Association

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Outlook and perspectives
In the course of ongoing research in our group, polycrystalline EuO tunnel contacts with a
native SiO 2 diffusion barrier on silicon have recently shown spin filter tunneling in electrical
DC transport. In view of silicon spintronics, this result is a proof of principle for the spin
filter effect in EuO tunnel barriers on silicon. This motivates a further improvement of the interface
chemical properties which enables a simultaneous heteroepitaxial integration of EuO
directly with Si (001). With regard to the thermal sensitivity of the passivated Si surface and
pronounced surface kinetics of Si and Eu atoms, we propose a reduction of the synthesis temperature.
Due to our thermodynamic analysis, a reduced temperature is suitable to maintain
the hydrogen passivation of the Si surface in order to avoid interfacial silicides – the main
antagonist to any tunnel functionality. Indeed, by a synthesis temperature of EuO as low as
200 ◦ C, very recently crystalline EuO could be stabilized. 209 We consider this approach as the
most promising route towards a further improvement of the chemical quality of the EuO/Si
heterointerface.
Moreover, coherent tunneling may be feasible in epitaxial tunnel contacts of EuO on Si (001).
In order to compare experimental findings of epitaxial EuO/Si (001) tunnel junctions – which
are experimentally feasible, as concluded by the thesis at hand – with predictions for the spin
filter efficiency, an appropriate modeling is needed. This motivates to launch spin-dependent
density functional theory calculations for coherent tunneling through epitaxial EuO/Si (001)
tunnel contacts.
The magnetic oxide EuO is a model system to propel fundamental research on localized 4f
ferromagnetism. One approach is strain engineering, which modifies the magnetic coupling
of EuO by lateral tension or compression, as shown in this thesis. This renders ultrathin
epitaxial EuO films interesting for further investigations using the MCD effect in hard X-
ray photoemission spectroscopy, since this technique yields element-specific information on
intra-atomic exchange coupling in EuO, and in addition permits a depth-selective profiling.
Using the MCD in hard X-ray photoemission, one can selectively investigate properties of
buried interfaces, the bulk EuO layer, or identify possible magnetic dead layers – thus providing
access to a complete set of the chemical and local magnetic properties of a layered
magnetic heterostructure.
Furthermore, recent calculations predict that epitaxial EuO becomes ferroelectric under biaxial
strain (tensile 4%, or compressive 3%). 21 Very recently, indeed a magnetization modulation
in EuO by ferroelectric polarization pinning is reported for strained ferromagnetic–
ferroelectric EuO/BaTiO 3 heterostructures. 20 This motivates further studies on epitaxial EuO
under sufficient biaxial strain by suited substrates. Thereby, EuO may even provide a route
to a multiferroic functionality, that combines ferromagnetic and ferroelectric order. For this,
high-quality EuO thin films need to be epitaxially integrated with cubic substrates providing
an appropriate biaxial strain, as has been successfully initiated in this thesis.
In that, future research on magnetic oxides like EuO offers exciting perspectives both for
fundamental physics and silicon spintronics applications.