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64 4. Results I: Single-crystalline epitaxial EuO thin films on cubic oxides
Now, we ask ourselves, which mechanisms increase the coercive field in perpendicular direction
of our single-crystalline EuO thin film. One microscopic origin may be anisotropy
of the magnetic Eu 4f orbitals, which have recently been shown to be not spherically symmetric
and thus provide a working point for local pinning in EuO. 152 During layer-by-layer
growth of EuO, the structural neighborhood parallel vs. perpendicular (i. e. in surface normal)
to growth direction may be slightly different, in particular due to the initially and finally
deposited Eu seed layers. These in-plane antiferromagnetic Eu environments may lead
to stronger magnetic pinning. This would yield a larger out-of-plane anisotropy.
We summarize, that the crystal quality of EuO plays a crucial role in magnetic switching behavior,
such that in a single-crystalline thin film the out-of-plane anisotropy is approximately
ten times smaller than for a polycrystalline sample. The out-of-plane saturation moment cannot
be reached with small external field. For the application side, the desired soft magnetic
switching and easily reached saturation magnetization only show up when magnetic fields
are applied in the surface plane (e. g. in [100] direction) of the single-crystalline EuO thin
film.
4.1.1. Thickness-dependent magnetic properties of EuO thin films
(a)
RHEED screen intensity
1.0
0.8
0.6
0.4
0.2
0
start
20
1st ml
40
2nd ml
60
80
process time (s)
off-specular spot
specular spot intensity
T ml = 27 s
4 ml = 1.03 nm EuO
3rd ml
100
4th ml
120
end
140
Figure 4.5.: Electron and X-ray diffraction of a 1 nm-thin EuO film. RHEED oscillations are a measure
for completed atomically smooth net planes (red in (a)), the off-specular intensity indicates begin
and end of synthesis. XRR (b) shows an EuO Kiessig fringe corresponding to 1.25 nm EuO.
The investigation and optimization of single-crystalline EuO in the thickness regime of few
nanometers is essential as a reference for fundamental studies regarding interfacial strain
effects and for EuO employed as ultrathin spin-functional tunnel barriers. † While bulklike
magnetic properties of thick single-crystalline EuO films (d 20 nm) were confirmed
in the last section, now we proceed towards growth and characterization of ultrathin EuO
layers with the aim to provide the same single-crystalline quality. As an evidence for the
successful growth of ultrathin EuO, RHEED oscillations and XRR Kiessig fringes identify four
perpendicular net planes of EuO (d EuO = 1.25 nm) in Fig. 4.5. This means, single-crystalline
EuO thin films ranging from one nanometer up to several tens of nanometers (bulk) are
available by our Oxide-MBE synthesis with persistently high structural quality.
as discussed in upcoming chapters 4.2 and 4.3 of epitaxial EuO on the cubic oxides LaAlO 3 and MgO.
† For the integration of EuO on Si, please see Ch. 5.