Polycapillary-boosted instrument performance in the extreme ultraviolet regime for inverse photoemission spectroscopy

A collimating polycapillary half lens, traditionally used in the medium and hard X-ray band, is operated at a photon energy of 36 eV for the first time. While the transmission still exceeds 50%, the measured and simulated spatial resolution and angular divergence approach 0.4 mm or less and at most 20 mrad, respectively. This unexpected, superior performance of the polycapillary optic in the extreme Ultraviolet could enable the design of an e cient, versatile and compact spectrometer for inverse photoemission spectroscopy (IPES): Its wavelength-dispersive component, a customized reflection zone plate, can maintain an energy resolution of 0.3 eV, whereas the sensitivity may be enhanced by more than one order of magnitude, compared to conventional spectrometers. Furthermore, the overall length of 0.9 m would allow for an eased alignment and evacuation. We see a significant potential for numerous polycapillary-based XUV / soft X-ray instruments in the future, in particular after further optimization for this long wavelength regime

Electronic structure and symmetry of valence states of epitaxial NiTiSn and NiZr0.5_{0.5}Hf0.5_{0.5}Sn thin films by hard x-ray photoelectron spectroscopy

The electronic band structure of thin films and superlattices made of Heusler compounds with NiTiSn and NiZr$_{0.5}$Hf$_{0.5}$Sn composition was studied by means of polarization dependent hard x-ray photoelectron spectroscopy. The linear dichroism allowed to distinguish the symmetry of the valence states of the different types of layered structures. The films exhibit a larger amount of {\it "in-gap"} states compared to bulk samples. It is shown that the films and superlattices grown with NiTiSn as starting layer exhibit an electronic structure close to bulk materials

Magnetic dichroism in angular-resolved hard X-ray photoelectron spectroscopy from buried layers

This work reports the measurement of magnetic dichroism in angular-resolved photoemission from in-plane magnetized buried thin films. The high bulk sensitivity of hard X-ray photoelectron spectroscopy (HAXPES) in combination with circularly polarized radiation enables the investigation of the magnetic properties of buried layers. HAXPES experiments with an excitation energy of 8 keV were performed on exchange-biased magnetic layers covered by thin oxide films. Two types of structures were investigated with the IrMn exchange-biasing layer either above or below the ferromagnetic layer: one with a CoFe layer on top and another with a Co$_2$FeAl layer buried beneath the IrMn layer. A pronounced magnetic dichroism is found in the Co and Fe $2p$ states of both materials. The localization of the magnetic moments at the Fe site conditioning the peculiar characteristics of the Co$_2$FeAl Heusler compound, predicted to be a half-metallic ferromagnet, is revealed from the magnetic dichroism detected in the Fe $2p$ states

Hard x-ray photoelectron spectroscopy of bulk and thin films of Heusler compounds

X-ray photoemission spectroscopy (XPS) is one of the most universal and powerful tools for investigation of chemical states and electronic structures of materials. The application of hard x-rays increases the inelastic mean free path of the emitted electrons within the solid and thus makes hard x-ray photoelectron spectroscopy (HAXPES) a bulk sensitive probe for solid state research and especially a very effective nondestructive technique to study buried layers.rnThis thesis focuses on the investigation of multilayer structures, used in magnetic tunnel junctions (MTJs), by a number of techniques applying HAXPES. MTJs are the most important components of novel nanoscale devices employed in spintronics. rnThe investigation and deep understanding of the mechanisms responsible for the high performance of such devices and properties of employed magnetic materials that are, in turn, defined by their electronic structure becomes feasible applying HAXPES. Thus the process of B diffusion in CoFeB-based MTJs was investigated with respect to the annealing temperature and its influence on the changes in the electronic structure of CoFeB electrodes that clarify the behaviour and huge TMR ratio values obtained in such devices. These results are presented in chapter 6. The results of investigation of the changes in the valence states of buried off-stoichiometric Co2MnSi electrodes were investigated with respect to the Mn content α and its influence on the observed TMR ratio are described in chapter 7.rnrnMagnetoelectronic properties such as exchange splitting in ferromagnetic materials as well as the macroscopic magnetic ordering can be studied by magnetic circular dichroism in photoemission (MCDAD). It is characterized by the appearance of an asymmetry in the photoemission spectra taken either from the magnetized sample with the reversal of the photon helicity or by reversal of magnetization direction of the sample when the photon helicity direction is fixed. Though recently it has been widely applied for the characterization of surfaces using low energy photons, the bulk properties have stayed inaccessible. Therefore in this work this method was integrated to HAXPES to provide an access to exploration of magnetic phenomena in the buried layers of the complex multilayer structures. Chapter 8 contains the results of the MCDAD measurements employing hard x-rays for exploration of magnetic properties of the common CoFe-based band-ferromagnets as well as half-metallic ferromagnet Co2FeAl-based MTJs.rnrnInasmuch as the magnetoresistive characteristics in spintronic devices are fully defined by the electron spins of ferromagnetic materials their direct measurements always attracted much attention but up to date have been limited by the surface sensitivity of the developed techniques. Chapter 9 presents the results on the successfully performed spin-resolved HAXPES experiment using a spin polarimeter of the SPLEED-type on a buried Co2FeAl0.5Si0.5 magnetic layer. The measurements prove that a spin polarization of about 50 % is retained during the transmission of the photoelectrons emitted from the Fe 2p3/2 state through a 3-nm-thick oxide capping layer.rnRöntgenphotoelektronenspektroskopie (XPS) ist eine der gebräuchlichsten und vielseitigsten Methoden, um die chemischen Zusammensetzung und elektronischen Struktur von Materialien zu bestimmen. Die Anwendung von harten Röntgenstrahlen erhöht die mittlere freie Weglänge der emittierten Elektronen im Festkörper. Somit ist hard X-ray Photoelektronenspektroskopie (HAXPES) eine ideale Methode um Volumeneigenschafen zerstörungsfrei zu bestimmen. rnDiese Arbeit fokussiert sich auf die Untersuchung von mehrschichtigen Strukturen, die als Bauelemente in Tunnelkontakten den magnetischen Tunnelwiderstand (TMR) nutzen wobei verschiedene Anwendungen der HAXPES Technik zum Einsatz kommen. Solche Spintronik-Bauelemente sind wichtige Komponenten der Nanotechnik. HAXPES ermöglicht die Untersuchung und das detaillierte Verständnis der Mechanismen und Eigenschaften der verwendeten magnetischen Materialien, bestimmt durch ihre Elektronenstruktur, die für die Leistung dieser Bauelemente bestimmend sind. rnAusführlich wurde die B-Diffusion von auf CoFeB basierenden Tunnelelementen als Funktion der Anlass-Temperatur und die daraus resultierenden Änderungen der elektronischen Struktur der CoFeB Elektroden untersucht, wodurch der hohe TMR-Wert erklärt werden kann. Diese Ergebnisse werden in Kapitel 6 dieser Arbeit erläutert. Die Änderung der Valenzzustände und des TMR von nicht stöchiometrischen Co2MnSi-Elektroden bei Änderung des Mangangehalts  wurden mittels hochauflösender HAXPES untersucht, die Ergebnisse sind in Kapitel 7 dargestellt.rnUm magnetoelektrische Eigenschaften wie die Austauschaufspaltung in Ferromagneten näher zu bestimmen, eignet sich die Untersuchung des magnetischen zirkularen Dichroismus (MCDAD) mittels Photoemission. Dabei tritt eine Asymmetrie in den Photoemissions-spektren auf, wenn entweder die Helizität bei fester Magnetisierung der Probe oder die Magnetisierungsrichtung bei fester Helizität verändert wird. Da diese Technik bis jetzt aufgrund niedriger Photonenergien nur zur Untersuchung von Oberflächen zu gebrauchen war, wurde diese Methode in HAXPES Experimente integriert, um einen Zugang zu den magnetischen Eigenschaften von komplexen Multilagensystem zu erlangen. Die Ergebnisse der MCDAD Messungen an CoFe und an vergrabenen Tunnelkontakten aus halbmetallischen Ferromagneten Co2FeAl mittels hochenergetischer Röntgenstrahlung sind in Kapitel 8 dargestellt.rnObwohl die magnetoresistiven Eigenschaften von Spintronik-Bauelementen durch den Spin der Elektronen der ferromagnetischen Materialien festgelegt sind, waren direkte Messungen der Spinpolarisation bis jetzt aufgrund der Oberflächensensitivität der klassischen Photoemission eingeschränkt. In Kapitel 9 werden die ersten Ergebnisse von spinaufgelösten HAXPES Experimenten an magnetischen Schichtsystemen aus Co2FeAl0.5Si0.5 mittels eines SPLEED Spin-Polarimeters präsentiert. Die Messungen belegen eine 50 % Spin-Polarisation der Photoelektronen der durch eine 3 nm dicke Oxiddeckschicht emittierten Photoelektronen aus Fe 2p3/2 Zuständen. Damit ist gezeigt, dass die Methode des Spin-HAXPES für die magnetische Charakterisierung von vergrabenen Schichten und inneren Grenzschichten möglich ist. Dies eröffnet für die Zukunft eine neue Klasse von Experimenten.r

Electronic and crystallographic structure, hard x-ray photoemission, and mechanical and transport properties of the half-metallic Heusler compound Co2MnGe

This work reports on the electronic and crystalline structure and the mechanical, magnetic, and transport properties of the polycrystalline Heusler compound Co2MnGe. The crystalline structure was examined in detail by extended x-ray absorption fine-structure spectroscopy and anomalous x-ray diffraction. The compound exhibits a well-ordered L21 structure as is typical for Heusler compounds with 2:1:1 stoichiometry. The low-temperature magnetic moment agrees well with the Slater-Pauling rule and indicates a half-metallic ferromagnetic state of the compound, as is predicted by ab initio calculations. Transport measurements and hard x-ray photoelectron spectroscopy were performed to explain the electronic structure of the compound. The obtained valence band spectra exhibit small energy shifts that are the result of the photoexcitation process, whereas electron-electron correlation in the ground state is negligible. The vibration and mechanical properties of the compound were calculated. The observed hardness values are consistent to a covalent-like bonding of Co2MnGe

Electronic and crystallographic structure, hard x-ray photoemission, and mechanical and transport properties of the half-metallic Heusler compound Co2MnGe

This work reports on the electronic and crystalline structure and the mechanical, magnetic, and transport properties of the polycrystalline Heusler compound Co2MnGe. The crystalline structure was examined in detail by extended x-ray absorption fine-structure spectroscopy and anomalous x-ray diffraction. The compound exhibits a well-ordered L21 structure as is typical for Heusler compounds with 2:1:1 stoichiometry. The low-temperature magnetic moment agrees well with the Slater-Pauling rule and indicates a half-metallic ferromagnetic state of the compound, as is predicted by ab initio calculations. Transport measurements and hard x-ray photoelectron spectroscopy were performed to explain the electronic structure of the compound. The obtained valence band spectra exhibit small energy shifts that are the result of the photoexcitation process, whereas electron-electron correlation in the ground state is negligible. The vibration and mechanical properties of the compound were calculated. The observed hardness values are consistent to a covalent-like bonding of Co2MnGe

Distinct Electronic Structure of the Electrolyte Gate-Induced Conducting Phase in Vanadium Dioxide Revealed by High-Energy Photoelectron Spectroscopy

The development of new phases of matter at oxide interfaces and surfaces by extrinsic electric fields is of considerable significance both scientifically and technologically. Vanadium dioxide (VO₂), a strongly correlated material, exhibits a temperature-driven metal-to-insulator transition, which is accompanied by a structural transformation from rutile (high-temperature metallic phase) to monoclinic (low-temperature insulator phase). Recently, it was discovered that a low-temperature conducting state emerges in VO₂ thin films upon gating with a liquid electrolyte. Using photoemission spectroscopy measurements of the core and valence band states of electrolyte-gated VO₂ thin films, we show that electronic features in the gate-induced conducting phase are distinct from those of the temperature-induced rutile metallic phase. Moreover, polarization-dependent measurements reveal that the V 3d orbital ordering, which is characteristic of the monoclinic insulating phase, is partially preserved in the gate-induced metallic phase, whereas the thermally induced metallic phase displays no such orbital ordering. Angle-dependent measurements show that the electronic structure of the gate-induced metallic phase persists to a depth of at least ∼40 Å, the escape depth of the high-energy photoexcited electrons used here. The distinct electronic structures of the gate-induced and thermally induced metallic phases in VO₂ thin films reflect the distinct mechanisms by which these states originate. The electronic characteristics of the gate-induced metallic state are consistent with the formation of oxygen vacancies from electrolyte gating