Many technological aspects of everyday life are based on practical applications of the magnetic properties of the materials. Miniaturization is a key technological aspect; electronic circuits and storage devices are nowadays steadily decreasing in size and will eventually reach molecular dimensions. The understanding and predictions of the properties of matter at atomic levels represents one of the great achievements of the last years in science. In the present thesis, the aim is to present a complete study of the electronic structure of selected materials, by means of experimental and theoretical methods. The class of materials which are presented in this thesis, are belonging to the magnetic molecules and intermetallic compounds. The electronic structure of the single molecule system named ferric star molecule has been studied. From the resonant X-ray emission study the trend observed for the FeFe3 star gives a signature for the high-spin structure, or more precisely of strong magnetic systems like FeO or Heusler alloys. For the case of intermetallic alloys and compounds, the Mn 2p core-level presents a visible split structure, which is arising from the exchange interactions between the core-hole and the unpaired 3d electrons. The interpretation of this splitting can be regarded as an evidence of local magnetic moments belonging to the Mn site.

This thesis gives a detailed overview about the electronic and magnetic structure of three different kinds of 3d-transition metal based materials with potential for possible future applications. The presented materials are a series of “high-k” rare-earth scandates, RScO3 (R=Pr, Nd, Sm, Eu, Gd, Tb and Dy), the muliferroic layered oxide LuFe2O4, and three iron-based magnetic polyoxometalates of the type {(M)M5}Fe30 (M =Mo,W). The samples are examined by several different x-ray spectroscopic techniques and complementary theoretical approaches, namely multiplet calculations and first principles electronic structure calculations, respectively. The occupied electronic states are determined by photoelectron spectroscopy (XPS) and x-ray emission spectroscopy (XES). The unoccupied states are investigated by x-ray absorption spectroscopy (XAS). X-ray magnetic circular dichroim (XMCD) is used to get the element specific magnetic moment of the materials, and these results are compared to SQUID measurements.

In this thesis the influence of different growth conditions on the structural and the magnetic properties of magnetite were analyzed. Therefore, ultrathin Fe3O4 films were grown on MgO(001) substrates, on NiO, and on Fe pre-covered MgO(001) substrates. In the first part of this thesis magnetite films with different film thicknesses were deposited directly on MgO by RMBE to investigate the thickness dependence of the anomalous strain and the in-plane magnetic anisotropy. Surface sensitive methods like XPS and LEED have shown that all films in the investigated thickness range are stoichiometric and epitactic magnetite. Bulk sensitive XRD experiments at the specular rod point to well-ordered films with homogenous film thickness indicated by the distinct Laue oscillations. However, the vertical layer distances are smaller than expected even for strained magnetite. Raman measurements were carried out to clarify this contradiction between surface sensitive and bulk sensitive measurements. While the 20 nm and 30 nm films exhibit the typical bands for magnetite, no distinct bands can be observed for the 7.6 nm film. Due to this results we first assume a partial formation of a thin maghemite layer on top of the uncapped magnetite film under ambient conditions. Nevertheless, additional XPS measurement exclude the complete oxidation of magnetite to maghemite since there is no significantly increased Fe3+-signal visible. Thus, the low vertical layer distance can be attributed to the presence of APBs causing an anomalous strain relaxation as reported in literature. Although all films feature ferromagnetic behavior there are differences in the characteristic of the magnetic in-plane anisotropy. The 7.6 nm film has an in-plane magnetic isotropy while the 20 nm and 30 nm film have an in-plane fourfold magnetic anisotropy. Here, the fourfold magnetic anisotropy is stronger for the 20 nm magnetite film than for the 30 nm film. The critical film thickness for the transition from magnetic isotropy to magnetic fourfold anisotropy may be influenced by film thickness and lattice strain induced by the substrate. The second part of this thesis features the thickness dependence of the structural quality of Fe3O4/NiO bilayers. Each film of the Fe3O4/NiO bilayer on MgO(001) have been successfully grown by RMBE. LEED and XPS experiments have proven that the surface near regions of the distinct films have high structural and stoichiometric properties. Here, too, the detailed ’bulk’ structural characterization of Fe3O4/NiO bilayers were carried out using XRD. It was shown that the Fe3O4 films grow homogeneously and smoothly on NiO films if the NiO film thickness is below 24 nm. Above this NiO film thickness the structural quality of the magnetite films gets distinctly worse. This behavior can be attributed to the fact that the interface roughness between NiO and Fe3O4 depends on the NiO film thickness. The roughness of the 3 nm NiO film is rather small and it is rising obviously with increasing NiO film thickness. Thus, the structural quality of the magnetite films grown on 30 nm NiO films is constantly reduced with increasing magnetite film thickness since the quality of the Fe3O4 films is influenced by the quality of the Fe3O4/NiO interface. In the third study the influence of initial iron buffer layers on the magnetic properties of magnetite grown on MgO(001) substrates has been investigated. In situ XPS and LEED indicate that the structural and stoichiometric properties of the surface near region of the magnetite films are not influenced by the iron buffer layer. However, the structural and magnetic properties of the whole film have changed compared to magnetite grown directly on MgO as shown by XRD and MOKE. The crystalline quality is poor and the magnetic easy axis of the magnetic in-plane anisotropy is rotated by 45◦ compared to magnetite films grown directly on MgO. Both crystalline quality and the rotation of the magnetic in-plane anisotropy show no dependence on the film thickness. However, the strength of the magnetocrystalline anisotropy decreases with increasing film thickness. XPS and XRD measurements have indicated that the iron buffer layer is completely oxidized during the second growth stage of the magnetite. The small Kerr rotation in the MOKE experiments of the samples with film thicknesses up to 28 nm confirms this result since a remaining iron film would cause a higher Kerr rotation. In the last part of this thesis the structural and magnetic properties of a partially oxidized, a completely oxidized and a metallic iron film are analyzed. The partially oxidized iron film is a bilayer with a metallic iron film and an iron oxide film. The surface near stoichiometry of both oxidized iron films correspond to magnetite as proven by XPS. The structural analysis by XRD reveals that though these magnetite films are crystalline, they have an inhomogeneous thickness. The magnetization curves of the partially oxidized film (bilayer) measured by MOKE exhibit a magnetic saturation which is comparable to the magnetic saturation of the single metallic iron film. However, the coercive field is higher compared to the single metallic iron film due to the high interface roughnesses. Considering the coercive field as a function of the sample rotation α the bilayer exhibits a fourfold anisotropy with eight maxima. However, the angular dependence of the magnetic remanence features a simple fourfold anisotropy with easy axes in h110i directions of MgO(001). Vector MOKE analysis displays that the magnetic reversal processes of the bilayer are similar to single metallic iron films. Thus, the bilayer exhibits mostly the magnetic properties of a single iron film. The bilayer has the same magnetic easy axis and a similar magnetic saturation. The magnetic easy axis of the magnetic in-plane anisotropy of the completely oxidized iron film is rotated by 45◦ compared to magnetite films grown directly on MgO as already observed in Ref. [148]. The completely oxidized iron film exhibits also a significantly increased coercive field due to high surface roughness. A magneto-dynamic investigation of the exchange-coupling of the Fe3O4/Fe bilayer system was carried out to yield all relevant magnetic parameters, such as anisotropies, as well as the coupling constant J1. Here, also a complex fourfold anisotropy was observed, which might be due to a coupling of a perpendicular spin-wave mode in the magnetite layer with the acoustical coupling mode. We have also successfully calculated the angular dependence of the resonance field of all films using the eighth order of the magnetocrystalline anisotropy. Although we have applied many measurement methods, we have found no explanation for the complex fourfold angular dependence of the coercive field of the bilayer. Nevertheless, this bilayer provides interesting properties for application in MTJs due to its enhanced magnetic properties like complex fourfold magnetic anisotropy and higher coercive field and remanence. All in all, we have shown that the structural and magnetic properties of magnetite films are strongly influenced by interlayers between film and substrate. While magnetite films directly deposited on MgO exhibit a homogeneous film thickness, both NiO interlayers with a thickness above 24 nm and initially grown iron films deteriorate the structural quality of the on top grown magnetite films. In addition, the magnetic fourfold anisotropy is rotated by 45◦ in comparison to magnetite films grown directly on MgO for the structurally disturbed magnetite on iron pre-covered MgO substrates. Here, further investigations are necessary to understand why this anisotropy rotation occurs and how we can improve the structural quality of magnetite on iron and NiO. The growth of magnetite by oxidizing previously deposited iron films leads to crystalline magnetite films but with inhomogeneous film thicknesses. In case of a Fe3O4/Fe bilayer, the magnetic properties are drastically changed due to the magnetic coupling between the iron and the magnetite film.

Diese Arbeit beschäftigt sich mit den niedrigdimensionalen Kohlenstoffmaterialien Fullerene, Kohlenstoffnanoröhren (CNTs) und der Diamantoberfläche. Es werden jeweils zwei der genannten Materialien kombiniert und im Hinblick auf die dadurch entstehenden magnetischen und elektronischen Eigenschaften untersucht. Durch die Füllung von Fullerenen in CNTs mit entsprechenden Durchmessern ergibt sich eine eindimensionale Anordnung der Fullerene. Diese Strukturen werden als peapods bezeichnet. Bei der Verwendung von endohedralen, magnetischen Fullerenen ergibt sich durch die Einschränkung der Dimension die Möglichkeit einer wohl-definierten Kopplung und dadurch eine mögliche Änderung der magnetischen Eigenschaften. Die hier betrachteten Moleküle sind die paramagnetischen Fullerene N@C60 und Er3N@C80 sowie das ferromagnetische Dy2ScN@C80-Fulleren, welches auch als ein Einzelmolekülmagnet (SMM) bezeichnet wird. Für die Herstellung von peapods wurde im Rahmen dieser Arbeit eine Füllanlage aufgebaut, welche die speziellen Anforderungen der Fullerene berücksichtigt und mit der auch CNTs auf Substraten gefüllt werden können. Der Erfolg der Füllung wird mit hochauflösender Transmisssions-Elektronenmikroskopie (HRTEM), energiedispersiver Röntgen-Spektroskopie (EDX) und optischer Emissions- Spektrometrie (OES) überprüft. Durch weitergehende Untersuchungen im HRTEM konnte eine Reaktion von metallischen Atomen aus den Fullerenen innerhalb der CNTs zu neuen, metallischen Clustern beobachtet werden. Für die Untersuchungen möglicher Änderungen der magnetischen Eigenschaften sind magnetische Messungen notwendig. Diese können mit der Methode des magnetischen zirkularen Röntgendichroismus (XMCD) durchgeführt werden. Für das Ziel von nanoskopischen XMCD-Messungen einzelner peapods wurden Rastertransmissions- Röntgenmikroskopie-Messungen (STXM) am Synchrotron durchgeführt. Diese wurden mit HRTEM-Messungen derselben Probenpositionen korreliert, um die spektroskopische mit der räumlichen Auflösung zu verknüpfen. Dabei konnte ein Röntgenabsorptions- Spektrum von einem dünnen peapod-Bündel gemessen werden. Zusätzlich wurde mit makroskopischen XMCD-Messungen von Er3N@C80-Fullerenen, gefüllt in CNTs, gezeigt, dass diese Methode auch für peapods anwendbar ist. Erste XMCDMessungen von ferromagnetischen Dy2ScN@C80-Fullerenen zeigen eine deutliche Änderung der magnetischen Eigenschaften durch die eindimensionale Anordnung in CNTs. Im Zusammenhang mit der eindimensionalen Anordnung von paramagnetischen Fullerenen wurde außerdem ein Konzept eines Quantenregisters betrachtet, welches auf N@C60- peapods basiert. Diese sollen dabei auf eine Diamantoberfläche deponiert werden, um oberflächennahe Stickstoff-Fehlstellen-Zentren zum Auslesen der Spinzustände der N@C60- Fullerene verwenden zu können. Die in diesem Fall auftretende elektronische Wechselwirkung zwischen unterschiedlich terminierten Diamantoberflächen und CNTs sowie Fullerenen wurde mit Kelvinsonden-Mikroskopie- Messungen (KPFM) untersucht. Dabei wurde erstmalig ein Elektronentransfer von der wasserstoffterminierten Diamantoberfläche in CNTs experimentell nachgewiesen, während dieser bei Sauerstoffterminierung nicht beobachtet wurde. Die präsentierten Messungen geben Auskunft über den auftretenden Ladungstransfer, indem Ladungen in C60- Fullerenen und CNTs lokal aufgelöst abgebildet werden. Zusammenfassend können die in dieser Arbeit gewonnenen Erkenntnisse helfen, zukünftige Bauelemente von klassischen Computern oder Quantencomputern, basierend auf niedrigdimensionalen Kohlenstoffmaterialien, zu entwickeln.

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The interferometric cantilever displacement detection in non-contact atomic force microscopy (NC-AFM) is in fundamental aspects explored and optimized. Furthermore, the opto-mechanical interaction of the light field with the cantilever is investigated in detail. Cantilevers are harmonic oscillators that are designed to have a high sensitivity for the detection of minute external forces typically originating from tip-sample interaction. In this work, however, the high sensitivity is used for detailed studies of opto-mechanical forces due to the radiation pressure of the light interacting with the cantilever. The interferometer in the NC-AFM setup consists of an optical cavity working similar to a Fabry-Pérot interferometer in combination with a reference interference arm working similar to a Michelson interferometer combining multi-beam interference with a reference beam resulting in a complex superposition of beams forming the interferometric intensity modulation signal. The character of the interferometer can be adjusted from predominant Michelson to predominant Fabry-Pérot characteristics by the optical loss inside the cavity. A systematic approach for accurate alignment, by using 3D intensity maps and intensity-over-distance curves, as well as the implications of deficient fiber-cantilever configurations are explored and the impact of the interferometer configuration on the detection system noise floor is investigated. A new physical property, namely, the Fabry-Perot enhancement factor is introduced that is a direct measure for the light intensity interacting with the cantilever compared to the reference beam intensity reflected back inside the fiber. The quantification of the optical loss yields an exact knowledge of the amount of light interacting with the cantilever that is crucial to understand opto-mechanical effects. The resulting opto-mechanical force varies sinusoidally during the course of one oscillation cycle. It is a key result of this work that the sinusoidal modification of the cantilever restoring force can be described analogue to the restoring force of a pendulum. This results in an observable amplitude dependent frequency shift of the cantilever oscillation, allowing a calculation of the ratio of the opto-mechanical force relative to the cantilever restoring force and thus allows an in-situ measurement of the cantilever stiffness with remarkable precision. Further investigation of the cantilever oscillation yields that other characteristic properties of the oscillation are significantly modified by the opto-mechanical interaction. The observed effective fundamental mode Q-factor drops significantly while the cantilever amplitude response to a certain excitation voltage increases. A discrete numerical model describing the cantilever as a 1D linear chain of mass points is implemented, yielding that the additional opto-mechanical force results in a partial pinning of the cantilever at the edges of the interferometric fringes. Pinning efficiently shifts energy from the fundamental mode to higher modes and modes of a pinned cantilever, resulting in a complex modal structure.

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