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3.4. Ex situ characterization techniques 55<br />
This expression is applied to determine the thickness of a possible reaction layer in EuO/Si<br />
heterostructures (e. g. EuSi 2 ) by the evaluation of Eu core-level spectra by HAXPES in this<br />
work.<br />
Choosing the excitation energy and a background treatment<br />
In the experiment, the excitation energy should be selected in a way to optimally investigate<br />
the buried interface of the EuO/Si heterostructures. In order to limit the information depth<br />
of Si photoemission to the top layer of the Si substrate, a reduced excitation energy is applied<br />
(e. g. the beamline minimum of the HAXPES endstation), while a high photon energy (hν <br />
4 keV) is chosen to fully record the photoemission of buried Eu atoms at the bottom of the<br />
EuO slab.<br />
Inelastic processes of photoelectrons in the solid generate secondary electrons and thereby<br />
generate a broad background intensity. Only photoelectrons carry the initial state information<br />
which are not inelastically scattered. In order to evaluate sharp peaks of unscattered<br />
photoelectrons quantitatively, it is necessary to define and subtract a realistic background of<br />
inelastically scattered photoelectrons. Therefore, we use the approach of Tougaard (1988) in<br />
this thesis, to simulate the inelastic background of secondary electrons, which is best suitable<br />
for inelastic backgrounds of rare earth compounds. 141–143<br />
3.4.5. Hard X-ray circular magnetic dichroism in photoemission<br />
Magnetic circular dichroism using hard X-ray photoemission spectroscopy (MCD-PE) combines<br />
the element-selective sensitivity with a magnetic contrast (intra-atomic exchange interactions:<br />
MCD) and a largely selective information depth up to ∼20 nm. Hence, MCD-PE is<br />
perfectly suited to probe buried interfaces of ultrathin magnetic layers.<br />
If a core-level shows an unequal population for every M quantum number, and the solid has<br />
magnetic order, then the magnetic circular dichroism (MCD) effect can occur. The photoemission<br />
variant of MCD is attractive, because it does not involve a spin-polarization analysis<br />
of the photoelectrons, but only a measurement of their intensity. Unlike X-ray absorption<br />
experiments, photoemission spectroscopy is usually (at least partially) constrained to certain<br />
emission angles. The geometry of an MCD experiment in photoemission is completely<br />
described by four vectors,<br />
{q, M, k e− , n}, (3.20)<br />
where q and M are the magnetic directions of the circularly polarized light and the sample,<br />
respectively. k e− is the photoelectron wave vector and n denotes the surface normal. The<br />
question, whether an MCD asymmetry in photoemission is observable at all, is answered by<br />
a rule given by Feder and Henk (1996): 144<br />
MCD in photoemission can exist, if there is no space-symmetry operation which<br />
only reverses the magnetization M but leaves the system unchanged otherwise.<br />
A discussion on forbidden geometries in order to observe MCD in photoemission is given in<br />
Starke (2000), 105 it comes out that only normal incidence combined with normal emission<br />
for an introduction of the MCD effect, please see Ch. 2.5.