Association

4.2. Lateral tensile strain: EuO on LaAl 3 (100) 77
Figure 4.18.: Axial and transversal elasticity of EuO on
LAO (100). The Poisson ratio ν can be calculated as
the ratio of axial and transversal change of length in
the crystalline thin film, in first order approximation.
16 nm
for the 16 nm EuO thick film of νEuO
=(0.01 ± 0.04). The uncertainty arises from the broadening
of the RSM peak of EuO/LAO (100) in Fig. 4.17b. Within the error bars, we consider
the Poisson ratio to be positive but near zero. When compared to other oxides (e. g. bulk
SiO 2 , ν =0.2), this small Poisson ratio is a remarkable feature of a strained EuO thin film
epitaxially integrated on LAO (100). Using the definition (4.2), we can state, that the volume
elasticity of the strained EuO thin film is extremely small (≈0).
The broadening of the two-dimensional diffraction peaks (Fig. 4.17c) allows one to judge the
crystal quality (according to Fig. 4.17 on p. 47). For LAO (2 0 3), the broadening is caused
by a certain structural inhomogeneity, yet we consider this value as small due to the very
high-quality surface of LAO as observed by in situ electron diffraction. A broadening is also
observed for the EuO (3 0 4) peak, which is interpreted as in-plane mosaicity. This means,
crystal domains are rotated with respect to neighboring domains by a small angle around an
axis normal to the surface. However, this broadening is as small as for the LAO substrate
which qualifies the mosaicity of EuO to be small, too.
In conclusion, our structural investigations of the strained epitaxial EuO/LAO (100) heterostructures
prove the superior crystal quality of EuO. EuO/LAO (100) constitutes a magnetic
model system which is stretched by 4.2% biaxial strain exclusively in two dimensions
and unchanged in the z dimension.
Magnetic characterization of epitaxial EuO thin films on LAO (100)
A magnetic characterization of the EuO/LAO (100) heterostructure by SQUID is given in
Fig 4.19 for two thicknesses of EuO, d = 4 nm and bulk-like d = 30 nm. The temperature
dependent magnetization shows a Brillouin-shaped curve without any features of antiferromagnetic
or paramagnetic phases. For ultrathin (4 nm) EuO/LAO (100), the Curie temperature
is clearly determined at 57 K, which is a reduction of 12 K according to the bulk value of
single-crystalline EuO/YSZ (100). This reduction is due to the lateral strain effect rather than
reduced dimensionality of the EuO slab, which is easily recognized when compared with the
thickness dependence of unstrained EuO/YSZ (see Fig. 4.6 on p. 65). The switching of the
magnetic moment due to magnetic field reversal is a measure for the crystal homogeneity.
For the strained EuO/LAO (100), the coercive field is 74 Oe which is double the minimum
value obtained for optimum EuO/YSZ (100). One explanation can be the in-plane mosaicity
determined by the reciprocal space map analysis. Moreover, the magnetization switches
gradually, as indicated by the rounded shape of the hysteresis, while for unstrained EuO/YSZ
rectangular shapes of the hysteresis are observed (see Fig. 4.6). A magnetic saturation moment
of nearly the bulk EuO value (7μ B expected) is observed at T =2K.