Magnetic oxide heterostructures : EuO on cubic oxides and on silicon

In the thesis at hand, we explore fundamental properties of ultrathin europium oxide (EuO) films. EuO is a model system of a localized 4$\textit{f}$ 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, upto now a challenging bulk and interface chemistry of EuO and Si has hampered a seamless integrationinto 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 bulk-like 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 $\textit{4d}$ photoemission multiplet. Thus, ultrathin EuO coherently grown on conductive YSZ allows us to explore fundamental magnetic and electronic properties of a $\textit{4f}$ 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:theMCDasymmetries 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 4$\textit{f}^{7}$ 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 hetero interface. In response to the extremely high chemical reactivity and pronounced surface kinetics ofEu, 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 spinfunctionalinterface, 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