Association

78 4. Results I: Single-crystalline epitaxial EuO thin films on cubic oxides
Figure 4.19.: Magnetic bulk properties
of epitaxial EuO on LAO
(100).
A Brillouin-shaped magnetization
curve is observed for
EuO/LAO (100) heterostructures
with 4.2% biaxial tensile
strain. The ordering temperature
for 4 nm EuO/LAO (100)
is reduced by 12 K with respect
to coherently grown epitaxial
EuO/YSZ.
A reduced dimensionality of the ultrathin EuO/LAO heterostructure may also affect the magnetic
ordering temperature, and thus we additionally investigate a bulk-like 30 nm EuO/LAO
(100) heterostructure, for which we have confirmed a persistent adaptation of the LAO lateral
lattice parameter, by SQUID (green curve in Fig. 4.19). The shape of the temperaturedependent
magnetization coincides with that of ultrathin EuO/LAO (100), however, the T C
is determined at 67.6 K, a reduction of 1.4 K with respect to unstrained bulk-like EuO/YSZ.
Thus, the reduction of long range magnetic order can almost completely be compensated
in laterally expanded EuO, when the unstrained z dimension is provided with a bulk-like
thickness of 30 nm.
Summarizing, the magnetic behavior of ultrathin EuO under 4.2% lateral tensile strain is
comparable to the bulk EuO behavior, only the T C is reduced by 12 K, which mainly originates
from the reduced J 1 coupling (eq. (2.9)) due to the increased distance of in-plane nearest
neighbors. This reduction can almost completely be compensated if a bulk-like thickness in
the unstrained vertical dimension is provided.
X-ray magnetic circular dichroism of epitaxial EuO on LAO (100)
Finally, we investigate 3 nm single-crystalline EuO on LAO (100) by means of the MCD effect
in core-level photoemission spectroscopy. In Fig. 4.20, the multiplets of Eu 3d3/2 and 4d photoemission
are well-resolved, similar to the reference sample EuO/cYSZ (100) (in Fig. 4.14
on p. 73). The maximum MCD amplitude is determined by the final states with largest m J
and is 8% for Eu 3d3/2 and 22% for Eu 4d. We compare these MCD amplitudes directly with
the reference heterostructure EuO/cYSZ of the last section, and find that the current strained
EuO/LAO heterostructure shows 44% and 58% maximum MCD of Eu 3d and 4d core-levels
with respect to the unstrained reference heterostructure. Thus, by tensile strain the intraatomic
coupling of Eu 2+ ions in EuO is more reduced in 3d 9 final states than in 4d 9 . This
may be explained by the weaker exchange coupling between 3d 9 and the magnetic open shell
4f 7 with configuration 8 S7/2.
Moreover, from magnetization curves by SQUID, we obtain a ratio of magnetization at T =
33 K of 84% when EuO/LAO (100) is compared with the reference sample EuO/YSZ (100)
(as seen from Figures 4.19 and 4.6). Thus, the MCD asymmetry (less than 50% with respect
to the reference sample) responds more sensitively on lateral tensile strain than the averaged