Association

74 4. Results I: Single-crystalline epitaxial EuO thin films on cubic oxides
m J . For 3 nm single-crystalline EuO/cYSZ (100), this maximum MCD difference is 18% for
Eu3d3/2 and 38% for Eu4d. These MCD values reach 95% of those measured from a 15 nm
thin film of single-crystalline EuO presented in Fig. 4.13. Thus, a bulk magnetization of
the EuO ultrathin film is almost realized, if we assume that the MCD amplitude is a good
representation of the macroscopic magnetization.
Here, we have presented a reference for single-crystalline unstrained EuO, which we will
compare with EuO under biaxial tensile strain and biaxial compressive strain in the following
sections 4.2 and 4.3.
4.2. Lateral tensile strain: EuO on LaAlO 3 (100)
Although the bulk properties of the EuO films are well-known, lateral strain from the interface
will significantly alter the electronic structure of epitaxial oxide heterostructures. 163
Biaxial lateral strain inherited from the underlying substrate has recently inspired several experimental
studies on epitaxial EuO, which conclude small changes in T C upon small (∼2%)
tensile strain. 27,33,64 Small lateral strain on EuO has been also applied by Liu et al. (2012).
They found a change (±2K) of T C due to the underlying substrate, either CaF 2 or MgO. 61
However, the authors investigated mainly polycrystalline EuO, making a systematic comparison
of the lattice strain difficult.
Taking advantage of the adsorption-controlled EuO synthesis ensuring the stoichiometry and
single-crystallinity (as presented in the last section), in the following we investigate singlecrystalline
EuO layers epitaxially integrated with LaAlO 3 (100) substrates (LAO) by an unchanged
synthesis. This induces 4.2% biaxial tensile strain to EuO, thus extending the Eu 2+
nearest-neighbor distance in the lateral dimension. First, we investigate the structural characteristics
of the strained epitaxial EuO top layer. Furthermore, we present the magnetic
behavior of EuO under lateral tension by LAO (100) using SQUID magnetometry. Finally, we
investigate Eu core-levels of the EuO/LAO (100) heterostructure by an analysis of magnetic
circular dichroism in core-level photoemission (MCD-PE).
Structural characterization of epitaxial EuO on LAO (100)
First, we investigate the structural quality of EuO which is deposited on LaAlO 3 (100) using
the adsorption-controlled growth by Oxide-MBE. The LAO crystal is usually referenced
with a large rhombohedral unit cell with a = b = 5.36 Å, these lattice parameters span the
cubic surface plane and provide 4.2% tensile strain to the EuO top layer. A pseudocubic representation
lattice is used for the quantitative analysis by X-ray diffraction presented in the
following, the conversion is illustrated in the appendix (Ch. A.1).
In Fig. 4.15, the in situ characterization of 16 nm EuO/LAO (100) is summarized. Before
EuO growth, we anneal the LAO (100) substrate similarly to the YSZ substrates (last section)
but with a temperature reduced below the structural displacement transition of LAO
at ∼540 ◦ C. 164 This yields a flat and single-crystalline cubic surface as indicated by sharp
A summary of substrates suited for epitaxial integration with EuO, including strain, is found in Tab. A.2 on
p. 128.