Association

2 1. Introduction
Ferromagnetic oxides closely match the electronic resistance with the silicon electrode. This
is one important prerequisite for high-efficiency spin injection. (iv) An advanced transport
concept is coherent tunneling, which relies on symmetry selection for spin-dependent tunneling
and requires a matching of the Bloch states at the interface with the electrode. 12,13
This requires single-crystallinity of the magnetic oxide and epitaxial integration with the
electrode.
Which materials fulfil this demanding set of requirements? Only few magnetic oxides are
known in nature (e. g. NiFe 2 O 4 , CoFe 2 O 2 , BiMnO 3 ,orY 3 Fe 5 O 12 ). Those materials provide
ferromagnetism and spin filter tunnel functionality at the same time. Among this material
class, we choose the binary magnetic oxide, Europium (II) oxide (EuO), which is an ideal
model system for a spin-functional magnetic oxide. EuO, in particular, is the only binary
magnetic oxide predicted to be thermodynamically stable in direct contact with silicon. 14 The
exchange splitting of the lower conduction band (2ΔE ex ≈ 0.6 eV) is largest in EuO among the
magnetic Europium chalcogenides (EuO, EuS, EuSe). Furthermore, a band match with silicon
is feasible due to the comparable bands gaps of EuO (1.12 eV) and Si (1.10 eV). Moreover, the
electronic conductance of EuO can be largely tuned 15 by a precise compositional control of
the synthesis by molecular beam epitaxy (MBE), which permits a conductance match with
silicon. Using MBE growth, EuO thin films have shown single crystalline quality on various
cubic substrates, even with large lattice mismatches up to 20%. 16 Thus, the moderate
lateral lattice mismatch between EuO and Si (001) of 5.5% is a promising starting point for
a heteroepitaxial integration of EuO tunnel contacts with Si. However, during EuO synthesis
at elevated temperatures, interdiffusion of Eu atoms with the silicon surface as well
as chemical reactions with oxygen will chemically degrade the functional EuO/Si interface.
Consequently, EuO directly on Si has revealed a polycrystalline or at best textured structure
until now. 17 This motivates our experimental study targeting on high-quality spin-functional
EuO/Si interfaces.
From a more fundamental perspective, EuO is a model system of a Heisenberg ferromagnet.
18,19 The half-filled 4f 7 shell exhibits a full spin alignment below T C , and is highly localized
inside the ionic Eu 2+ core. A nearest-neighbor interaction between the 4f 7 shells via
5d bands is the origin of the ferromagnetic order, which is an example of localized 4f ferromagnetism.
This is very rare in nature, and indeed, the existence of ferromagnetic insulators
was controversial until the mid-1950s. Until now, several indirect exchange mechanism have
been proposed theoretically for the magnetic coupling between the Eu 2+ ions of EuO. Experimentally,
this coupling can be tuned by either external parameters like pressure and biaxial
strain, or internal factors like electron doping. Taking advantage of the well-established
single-crystalline growth of EuO thin films on lattice-matched substrates, 15,16,22,23 we decide
to propel the research on single-crystalline EuO in the direction of inducing biaxial strain to
epitaxial EuO thin films. In this way, we focus on the expansion and compression of the lateral
lattice parameter of EuO by biaxial tension from selected underlying cubic substrates.
Thereby, we alter the electronic structure and inter-atomic exchange of the EuO single-crystal
in a controlled way – this provides insight into fundamental ferromagnetic properties of EuO.
Very recently, magnetization modulation in EuO by ferroelectric polarization pinning has been reported for
strained ferromagnetic–ferroelectric EuO/BaTiO 3 heterostructures. 20,21