Association

Abstract
In the thesis at hand, we explore fundamental properties of ultrathin europium oxide (EuO) films.
EuO is a model system of a localized 4f Heisenberg ferromagnet, in which the ferromagnetic coupling
– provided a high crystalline quality – can be tuned by biaxial lattice strain. Moreover, the magnetic
oxide EuO is perfectly suited as a spin-functional tunnel contact for silicon spintronics. However, up
to now a challenging bulk and interface chemistry of EuO and Si has hampered a seamless integration
into functional silicon heterostructures.
In order to investigate fundamental aspects of the magnetic and electronic structure of ultrathin EuO,
in the first part of this thesis, we synthesize EuO thin films on conductive YSZ substrates from bulklike
thicknesses down to one nanometer by oxide molecular beam epitaxy (MBE). The EuO thin films
are of textbook-like single-crystalline quality, and show bulk-like magnetic properties. We control the
stoichiometry of buried EuO thin films by hard X-ray photoemission spectroscopy (HAXPES); even a
1 nm ultrathin EuO film exhibits no valence change or interface shifts. Furthermore, we conduct an
advanced magnetic characterization by the magnetic circular dichroism (MCD) of Eu core-levels in
photoemission, this gives us insight into the intra-atomic exchange coupling of EuO thin films. The
MCD reveals large asymmetries of up to 49% in the well-resolved Eu 4d photoemission multiplet.
Thus, ultrathin EuO coherently grown on conductive YSZ allows us to explore fundamental magnetic
and electronic properties of a 4f magnetic oxide.
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. We apply +4.2% tensile biaxial strain
to EuO by epitaxial EuO/LaAlO 3 (100) heterostructures. EuO seamlessly adapts the lateral lattice
parameter of LaAlO 3 , while the perpendicular parameter of EuO is the unchanged EuO bulk value,
thus the strained EuO thin film shows a Poisson ratio of ν EuO ≈ 0. The tensile strain reduces the Curie
temperature significantly by 12.3 K. The MCD effect provides an advanced magnetic characterization:
the MCD asymmetries in Eu core-level photoemission reveal a larger reduction due to the tensile strain
than obtained from bulk-averaging SQUID measurements. Thus, the mechanism of tensile strain on
intra-atomic exchange (indicated by MCD) is significantly different than on the spin order of the 4f 7
shell (indicated by SQUID). Experiments on EuO by MCD, thereby, reveal exciting perspectives for
studying fundamental magnetic properties of EuO.
In the second part of this thesis, we explore how to integrate EuO directly with Si (001). We focus on
interface engineering of structural and chemical properties of the EuO/Si (001) spin-functional heterointerface.
In response to the extremely high chemical reactivity and pronounced surface kinetics of
Eu, EuO, and Si during EuO synthesis at elevated temperatures, we initially conduct a thermodynamic
analysis of the EuO/Si interface. In this way, we decide to investigate three in situ passivation techniques
for the Si (001) surface, in order to prevent metallic and oxide contaminations at the EuO/Si
interface – both being main antagonists for spin-selective tunneling. We conduct a comprehensive optimization
study of the EuO/Si heterointerface by tuning the passivation parameters of the Si (001)
surface and the growth parameters of EuO. Using HAXPES, we evaluate Si and Eu core-level spectra
and determine the minimum of interface contaminants as d opt (SiO x ) = 0.69 nm concomitant with
d opt (EuSi 2 ) = 0.20 nm, both of which are clearly in the subnanometer regime.
In conclusion, our ultrathin EuO/Si (001) heterostructures reveal a high chemical quality of the spinfunctional
interface, combined with magnetic properties of the EuO layer akin to bulk. By selected
interface passivation methods, we achieve a reduction of residual contaminations to clearly below a
closed interface coverage. Thus, we could confirm a heteroepitaxial integration of EuO on Si (001),
which is the experimental basis for possible band-matched coherent tunneling. This is the first time
that a direct integration of high quality EuO on silicon was experimentally realized – without insertion
of additional oxide buffer layers. Such optimized EuO/Si (001) heterointerfaces are paving the
pathway for near-future spin-functional devices using EuO tunnel contacts.
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