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84 4. Results I: Single-crystalline epitaxial EuO thin films on cubic oxides
4.4. Summary
In order to provide high-quality ultrathin EuO for our fundamental studies, we synthesized
EuO thin films from bulk-like thicknesses down to one nanometer by reactive molecular
beam epitaxy specialized for oxide growth (Oxide-MBE).
The EuO thin films are of textbook-like single-crystalline quality, as confirmed by electron
diffraction techniques. We developed a procedure to obtain a certain metallicity in YSZ substrates
(“conductive YSZ”), which allows for experimental techniques involving high electron
yields, e. g. hard X-ray photoemission spectroscopy (HAXPES) or transmission electron microscopy
(TEM). On conductive YSZ, we confirmed epitaxial layer-by-layer growth of EuO.
A sharp and well-defined EuO/cYSZ interface as well as a homogenous crystallinity of the
EuO film are observed by high-resolution TEM. Our magnetic oxide EuO thin films exhibit a
bulk-like magnetization down to 2 nm thickness. The stoichiometric chemical quality of EuO
in Si/EuO/cYSZ (100) heterostructures was confirmed by HAXPES for various thicknesses.
Even for 1 nm ultrathin EuO, no valence change or interface shifts are identified from Eu
core-level peaks. We conducted an advanced magnetic characterization by magnetic circular
dichroism (MCD) in core-level photoemission, providing insight into the intra-atomic exchange
coupling of EuO. This reveals large MCD asymmetries of up to 44% in the Eu 3d and
49% in the Eu 4d photoemission multiplets. The temperature dependence of the MCD describes
a Brillouin function with T C = 64 K. Thus, EuO thin films on conductive YSZ provide
a fundament for ideal EuO heterostructures of reference quality. This allows to explore advances
of selected physical properties of ultrathin EuO layers – like a tuned ferromagnetism.
Biaxial lateral strain applied to single-crystalline EuO is of fundamental interest, since it
alters the electronic structure and magnetic coupling in a controlled way. In order to investigate
epitaxial EuO under 4.2% tensile biaxial strain, we synthesized high quality EuO/LAO
(100) heterostructures by Oxide-MBE. A seamless heteroepitaxy of EuO is observed by electron
diffraction. For 16 nm epitaxial EuO/LAO (100), reciprocal space maps reveal the adaption
of the lateral LAO lattice parameter, while the perpendicular parameter c of EuO is the
unchanged EuO bulk value, which corresponds to a Poisson ratio of ν EuO ≈ 0. While the
magnetization of 4 nm EuO/LAO (100) heterostructures shows a Brillouin-shape as expected
for a Heisenberg ferromagnet with a bulk-like saturation moment, the Curie temperature is
reduced by 12.3 K with respect to the EuO reference value. However, a EuO/LAO (100) heterostructure
of bulk thickness shows a T C reduction of only 1.7 K. Therefrom we anticipate,
that a large thickness in the unstrained z dimension provides sufficient long range magnetic
exchange for a bulk-like Curie temperature in laterally expanded EuO. MCD asymmetries
in Eu 3d and 4d core-level photoemission multiplets show a maximum asymmetry of 58%
relative to the unstrained EuO reference. This reduction is significantly larger than expected
from SQUID measurements, which we interpret as a larger effect of the tensile strain on
intra-atomic exchange (MCD) than on the long range order of the 4f 7 magnetic moments
(SQUID).