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60 4. Results I: Single-crystalline epitaxial EuO thin films on cubic oxides
4.1. Coherent growth: EuO on YSZ (100)
The difficulty lies in obtaining high quality,
stoichiometric, ultrathin EuO films.
(J. S. Moodera on EuO, 2007)
In order to provide high-quality ultrathin EuO films without any strain or chemical intermixing,
first we stabilize EuO on yttria-stabilized zirconia (YSZ), as discussed in the following.
In response to the extremely narrow parameter regime in which the stoichiometric phase of
EuO can be obtained via reactive MBE, the O 2 supply has to be under a meticulous control.
In Fig. 4.2, we investigate EuO thin films synthesized in an O 2 pressure range around the stoichiometric
EuO phase by electron diffraction and SQUID magnetometry. All oxygen partial
pressures lie in the lower 10 -9 Torr range, coinciding with the stability region for all Eu oxide
phases in the Ellingham diagram (Fig. 4.1 on p. 58).
Overoxidized Eu 1−x O x phases form next to EuO, if 3 × 10 −9 Torr oxygen partial pressure
is supplied (Fig. 4.2a). Here, the Eu 1−x O x film is insulating as observed in LEED, and the
magnetization exhibits mainly paramagnetic contributions of Eu 2 O 3 which are identified by
a tail below 5 K. 147–149
Stoichiometric EuO. A slight reduction of the oxygen partial pressure to values below
1.7 × 10 −9 Torr yields stoichiometric EuO, which exhibits the expected fcc rocksalt lattice
and magnetization curves with a Brillouin shape as expected for a Heisenberg ferromagnet
(Fig. 4.2b–d). We identify this oxygen partial pressure (1.7 × 10 −9 Torr) as the limit, at which
stoichiometric EuO can be obtained (Fig. 4.2b). No excess Eu for the Eu distillation condition
is left in order to ensure the EuO stoichiometry. This is reflected by the formation of a small
fraction of higher oxidized Eu 1−x O x phases in the RHEED pattern of Fig. 4.2b, as well as in
the small paramagnetic feature of the magnetic moment at T → 0 K. In the range of mainly
stoichiometric EuO and Eu distillation growth, we establish the growth at 1.5 × 10 −9 Torr for
stoichiometric EuO with a perfect fcc structure and a magnetization curve identical with a
Brillouin function. Thus, the oxygen supply as presented in Fig. 4.2c is considered as the
optimum for stoichiometric EuO.
Europium-rich EuO 1−x . A further reduction of the oxygen partial pressure also yields mainly
stoichiometric EuO (Fig. 4.2d). We observe even an improved surface crystal structure, as indicated
by the LEED pattern, probably due to the high mobility of the excess Eu during
Eu-rich growth. Nonetheless, this composition does not match the exact stoichiometry of
EuO: in the SQUID characterization, we observe a paramagnetic feature near T = 0 K and
a magnetization tail extending up to T C (EuO 1−x ) = 150 K – both indications of metallic Eu
contributions in the EuO thin film. 15,23
A summary of substrates suited for epitaxial integration with EuO, including strain, is found in Tab. A.2 on
p. 128.