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The journal of physical chemistry / C 125(42), 23327 - 23337 (2021). doi:10.1021/acs.jpcc.1c05274, In this work, we present a study about the chemical and magnetic properties of thin magnetite/cobalt ferrite bilayers deposited on MgO(001). Two series of samples with different CoxFe$_{3���x}$O$_4$ stoichiometries (x = 1 and x = 0.5) in combination with Fe$_3$O$_4$ layers of varying thickness were prepared by reactive molecular beam epitaxy. The quality of the respective films were controlled by means of in situ X-ray photelectron spectroscopy and low energy electron diffraction. Stoichiometry and electronic structure were carried out by hard X-ray photoelectron spectroscopy. To determine the cationic distribution and magnetic moments, X-ray magnetic circular dichroism measurements were performed and charge transfer multiplet and sum rule calculations were applied. Here we find an enhanced interface magnetization for the bilayers. Additionally, superconducting quantum interference device measurements showed characteristic exchange-spring behavior., Published by Soc., Washington, DC

In order to explore an alternative pathway to prepare ultrathin CoFe2O4 films, epitaxial CoO/Fe3O4 bilayers with varying film thickness of the CoO film were grown on Nb-doped SrTiO3(001) substrates...

Detailed in operando studies of electrochemically induced PbBrF deposition at the liquid mercury/liquid electrolyte interface are presented. The nucleation and growth were monitored using time-reso...

Journal of synchrotron radiation 27(5), 1200 - 1208 (2020). doi:10.1107/S1600577520009789, Nanoprobe X-ray diffraction (nXRD) using focused synchrotron radiation is a powerful technique to study the structural properties of individual semiconductor nanowires. However, when performing the experiment under ambient conditions, the required high X-ray dose and prolonged exposure times can lead to radiation damage. To unveil the origin of radiation damage, a comparison is made of nXRD experiments carried out on individual semiconductor nanowires in their as-grown geometry both under ambient conditions and under He atmosphere at the microfocus station of the P08 beamline at the third-generation source PETRA III. Using an incident X-ray beam energy of 9 keV and photon flux of 10$^{10}$ s$^{−1}$, the axial lattice parameter and tilt of individual GaAs/In$_{0.2}$Ga$_{0.8}$As/GaAs core–shell nanowires were monitored by continuously recording reciprocal-space maps of the 111 Bragg reflection at a fixed spatial position over several hours. In addition, the emission properties of the (In,Ga)As quantum well, the atomic composition of the exposed nanowires and the nanowire morphology were studied by cathodoluminescence spectroscopy, energy-dispersive X-ray spectroscopy and scanning electron microscopy, respectively, both prior to and after nXRD exposure. Nanowires exposed under ambient conditions show severe optical and morphological damage, which was reduced for nanowires exposed under He atmosphere. The observed damage can be largely attributed to an oxidation process from X-ray-induced ozone reactions in air. Due to the lower heat-transfer coefficient compared with GaAs, this oxide shell limits the heat transfer through the nanowire side facets, which is considered as the main channel of heat dissipation for nanowires in the as-grown geometry., Published by Wiley-Blackwell, [S.l.]

It is demonstrated by a detailed structural analysis that the crystallinity and the efficiency of small molecule based organic photovoltaics can be tuned by solvent vapor annealing (SVA). Blends made of the small molecule donor 2,2′-[(3,3′″,3″″,4′-tetraoctyl[2,2′:5′,2″:5″,2′″:5′″,2″″-quinquethiophene]-5,5″″-diyl)bis[(Z)-methylidyne(3-ethyl-4-oxo-5,2-thiazolidinediylidene)]]bis-propanedinitrile (DRCN5T) and the acceptor [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) were annealed using solvent vapors with either a high solubility for the donor (tetrahydrofuran), the acceptor (carbon disulfide) or both (chloroform). The samples were analyzed by grazing-incidence wide-angle X-ray scattering (GIWAXS), electron diffraction, X-ray pole figures, and time-of-flight secondary ion mass spectrometry (ToF-SIMS). A phase separation of DRCN5T and PC71BM is induced by SVA leading to a crystallization of DRCN5T and the formation of a DRCN5T enriched layer. The DRCN5T crystallites possess the two dimensional oblique crystal system with the lattice parameters a = 19.2 Å, c = 27.1 Å, and β = 111.1° for the chloroform case. No major differences in the crystal structure for the other solvent vapors were observed. However, the solvent choice strongly influences the size of the DRCN5T enriched layer. Missing periodicity in the [010]-direction leads to the extinction of all Bragg reflections with k ≠ 0. The annealed samples are randomly orientated with respect to the normal of the substrate (fiber texture).

ACS catalysis 9(7), 6530 - 6539 (2019). doi:10.1021/acscatal.9b01402, Sophisticated $\mathrm{IrO_{2}(110)}$-based model electrodes are prepared by deposition of a 10 nm thick single-crystalline $\mathrm{IrO_{2}(110)}$ layer supported on a structure-directing $\mathrm{IrO_{2}(110)-RuO_{2}(110)/Ru(0001)}$ template, exposing a regular array of mesoscopic rooflike structures. With this model electrode together with the dedicated in situ synchrotron based techniques (SXRD, XRR) and ex situ characterization techniques (SEM, ToF-SIMS, XPS), the corrosion process of $\mathrm{IrO_{2}(110)}$ in an acidic environment (pH 0.4) is studied on different length scales. Potential-induced pitting corrosion starts at 1.48 V vs SHE and is initiated at so-called surface grain boundaries, where three rotational domains of $\mathrm{IrO_{2}(110)}$ meet. The most surprising result is, however, that even when the electrode potential is increased to 1.94 V vs SHE 60–70% of the $\mathrm{IrO_{2}}$ film still stays intact down to the mesoscale and atomic scale and no uniform thinning of the $\mathrm{IrO_{2}(110)}$ layer is encountered. Neither flat$\mathrm{IrO_{2}(110)}$ terraces nor single steps are attacked. Ultrathin single-crystalline $\mathrm{IrO_{2}(110)}$ layers seem to be much more stable to anodic corrosion than hitherto expected., Published by ACS, Washington, DC

We report on the influence of oxygen partial pressure for the development of surface oxides covering the industrial aluminum alloy standard 6063 at temperatures ranging from room temperature to 500° C. Using an array of synchrotron-based techniques, we followed the change in oxide thickness, chemical composition, and the lateral distribution of alloying elements. The impact of the oxygen chemical potential is most visible at high temperatures where the oxide composition changes from mostly Al based to mostly Mg based. This is in stark contrast to the ultra-high vacuum (UHV) conditions where only a partial compositional transition is observed. The microscopy data demonstrate that in the UHV case, Mg segregation onto the surface occurs firstly at grain boundaries at 300° C and secondly at sites over the entire surface at 400° C. Further, the initial oxide thickness is 45 A, as determined by XPS and XRR, decreases in all observed cases after heating to 300° C. At higher temperatures, however, the oxygen partial pressure highly influences the resulting oxide thickness as evident from our X-ray reflectivity data. (Less)

Nature Communications 12(1), 5046 (2021). doi:10.1038/s41467-021-25320-0, The original version of this Article contained an error in the Acknowledgements section, which was previously incorrectly given as���The authors gratefully acknowledge funding by the Austrian Science Fund (FWF) through project P4509-N16 as well as DESY(Hamburg, Germany), a member of the Helmholtz Association HGF, for the allocation of beamtime and provision of experimentalfacilities.���. The correct version states ���The authors gratefully acknowledge funding by the Austrian Science Fund (FWF) through projectF4509-N16 as well as DESY (Hamburg, Germany), a member of the Helmholtz Association HGF, for the allocation of beamtime andprovision of experimental facilities.��� in place of the incorrect text. This has been corrected in both the PDF and HTML versions of theArticle., Published by Nature Publishing Group UK, [London]

The catalytic oxidation of CO on transition metals, such as Pt, is commonly viewed as a sharp transition from the CO-inhibited surface to the active metal, covered with O. However, we find that minor amounts of O are present in the CO-poisoned layer that explain why, surprisingly, CO desorbs at stepped and flat Pt crystal planes at once, regardless of the reaction conditions. Using near-ambient pressure X-ray photoemission and a curved Pt(111) crystal we probe the chemical composition at surfaces with variable step density during the CO oxidation reaction. Analysis of C and O core levels across the curved crystal reveals that, right before light-off, subsurface O builds up within (111) terraces. This is key to trigger the simultaneous ignition of the catalytic reaction at different Pt surfaces: a CO-Pt-O complex is formed that equals the CO chemisorption energy at terraces and steps, leading to the abrupt desorption of poisoning CO from all crystal facets at the same temperature., We acknowledge financial support from the Spanish Ministry of Science and Innovation (Grants MAT-2017-88374-P, PID2019-107338RB-C63, PID2019-107338RB-C66), the Basque Government (Grants IT-1255-19, IT-1246-19), the Knut and Alice Wallenberg foundation (DNR KAW 2015.0058 „Atomistic Design of new Catalysts“) and the Swedish Research Council (DNR 349-2011-6491 „Catalysis on the atomic scale“).

The mechanism behind the stability of organic nanoparticles prepared by liquid antisolvent (LAS) precipitation without a specific stabilizing agent is poorly understood. In this work, we propose that the organic solvent used in the LAS process rapidly forms a molecular stabilizing layer at the interface of the nanoparticles with the aqueous dispersion medium. To confirm this hypothesis, n-octadecyltrichlorosilane (OTS)-functionalized silicon wafers in contact with water-solvent mixtures were used as a flat model system mimicking the solid-liquid interface of the organic nanoparticles. We studied the equilibrium structure of the interface by X-ray reflectometry (XRR) for water-solvent mixtures (methanol, ethanol, 1-propanol, 2-propanol, acetone, and tetrahydrofuran). The formation of an organic solvent-rich layer at the solid-liquid interface was observed. The layer thickness increases with the organic solvent concentration and correlates with the polar and hydrogen bond fraction of Hansen solubility parameters. We developed a self-consistent adsorption model via complementing adsorption isotherms obtained from XRR data with molecular dynamics simulations.

Nanotechnology 31(21), 1-7 (2020). doi:10.1088/1361-6528/ab7590, We report on the direct correlation between the structural and optical properties of single, as-grown core-multi-shell GaAs/In$_{0.15}$Ga$_{0.85}$As/GaAs/AlAs/GaAs nanowires. Fabricated by molecular beam epitaxy on a pre-patterned Si(111) substrate, on a row of well separated nucleation sites, it was possible to access individual nanowires in the as-grown geometry. The polytype distribution along the growth axis of the nanowires was revealed by synchrotron-based nanoprobe x-ray diffraction techniques monitoring the axial 111 Bragg reflection. For the same nanowires, the spatially-resolved emission properties were obtained by cathodoluminescence hyperspectral linescans in a scanning electron microscope. Correlating both measurements, we reveal a blueshift of the shell quantum well emission energy combined with an increased emission intensity for segments exhibiting a mixed structure of alternating wurtzite and zincblende stacking compared with the pure crystal polytypes. The presence of this mixed structure was independently confirmed by cross-sectional transmission electron microscopy., Published by IOP Publ., Bristol

Applied physics letters 117(1), 011601 - (2020). doi:10.1063/5.0013925, Ni$_x$Fe$_{3���x}$O$_4$ thin films with varying Ni amount (0 ��� x ��� 1.5) were deposited on MgO(001) via reactive molecular beam epitaxy. The growth process was monitored during film deposition by means of X-ray diffraction. All prepared films exhibit a well-ordered structure with complete vertical crystallinity throughout the whole film growth and flat surfaces of the final films independent of the Ni amount. An enhancement of the vertical compression in the initial growth continuously decreases up to a film thickness of 8 nm. During further growth, all films exhibit residual and constant vertical compression with lateral adaption of the final films to the substrate lattice, as observed by high energy surface X-ray diffraction experiments. Hard X-ray photoelectron spectroscopy measurements of the final films reveal increasing Fe$^{3+}$:Fe$^{2+}$ ratios for higher Ni content and point to additional NiO agglomerations within the films exceeding the stoichiometric Ni amount of x = 1., Published by American Inst. of Physics, Melville, NY

Typically, core–shell–shell semiconductor nanowires (NWs) made from III–V materials with low lattice mismatch grow pseudomorphically along the growth axis, i.e. the axial lattice parameters of the core and shell materials are the same. Therefore, both the structural composition and interface strain of the NWs are encoded along directions perpendicular to the growth axis. Owing to fluctuations in the supplied growth species during molecular beam epitaxy (MBE) growth, structural parameters such as local shell thickness, composition and strain may differ between NWs grown onto the same substrate. This requires structural analysis of single NWs instead of measuring NW ensembles. In this work, the complete structure of single GaAs/(In,Ga)As/GaAs core–shell–shell NW heterostructures is determined by means of X-ray nanodiffraction using synchrotron radiation. The NWs were grown by MBE on a prepatterned silicon (111) substrate with a core diameter of 50 nm and an (In,Ga)As shell thickness of 20 nm with a nominal indium concentration of 15%, capped by a 30 nm GaAs outer shell. In order to access single NWs with the X-ray nanobeam being incident parallel to the surface of the substrate, a single row of holes with a separation of 10 µm was defined by electron-beam lithography to act as nucleation centres for MBE NW growth. These well separated NWs were probed sequentially by X-ray nanodiffraction, recording three-dimensional reciprocal-space maps of Bragg reflections with scattering vectors parallel (out-of-plane) and perpendicular (in-plane) to the NW growth axis. From the out-of-plane 111 Bragg reflection, deviations from hexagonal symmetry were derived, together with the diameters of probed NWs grown under the same conditions. The radial NW composition and interface strain became accessible when measuring the two-dimensional scattering intensity distributions of the in-plane 2{\overline 2}0 and 22{\overline 4} reflections, exhibiting well pronounced thickness fringes perpendicular to the NW side planes (truncation rods, TRs). Quantitative values of thickness, composition and strain acting on the (In,Ga)As and GaAs shells were obtained via finite-element modelling of the core–shell–shell NWs and subsequent Fourier transform, simulating the TRs measured along the three different directions of the hexagonally shaped NWs simultaneously. Considering the experimental constraints of the current experiment, thicknesses and In content have been evaluated with uncertainties of ±2 nm and ±0.01, respectively. Comparing data taken from different single NWs, the shell thicknesses differ from one to another.

Anodization of aluminum alloys AA 6082 and AA 7075 was investigated in-situ with integrated electrochemical and synchrotron-based X-ray reflectivity (XRR) methods providing complementary information about the anodic processes taking place on the alloys. The stepwise potentiostatic polarization measurements reveal dynamic processes of the anodic oxide formation and dissolution, and the following electrochemical impedance spectroscopy measurements detect the break of the native oxide and the growth of typical two-layer anodic oxide film, while the XRR measurements show the growth of entire anodic oxide film whose thickness increases linearly with the increasing applied potential. The results indicate that while a stable anodic oxide can be formed on the both alloys with a similar growth factor, AA 7075 shows a thinner thickness of the barrier layer and a lower resistance of the oxide film. The electrochemical results suggest both localized and uniform anodic dissolution processes, which are more pronounced on AA 7075, demonstrating the effect of alloying elements on the growth of anodic oxides.

The journal of physical chemistry letters 10(7), 1634 - 1638 (2019). doi:10.1021/acs.jpclett.9b00331, Films of carbon tetrachloride (CCl$_4$) confined in slit geometry between two flat diamond substrates down to a few tens of Angstroms are studied by combining X-ray reflectivity with in-plane and out-of-plane X-ray scattering. The confined films form a heterogeneous structure with coexisting regions of liquid and crystalline phases. The liquid phase shows short-range ordering normal to the surfaces of the substrates. The experiments directly show the ability of the confinement to induce crystal objects, which is a long-discussed issue in the literature. The surface structure and morphology of the substrates may influence the actual realization of the crystalline phase in confinement., Published by ACS, Washington, DC

Applied physics letters 118(8), 089901 (2021). doi:10.1063/5.0045052, Published by American Inst. of Physics, Melville, NY

The journal of chemical physics 152(11), 114705 (1-9) (2020). doi:10.1063/1.5142558, We have investigated the structure of an ultrathin iron oxide phase grown on Ag(100) using surface x-ray diffraction in combination with Hubbard-corrected density functional theory (DFT+U) calculations. The film exhibits a novel structure composed of one close-packed layer of octahedrally coordinated Fe$^{2+}$ sandwiched between two close-packed layers of tetrahedrally coordinated Fe$^{3+}$ and an overall stoichiometry of Fe$_3$O$_4$. As the structure is distinct from bulk iron oxide phases and the coupling with the silver substrate is weak, we propose that the phase should be classified as a metastable two-dimensional oxide. The chemical and physical properties are potentially interesting, thanks to the predicted charge ordering between atomic layers, and analogy with bulk ferrite spinels suggests the possibility of synthesis of a whole class of two-dimensional ternary oxides with varying electronic, optical, and chemical properties., Published by American Institute of Physics, Melville, NY

Journal of synchrotron radiation 25, 1664 - 1672 (2018). doi:10.1107/S1600577518013218, Since the properties of functional materials are highly dependent on their specific structure, and since the structural changes, for example during crystallization, induced by coating and annealing processes are significant, the study of structure and its formation is of interest for fundamental and applied science. However, structure analysis is often limited to ex situ determination of final states due to the lack of specialized sample cells that enable real-time investigations. The lack of such cells is mainly due to their fairly complex design and geometrical restrictions defined by the beamline setups. To overcome this obstacle, an advanced sample cell has been designed and constructed; it combines automated doctor blading, solvent vapor annealing and sample hydration with real-time grazing-incidence wide- and small-angle scattering (GIWAXS/GISAXS) and X-ray reflectivity (XRR). The sample cell has limited spatial requirements and is therefore widely usable at beamlines and laboratory-scale instruments. The cell is fully automatized and remains portable, including the necessary electronics. In addition, the cell can be used by interested scientists in cooperation with the Institute for Crystallography and Structural Physics and is expandable with regard to optical secondary probes. Exemplary research studies are presented, in the form of coating of P3HT:PC$_{61}$PM thin films, solvent vapor annealing of DRCN5T:PC$_{71}$BM thin films, and hydration of supported phospholipid multilayers, to demonstrate the capabilities of the in situ cell., Published by IUCr, [S.l.]

The journal of physical chemistry / A 122(49), 28075 - 28084 (2018). doi:10.1021/acs.jpcc.8b08205, Doping with halide ions is a popular method toalter the properties of metal phthalocyanines (MPcs),particularly magnetic and electrical nature of organic semiconductorsfor applications in spintronic or electronic devices.Doping can cause a structural rearrangement in MPc packing,and the physical properties may be correlated with molecularpacking. Films of a planar and magnetic MPc, manganese-(II)phthalocyanine (MnPc), are chosen for iodine dopingstudy. The optical, magnetic, and electrical properties ofpristine- and iodine-doped MnPc thin films are investigatedand can be directly associated with their molecular structure.Two-dimensional grazing incidence synchrotron X-ray diffractionreveals structural disorder in MnPc films upon iodineinfusion induced by the reorientation of ordered, edge-on molecular configuration to tilted and face-on configurations in arandom fashion. The film morphology changes accordingly, where in the uniform crystallites reorganize in a disordered manner.The ferromagnetic nature of the pristine film gets weakened because of iodine species and favors antiferromagnetic coupling.The study of electrical properties at room temperature by conducting atomic force microscopy reveals that the conductance isenhanced independently of the film thickness because of the disorder induced by iodine inclusion., Published by ACS, Washington, DC

Journal of synchrotron radiation 25(5), 1389 - 1394 (2018). doi:10.1107/S1600577518010597, High-energy surface X-ray diffraction (HESXRD) provides surface structural information with high temporal resolution, facilitating the understanding of the surface dynamics and structure of the active phase of catalytic surfaces. The surface structure detected during the reaction is sensitive to the composition of the gas phase close to the catalyst surface, and the catalytic activity of the sample itself may affect the surface structure, which in turn may complicate the assignment of the active phase. For this reason, planar laser-induced fluorescence (PLIF) and HESXRD have been combined during the oxidation of CO over a Pd(100) crystal. PLIF complements the structural studies with an instantaneous two-dimensional image of the CO$_2$ gas phase in the vicinity of the active model catalyst. Here the combined HESXRD and PLIF operando measurements of CO oxidation over Pd(100) are presented, allowing for an improved assignment of the correlation between sample structure and the CO$_2$ distribution above the sample surface with sub-second time resolution., Published by Wiley-Blackwell, [S.l.]

The spin polarization of Pt in Pt/NiFe2O4 and Pt/Fe bilayers is studied by interface-sensitive x-ray resonant magnetic reflectivity to investigate static magnetic proximity effects. The asymmetry ratio of the reflectivity is measured at the Pt L3 absorption edge using circular polarized x-rays for opposite directions of the magnetization at room temperature. The results of the 2% asymmetry ratio for Pt/Fe bilayers are independent of the Pt thickness between 1.8 and 20 nm. By comparison with ab initio calculations, the maximum magnetic moment per spin polarized Pt atom at the interface is determined to be (0.6±0.1) μB for Pt/Fe. For Pt/NiFe2O4 the asymmetry ratio drops below the sensitivity limit of 0.02 μB per Pt atom. Therefore, we conclude, that the longitudinal spin Seebeck effect recently observed in Pt/NiFe2O4 is not influenced by a proximity induced anomalous Nernst effect.

Iron oxide films with different thicknesses (7.6–30 nm) were grown on clean MgO(001) substrates using reactive molecular beam epitaxy at 250 °C depositing Fe in a 5 × 10−5 mbar oxygen atmosphere. X-ray photoelectron spectra and low energy electron diffraction experiments indicate the stoichiometry and the surface structure of magnetite (Fe3O4). Film thicknesses and the lattice constants were analyzed ex situ by x-ray reflectometry and x-ray diffraction, respectively. These experiments reveal the single crystalline and epitactic state of the iron oxide films. However, the obtained vertical layer distances are too small to be strained magnetite and would rather suit to maghemite. Although Raman spectroscopy carried out to analyze the present iron oxide phase showed that the films might have slightly been oxidized in ambient conditions, a posteriori performed XPS measurements exclude a strong oxidation of the surface. Therefore we consider the presence of anti phase boundaries to explain the low vertical layer distances of the magnetite films. Further magnetooptic Kerr measurements were performed to investigate the magnetic properties. While the thinnest film shows a magnetic isotropic behavior, the thicker films exhibit a fourfold magnetic in-plane anisotropy. The magnetic easy axes are in the Fe3O4 directions. We propose that the magnetocrystalline anisotropy is too weak for very thin iron oxide films to form fourfold anisotropy related to the cubic crystal structure.

We present results from measurements of the native oxide film thickness on four different industrial aluminum alloys and three different aluminum single crystals. The thicknesses were determined using X-ray reflectivity, X-ray photoelectron spectroscopy, and electrochemical impedance spectroscopy. In addition, atomic force microscopy was used for micro-structural studies of the oxide surfaces. The reflectivity measurements were performed in ultra-high vacuum, vacuum, ambient, nitrogen and liquid water conditions. The results obtained using X-ray reflectivity and X-ray photoelectron spectroscopy demonstrate good agreement. However, the oxide thicknesses determined from the electrochemical impedance spectroscopy show a larger discrepancy from the above two methods. In the present contribution the reasons for this discrepancy are discussed. We also address the effect of the substrate type and the presence of water on the resultant oxide thickness.

International audience; We present results from the anodization of an aluminum single crystal [Al(111)] and an aluminum alloy [Al 6060] studied by in situ x-ray reflectivity, in situ electrochemical impedance spectroscopy and ex situ scanning electron microscopy. For both samples, a linear increase of oxide film thickness with increasing anodization voltage was found. However, the slope is much higher in the single crystal case, and the break-up of the oxide film grown on the alloy occurs at a lower anodization potential than on the single crystal. The reasons for these observations are discussed as are the measured differences observed for x-ray reflectivity and electrochemical impedance spectroscopy. (C) 2014 Author(s)

It is shown that ultrathin, highly ordered, continuous films of cerium dioxide may be prepared on silicon following substrate prepassivation using an atomic layer of chlorine. The as-deposited, few-nanometer-thin Ce2O3 film may very effectively be converted at room temperature to almost fully oxidized CeO2 by simple exposure to air, as demonstrated by hard X-ray photoemission spectroscopy and X-ray diffraction. This post-oxidation process essentially results in a negligible loss in film crystallinity and interface abruptness.

X-ray photoelectron emission microscopy (XPEEM) was used in combination with other microscopic and spectroscopic techniques to follow the surface development of an aluminum brazing sheet during heating. The studied aluminum alloy sheet is a composite material designed for vacuum brazing. Its surface is covered with a native aluminum oxide film. Changes in the chemical state of the alloying elements and the composition of the surface layer were detected during heating to the melting temperature. It was found that Mg segregates to the surface upon heating, and the measurements indicate the formation of magnesium aluminate. During the heating the aluminum oxide as well as the silicon is observed to disappear from the surface. Our measurements is in agreement with previous studies observing a break-up of the oxide and the outflow of the braze cladding onto the surface, a process assisted by the Mg segregation and reaction with surface oxygen. This study also demonstrates how XPEEM can be utilized to study complex industrial materials.

Journal of applied physics 109(9), 093907 (2011). doi:10.1063/1.3576135, Thin Co films of different thickness deposited on glass are investigated by magnetooptic Kerr effect to study the uniaxial magnetic anisotropy of these films. The direction of the uniaxial magnetic anisotropy is determined from the azimuthal dependence of the magnetic remanence and differs with increasing thickness of the Co film investigated by x-ray reflectivity. Our experiments reveal that preparation conditions like temperature, deposition rate, or obliqueness of deposition cannot be the reason for this rotation effect of the uniaxial magnetic anisotropy. Also, strain in the substrate and possible textures in the filmstructure can be excluded as the origin of the magnetic behavior as studied by grazing incidence wide angle x-ray scattering. Thus, probably only the substrate shape in connection with the amorphous or polycrystalline filmstructure can explain the rotation of the uniaxial magnetic anisotropy., Published by American Inst. of Physics, Melville, NY

Iron oxide films were reactively grown on iron buffer films, which were deposited before on MgO(001) substrates to analyze the influence of the initial iron buffer layers on the magnetic properties of the magnetite films. X-ray photoelectron spectroscopy and low energy electron dif-fraction showed that magnetite films of high crystalline quality in the surface near region were formed by this two-step deposition procedure. The underlying iron film, however, was completely oxidized as proved by x-ray reflectometry and diffraction. The structural bulk quality of the iron oxide film is poor compared to magnetite films directly grown on MgO(001). Although the iron film was completely oxidized, we found drastically modified magnetic properties for these films using the magnetooptic Kerr effect. The magnetite films had strongly increased coercive fields, and their magnetic in-plane anisotropy is in-plane rotated by 45° compared to magnetite films formed directly by one step reactive growth on MgO(001).

Journal of applied physics 105, 124108 (2009). doi:10.1063/1.3152796, Films of hexagonal praseodymium sesquioxide (h-Pr$_2$O$_3$) were deposited on Si(111) by molecular beam epitaxy and thereafter annealed in 1 atm oxygen at different temperatures, ranging from 100 to 700 °C. The films of the samples annealed at 300 °C or more were transformed to PrO2 with B-oriented Fmˉ3m structure, while films annealed at lower temperatures kept the hexagonal structure. The films are composed of PrO2 and PrO2−δ species, which coexist laterally and are tetragonally distorted due to the interaction at the interface between oxide film and Si substrate. Compared to PrO$_2$, PrO$_2$−δ has the same cubic structure but with oxygen vacancies. The oxygen vacancies are partly ordered and increase the vertical lattice constant of the film, whereas the lateral lattice constant is almost identical for both species and on all samples. The latter lattice constant matches the lattice constant of the originally crystallized hexagonal praseodymium sesquioxide. That means that no long range reordering of the praseodymium atoms takes place during the phase transformation, Published by American Institute of Physics, Melville, NY

Ultra thin heteroepitaxial PrO2 films on Si(111) were annealed under UHV conditions and investigated by x-ray diffraction (XRD), x-ray reflectometry (XRR) and spot profile analysis low energy electron diffraction (SPALEED) with regard to structural stability and phase transitions due to the high oxygen mobility of the oxide. This gives information about the manageability of the material and its application as a model catalyst system in surface science. While the samples are stable in UHV at room temperature, annealing at 300 ◦ C exhibits a terminated phase transition from PrO2 and PrO2−� to cub-Pr2O3 with an increase in the silicate at the interface and a decrease in the crystalline praseodymia layer mainly due to atomic diffusion of silicon into the oxide film. Strain effects during the phase transition also cause mosaic formation at the surface. Further annealing up to 600 ◦ C shows only little change in the film structure. This will finally lead to a model of the film structure during the annealing process. (Some figures in this article are in colour only in the electronic version)