Association

70 4. Results I: Single-crystalline epitaxial EuO thin films on cubic oxides
(a)
160x10 3
intensity (counts)
110
60
bulk vs. ultrathin EuO on cYSZ(001)
Eu 3d hv=5.9 keV, normal emission,
dE=0.2 eV, T=63 K
Eu 2+ 3d 3/2
ultrathin (1 nm) EuO
bulk-like (4 nm) EuO
Si
plasmon
(Eu 3+ )
Eu 2+ 3d 5/2
1160 1150 1140 1130 1120
binding energy (eV)
(b)
bulk vs. ultrathin EuO on cYSZ(001)
50x10 3 hv=5.9 keV, normal emission,
dE=0.2 eV, T=63 K
intensity (counts)
30
Eu 4d
ultrathin (1 nm) EuO
bulk-like (4 nm) EuO
7
DJ
(J=1..5)
9
DJ
(J=2..6)
145 140 135 130 125
binding energy (eV)
Figure 4.12.: HAXPES of ultrathin single-crystalline EuO/cYSZ (100) recorded at 63 K. Large crosssections
of Eu 3d and 4d levels in (a) and (b) permit one to resolve even small intensity changes
in the multiplet structure.
peaks at low binding energy, as depicted in Fig. 4.11c. They resolve a complex multiplet
structure. The 4d orbital shares the same principal quantum number with the 4f valence
level which involves a strong d–f exchange interaction due to radial overlap. Therefore, we
cannot separate 4d spectra into 4d J=3/2 and 4d J=5/2 components, but rather to a J = L − S
multiplet splitting, and assign the peaks to the 7 D J and 9 D J multiplets, respectively. 114,115
Eu 4d final states are well described by the intermediate coupling scheme (LSJ) in which
both spin-orbit and exchange splitting are treated as perturbations. 112 This leads to an intensity
ratio of about 1:3 for the septet to the nonet peaks 161 of the final state configuration
∣
∣4f 7 : S7/2〉∣ 8 ∣∣4d
〉∣ 9 : 9 D J , 7 D ∣∣ɛl
〉
J . Once more, we observe a clear Eu 2+ valency in EuO, indicative
for the integral divalent state of EuO.
In the HAXPES spectrum (Fig. 4.11c), the fine structure of the 7 D final state is not resolved,
whereas the J = 2–6 components in the 9 D state are easily identified. Energy losses due to
the excitation of the 5p or the 4f electrons during photoemission are predicted to lead to
satellites at 5–10 eV higher binding energy from the main line. 162 . Furthermore, smaller
contributions of the 7 D final state exist at up to 20 eV higher binding energy than the 7 D
main peak. 114 However, these higher binding energy features overlap within intensities of
an extrinsic plasmon of silicon and the background of inelastic electron scattering.
In a further step, we investigate the two core-levels from ultrathin single-crystalline EuO, and
compare a bulk-like EuO thin film (4 nm) with a quasi-two dimensional EuO layer (1 nm).
Such thicknesses are typical for tunnel barriers in future spin-selective tunnel contacts. In
Fig. 4.12a, b, the HAXPES spectra of Eu 3d and Eu 4d core-levels are taken at LN 2 temperature
in order to reduce thermal broadening. We observe a line shape of the core-level spectra
comparable with thicker EuO, even the multiplet splitting originating from the single atomic
angular momenta J is resolved with an identical distribution of spectral weight.
In the spectra of 1 nm EuO, a small deviation can be observed at 8 eV shift towards higher
binding energy with respect to the main multiplet features. We observe this small feature
consistently in both the Eu 3d and 4d spectra, and identify this as chemical shift of Eu 3+
ions. 1 An explanantion may be ionic oxygen provided by the cYSZ substrate, to which the
ultrathin EuO layer reacts sensitively with oxidation.
In conclusion, we investigated Eu core-levels of heterostructures with single-crystalline EuO
thin films in the range 1–20 nm by HAXPES. We selected the Eu 3d and the 4d orbitals for