Association

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