Electronic Structure and Magnetism of EuO Films on the Nanometer Scale

This thesis deals with thin films of the ferromagnetic semiconductor EuO. These are prepared by molecular beam epitaxy under ultra high vacuum conditions and their properties are investigated in-situ by means of low energy electron diffraction, magneto-optical Kerr effect (MOKE), scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). The experiments are carried out in a low temperature STM apparatus with a base temperature of 5.3K. The development of a highly sensitive MOKE instrument is in the technical focus of this work. EuO is grown in (100) orientation on a ferromagnetic Ni(100) single crystal as well as on single crystalline graphene on Ir(111). An antiferromagnetic coupling between the magnetic moments of the Ni(100) and the EuO(100) film turns out. The 3.3nm thin stoichiometric films of EuO on graphene show a Curie temperature Tc=75K, which is increased with respect to films of the same thickness grown on other substrates and even with respect to bulk EuO. Point defects in EuO films are analyzed with help of atomically resolved STM images. The defect concentration can be quantified using highly-resolved STM topography images. The mobility of the oxygen vacancies is shown by a series of topography STM images and a mobility of the vacancies along the EuO directions is found. Investigations of the electronic structure of the EuO(100) surface are performed by STM and STS, which require a high sample quality, i.e, a topographically very well ordered and flat surface with a minimum of defects. The ultra thin films of EuO on graphene described in thesis are of such high quality. A local n-doping and an upward band bending is found at the oxygen vacancy sites. Tunneling spectroscopy maps of the ferromagnetic EuO(100) surface show standing wave patterns, which are the characteristic fingerprint of the surface state in EuO(100). The energy vs. momentum space of the surface state is explored by evaluating the surface state scattering patterns. A dispersion relation determined this way of the surface state is free electron like, with an effective mass of the mass of a free electron. The same measurements are performed in the paramagnetic phase of EuO at 81K. Thereby the surface state is shifted in energy. The shift indicates indirectly the spin degeneracy of the surface state in the paramagnetic phase, which is lifted in the ferromagnetic phase of EuO. This behavior is analogous to the well known spin splitting of the EuO conduction band. The spin splitting of the surface state (measured with this work) amounts to 0.66eV