Europium oxide (EuO) based materials exhibit a wealth of spectacular phenomena, including half-metallic ferromagnetism, metal-insulator transition, colossal magneto-resistance, large magneto-optical Kerr effect, tunable ferromagnetic ordering temperatures, and large and long-lived photo-induced conductivity. These extraordinary properties make EuO an ideal candidate for implementation in device applications, in particular, for spintronics. Most of the work in the past has been carried out on bulk EuO, but for device applications it is preferred to have the EuO in thin film form. A flurry of studies have therefore emerged in the last decade in order to explore a wide variety of preparation routes and to investigate the properties of the resulting EuO thin films. A recent highlight is the demonstration that doped EuO films can be fabricated on Si and GaN, thereby exhibiting the expected spin-polarized transport effects [1]. Nevertheless, it is still far from a trivial task to prepare EuO thin films with well defined properties. For bulk EuO, it is already known that stoichiometry is the key issue, and that the presence of small amounts of defects or impurities quickly lead to very large deviations of the material properties. In fact, to make bulk EuO to be stoichiometric one needs temperatures as high as 1800 C. It is obvious that such high temperatures are not compatible with device engineering processes. The preparation of thin films must therefore involve much lower temperatures, preferably not higher than 400-500 C. The consequences are very dear. It turned out that many of the recent studies on EuO thin films are suffering from sample quality problems, due to the presence of, e.g., trivalent Eu species (Eu3O4, Eu2O3), oxygen vacancies, or even Eu metal clusters. Controlled doping of the EuO with trivalent rare-earth ions is also not trivial, since most often even the actual doping concentrations were not known. In fact, one could also question in this respect the quality of many of the doped EuO samples used in the past bulk studies. The focus of this thesis is on the preparation and the properties of high-quality single-crystalline EuO and Gd-doped EuO thin films. The so-called Eu-distillation-assisted molecular beam epitaxy (MBE) has been employed to achieve full control of the stoichiometry. The films have been epitaxially grown on yttria-stabilized cubic zirconia (YSZ) (001) substrates. By a systematic variation of the oxygen deposition rates, we have been able to observe sustained oscillations in the intensity of the reflection high-electron energy diffraction (RHEED) pattern during growth. We thus have demonstrated that layer-by-layer growth has been achieved for the first time. We also have confirmed that YSZ indeed supplies oxygen during the initial stages of growth, yet the EuO stoichiometry can still be well maintained. In the case of Gd-doped EuO films, the presence of Gd even helps to stabilize the layer-by-layer growth mode. It is important to achieve this growth mode, since it enables the preparation of films with very smooth and flat surfaces. This in turn facilitates the capping of the films with a thin Al overlayer in order to protect the films against degradation under ambient conditions. More important, the smoothness of the film will enable the preparation of high quality device structures. By using ex-situ soft x-ray absorption spectroscopy (XAS) at the Eu and Gd M4,5 edges, we have confirmed that the films are completely free from Eu3+ contaminants, and we were able to determine reliably the actual Gd concentration. This actual Gd concentration could in fact significantly deviate from the nominal Gd/Eu evaporation ratio. From magnetization and susceptibility measurements, we found the Curie temperature to increase smoothly as a function of doping from 69 K up to a maximum of 125 K, all with a saturation moment of 7 Bohr magneton. A threshold behavior was not observed for Gd concentrations as low as 0.2%. Analysis of the data also shows that the Gd-doped EuO films can well be described as a 3D, S=7/2, Heisenberg ferromagnet. Using magneto circular dichroic soft x-ray measurements we also proved that the Gd magnetic moments couples ferromagnetically to that of Eu. [1] A. Schmehl, et al., Nature Mater. 6, 882 (2007)
