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4.1. Coherent growth: EuO on YSZ (100) 71
an investigation at low temperature yielding high resolution spectra. Multiplet structures (J
final states splitting) in agreement with literature were resolved, and exclusively the Eu 2+
oxidation state and an unaltered multiplet structure was observed for EuO films of 20, 4 and
1 nm thickness (with a small deviation for 1 nm EuO). Here, the HAXPES analysis of buried
EuO thin films proves its potential to characterize the chemical properties and detailed multiplet
structure of a buried layer of a spin-functional magnetic oxide.
4.1.4. Magnetic circular dichroism of single-crystalline EuO/cYSZ (100)
Magnetic circular dichroism is of fundamental interest, as its analysis can elucidate the
intra-atomic interaction between core-levels and the magnetic open shell. Moreover, its
Boltzmann-averaged amplitude is a direct measure for the magnetization of the EuO thin
film. This section summarizes the first successful X-ray magnetic circular dichroism experiment
in core-level photoemission (MCD-PE) on EuO thin films. We investigate singlecrystalline
epitaxial EuO thin films on conductive YSZ (100). This system allows one to investigate
a magnetic oxide with 4f spin-only magnetic order, textbook-like crystalline quality
and chemically well-defined and atomically sharp interfaces.
Magnetically oriented 8 S J states in the Eu 4f orbital form the basis of ferromagnetism in the
magnetic oxide EuO. These oriented spins interact via d–f exchange with core-levels like
4d and 3d, both of which showing a resolvable final state multiplet structure. The J final
state components of these core-levels are of largely different strength, as presented in the last
section. This is the basis for the observation of magnetic circular dichroism in photoemission
final states of these core-levels, as introduced theoretically in Ch. 2.5. We conducted MCD-
PE measurements of Eu 3d and 4d core-levels at beamline P09 of the high-brilliance facility
PETRA III (see Ch. 3.4.3). The polarization vector of the circularly polarized light q and the
magnetization of the thin film M are collinear during the photoemission experiment.
In Fig. 4.13, MCD-PE spectra of Eu 3d and 4d core-levels are summarized. The spectra clearly
resolve the single final states of m J =2..5 and m J =1..6 for the 3d3/2 and 3d5/2 mutliplets, where
jj-coupling is assumed. The most dominant splitting of single angular momenta is observed
in the 4d final states 9 D J , J =2..6, where LS-coupling is assumed. The resolved J components
match the energy positions of calculated values from literature very well, as illustrated with
grey markers. 110,114,115
Consistently in the MCD of Eu 3d and 4d core-levels, we identify the largest MCD asymmetry
for the final state components with largest m J . The MCD difference can be interpreted as
the product of core-shell angular momentum l after photoexcitation with the spin moment
S of the open 4f shell, therefore 〈l, S〉. When we identify the sign of the MCD asymmetry as
vector of l, then we can correlate its direction with the fixed spin moment of S 4f , which is
indicated in the MCD spectra (Fig. 4.13). We recognize, that the multiplet lines with largest
m J have parallel angular orientation with S 4f . At a minimum temperature of the sample
(T min = 33 K), maximum MCD asymmetries of 44% for Eu3d and 49% for Eu4d could be
observed for these maximum J lines. The alignment of multiplet lines reverses with lower
J: The m J =1..4 lines and the multiplet satellite of 3d 5
2 are antiparallel with respect to S 4f
For intra-atomic coupling of angular momenta in Eu core-levels, please see Ch. 2.5.2.