR, Trieste) and K.Fauth/G.Schütz (MPI for Metal Research, Stuttgart) at the synchrotrons ELETTRA (Trieste), ESRF (Grenoble), and BESSY II (Berlin). In the experiments the high intensity of the synchrotron light is essential to push the detection limit for adsorbates to lowest values. Only with this light source magnetic studies of nanostructures become possible. Our studies allowed the analysis of Fe structures even at coverages around 0.15ML (BESSY beamline PM-3) and below 0.05ML (ESRF beamline ID 08).