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It is shown how soft X-ray nanospectroscopy maps of powders (particle size ∼200 nm) can be analyzed to quantitatively retrieve X-ray linear dichroism (XLD) parameters. A computational modeling procedure is described that can be used in conjunction with Monte Carlo simulations to prove statistical dissimilarity of XLD parameters between different samples., X-ray linear dichroism (XLD) is a fundamental property of many ordered materials that can for instance provide information on the origin of magnetic properties and the existence of differently ordered domains. Conventionally, measurements of XLD are performed on single crystals, crystalline thin films, or highly ordered nanostructure arrays. Here, it is demonstrated how quantitative measurements of XLD can be performed on powders, relying on the random orientation of many particles instead of the controlled orientation of a single ordered structure. The technique is based on a scanning X-ray transmission microscope operated in the soft X-ray regime. The use of a Fresnel zone plate allows X-ray absorption features to be probed at ∼40 nm lateral resolution – a scale small enough to probe the individual crystallites in most powders. Quantitative XLD parameters were then retrieved by determining the intensity distributions of certain diagnostic dichroic absorption features, estimating the angle between their transition dipole moments, and fitting the distributions with four-parameter dichroic models. Analysis of several differently produced ZnO powders shows that the experimentally obtained distributions indeed follow the theoretical model for XLD. Making use of Monte Carlo simulations to estimate uncertainties in the calculated dichroic model parameters, it was established that longer X-ray exposure times lead to a decrease in the amplitude of the XLD effect of ZnO.

A high-spin state of SrRuO3 at the interface and large magnetic coupling were observed in SrRuO3/LaNiO3 superlattices. X-ray absorption spectroscopy and X-ray linear dichroism results show preferential occupation of the d3z2-r2 orbitals. The origin of the high-spin state in SrRuO3 and the large-scale magnetic coupling is closely related to the orbital reconstruction. The reduced energy of the d3z2−r2 orbitals and enhanced densities of the eg of SrRuO3 contribute to the interface magnetic moment. The orbital occupancy is strongly correlated with the interface magnetic properties, thereby extending the concept of orbital degrees of freedom modulation on magnetic coupling properties. Impact statement A high-spin state in SRO at the interface and large magnetic coupling is observed in SRO/LNO superlattices and the origin of these properties is closely related to orbital reconstruction.

Understanding and controlling phase transition in the transition-metal oxides is interesting from the fundamental physics point of view. The pd hybridization are of considerable issue to tune the phase transition temperature. Here, the transport properties of the NdNiO3 with artificial ultra-thin NiO insert layer have been investigated. The relationship between the essential parameters of the pd hybridization (T) and the metal-insulator transition (MIT) temperature has been clearly elucidated a monotone decreasing relationship. This work realized the combination of the engineering hybridization and the phase transition temperature, which might be significant for the development of multifunctional materials in the transition-metal oxides.

Soft X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) have become essential experimental tools for the investigations the complex physics of transition metal oxide (TMO) heterostructures. XAS has been long used to determine the valence, the orbital and magnetic properties of transition metals. More recently, linear and circular dichroism in XAS have been widely applied to determine the crystal field splitting, the atomic orbital and spin moments, and the magnetic order of 3d-states, in bulk sample, in thin films and at atomically-sharp interfaces. Although less common, RIXS is also gaining popularity for its capability of accessing local and collective excitations at a time; the recent technical advances have been established RIXS as an important method for the determination of the electronic and magnetic properties of TMOs. This chapter is a brief review of the salient XAS and RIXS results on TMO and TMO heterostructures published in the last 15 years.

Iron and cobalt phthalocyanines assemble on the Au(110) surface lying parallel to the surface, as deduced by near-edge X-ray absorption fine structure (NEXAFS) taken with linearly polarized radiation at the C and N K edges. The molecular chains, firmly anchored to the underlying metal surface, arrange into long-range ordered rows with a (5 × 3) symmetry along the [001] azimuthal direction at completion of the first single layer. The interaction process is mainly determined by the d orbitals associated with the central Fe and Co atoms, as observed by valence band photoemission and NEXAFS at the Fe and Co L2,3 edges. The spin and orbital configuration of the FePc and CoPc molecules is strongly influenced by the interface with a charge transfer from the underlying metal to the out-of-plane empty states located at the Fe and Co centers of the molecules. This interaction process induces electronic states located at the interface, localized on the central metal atoms and close to the Fermi level (0.2 eV binding energy for FePc and 0.7 eV for CoPc) without energy dispersion, as deduced by angular-resolved photoemission. On the contrary, a delocalized state has been observed with dispersion along the molecular chains, mainly due to the overlapping of the ? charge of the macrocycles ligands mediated by the Au substrate.

The two wide band gap dilute magnetic semiconductors (DMS) Gd:GaN and Co:ZnO are among the most favored materials for spintronic applications. Despite intense research efforts during the last years, the origin of the magnetic order is still under debate. This work reports structural and magnetic investigations on these DMS materials employing several complementary techniques. The X-ray linear dichroism (XLD) has been used to gain element-specific insight into the local structure of dopants and cations. X-ray diffraction (XRD) was used to probe the global structural properties. Magnetic characterization by superconducting quantum interference device (SQUID) has been complemented by electron spin resonance (ESR) and X-ray magnetic circular dichroism (XMCD). Gd:GaN samples were fabricated by focus ion beam (FIB) implantation and molecular beam epitaxy (MBE). Room temperature ferromagnetic-like behavior as found for some of our samples by SQUID could not be reliably reproduced. Instead XMCD measurements at the Gd L3-edge reveal paramagnetic behavior of the dopant. Additionally a possible magnetic polarization of Ga atoms of the host crystal is shown to be too small to explain the total magnetization of these samples. In some samples the formation of Gd and GdN clusters was evidenced by ESR measurements. Intrinsic room temperature ferromagnetism of this material as seen by SQUID cannot be confirmed by any other technique - neither ESR nor XMCD. Co:ZnO samples were predominantly grown by reactive magnetron sputtering (RMS). 95\% of the Co atoms are incorporated on substitutional Zn-sites in samples of high structural quality. These samples show paramagnetic behavior as found by SQUID, XMCD and ESR. RMS growth of Co:ZnO with reduced oxygen partial pressure yields a magnetic behavior known from ferromagnetic nanoclusters. The X-ray near edge absorption spectroscopy (XANES) and XMCD at the Co K-edge support an increased fraction of Co atoms with metallic character in these samples. A reduced XLD signal indicates less substitutional Co-atoms. These samples were annealed either under O2 atmosphere or high vacuum (HV) conditions. While the latter strongly enhances ferromagnetic-like properties, they vanish upon O2 annealing. XANES and XLD analyses show that non-substitutional Co atoms are oxidized partially to Co3O4 by annealing in an O2 atmosphere, whereas HV annealing increases the fraction of a metallic Co phase. ESR measurements consistently show signatures of superparamagnetic ensembles. Samples of high structural quality, are inert to annealing procedures. Die verdünnten magnetischen Halbleiter (DMS), Co:ZnO und Gd:GaN, gehören zu den favorisierten Materialien für mögliche Spintronik-Anwendungen. Der Gegenstand dieser Arbeit sind strukturelle und magnetische Untersuchungen dieser Materialen mit komplementären Methoden. In Ergänzung der Röntgendiffraktometrie (XRD) wurde der Röntgen-Linear-Dichroismus (XLD) genutzt, um elementspezifisch lokale strukturelle Informationen über Dotieratom und Wirtskation zu erhalten. Die SQUID-Magnetometrie wurde durch Messung der Elektronenspinresonanz (ESR) und des Röntgenzirkulardichroismus (XMCD) komplettiert. Gd:GaN wurde durch Gd-Ionen Implantation und Molekularstrahlepitaxie (MBE) gewachsen. Bei 300 K zeigten nur wenige Proben scheinbar ferromagnetisches Verhalten in SQUID-Messungen, welches nicht zuverlässig reproduziert werden konnte. Stattdessen konnte an der Gd L3-Kante mittels XMCD paramagnetisches Verhalten des Dotieratoms nachgewiesen werden. Ergänzend konnte gezeigt werden, dass eine mögliche magnetische Polarisation des Ga-Untergitters zu gering ist, um die Gesamtmagnetisierung der Probe zu erklären. Somit müssen extrinsische Ursachen für gelegentliche ferromagnetische Signaturen in integralen Magnetisierungsmessungen in Betracht gezogen werden. Die Bildung von Gd und GdN Nanoclustern, wurde durch ESR Messungen gezeigt. Intrinsischer Ferromagnetismus bei T = 300 K konnte weder mit ESR noch mit XMCD bestätigt werden. Die Co:ZnO Proben wurden vorwiegend durch reaktives Magnetronsputtern (RMS) hergestellt. 95\% der Co Atome besetzen substitutionelle Zn-Plätze in Proben von hoher struktureller Qualität. Diese Proben zeigen paramagnetisches Verhalten. RMS Wachstum unter reduziertem O2-Partialdruck induziert magnetisches Verhalten, wie es von ferromagnetischen Nanoclustern bekannt ist. Röntgennahkantenabsorptionsspektroskopie (XANES) und XMCD bestätigen einen erhöhten metallischen Co Anteil in diesen Proben. Entsprechend ist das XLD Signal, welches ein Maß für substitutionelles Co darstellt, reduziert. Diese Proben wurden unter O2-Atmosphäre oder Hochvakuum (HV) getempert. Während Letzteres die ferromagnetischen Eigenschaften der Proben verstärkt, führt das O2-Tempern zu einem Verschwinden der ferromagnetischen Signaturen. XANES und XLD Analysen zeigen, dass nicht-substitutionelle Co Atome teilweise zu Co3O4 oxidiert werden, wohingegen HV-Tempern eine Erhöhung des metallischen Co Anteils bewirkt. ESR Messungen zeigen Signaturen von superparamagnetischen Ensemblen. Proben von hoher struktureller Qualität sind inert gegenüber Temperprozeduren.