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Integration of solvothermal reaction products into complex thin-layer architectures is frequently achieved by combinations of layer transfer and subtractive lithography, whereas direct additive substrate patterning with solvothermal reaction products has remained challenging. We report reactive additive capillary stamping under solvothermal conditions as a parallel contact-lithographic access to patterns of solvothermal reaction products in thin-layer configurations. To this end, corresponding precursor inks are infiltrated into mechanically robust mesoporous aerogel stamps derived from double-network hydrogels. The stamp is then brought into contact with a substrate to be patterned under solvothermal reaction conditions inside an autoclave. The precursor ink forms liquid bridges between the topographic surface pattern of the stamp and the substrate. Evaporation-driven enrichment of the precursors in these liquid bridges, along with their liquid-bridge-guided conversion into the solvothermal reaction products, yields large-area submicron patterns of the solvothermal reaction products replicating the stamp topography. For example, we prepared thin hybrid films, which contained ordered monolayers of superparamagnetic submicron nickel ferrite dots prepared by solvothermal capillary stamping surrounded by nickel electrodeposited in a second orthogonal substrate functionalization step. The submicron nickel ferrite dots acted as a magnetic hardener, halving the remanence of the ferromagnetic nickel layer. In this way, thin-layer electromechanical systems, transformers, and positioning systems may be customized.

Materials 15(1), 46 (2022). doi:10.3390/ma15010046, Here, we present the (element-specific) magnetic properties and cation ordering for ultrathin Co-rich cobalt ferrite films. Two Co-rich Co$_x$Fe$_{3−x}$O$_4$ films with different stoichiometry (x=1.1 and x=1.4) have been formed by reactive solid phase epitaxy due to post-deposition annealing from epitaxial CoO/Fe$_3$O$_4$ bilayers deposited before on Nb-doped SrTiO$_3$(001). The electronic structure, stoichiometry and homogeneity of the cation distribution of the resulting cobalt ferrite films were verified by angle-resolved hard X-ray photoelectron spectroscopy. From X-ray magnetic circular dichroism measurements, the occupancies of the different sublattices were determined using charge-transfer multiplet calculations. For both ferrite films, a partially inverse spinel structure is found with increased amount of Co$^{3+}$ cations in the low-spin state on octahedral sites for the Co$_{1.4}$Fe$_{1.6}$O$_4$ film. These findings concur with the results obtained by superconducting quantum interference device measurements. Further, the latter measurements revealed the presence of an additional soft magnetic phase probably due to cobalt ferrite islands emerging from the surface, as suggested by atomic force microscope measurements., Published by MDPI, Basel

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...

In spin-transport experiments with spin currents propagating through an antiferromagnetic (AFM) material, the antiferromagnet is mainly treated as a passive spin conductor not generating nor adding any spin current to the system. The spin current transmissivity of the AFM NiO is affected by magnetic fluctuations, peaking at the N\'eel temperature and decreasing by lowering the temperature. To study the role of antiferromagnetism in local and nonlocal spin-transport experiments, we send spin currents through NiO of various thicknesses placed on ${\mathrm{Y}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$. The spin currents are injected either electrically or by thermal gradients and measured at a wide range of temperatures and magnetic field strengths. The transmissive role is reflected in the sign change of the local electrically injected signals and the decrease in signal strength of all other signals by lowering the temperature. The thermally generated signals, however, show an additional upturn below $100\phantom{\rule{4pt}{0ex}}\mathrm{K}$ that is unaffected by an increased NiO thickness. A change in the thermal conductivity could affect these signals. The temperature and magnetic field dependence are similar to those for bulk NiO, indicating that NiO itself contributes to thermally induced spin currents.

We report an optimized two-step thermopolymerization process carried out in contact with micropatterned molds that yields porous phenolic resin dual-use stamps with topographically micropatterned contact surfaces. With these stamps, two different parallel additive substrate manufacturing methods can be executed: capillary stamping and decal transfer microlithography. Under moderate contact pressures, the porous phenolic resin stamps are used for nondestructive ink transfer to substrates by capillary stamping. Continuous ink supply through the pore systems to the contact surfaces of the porous phenolic resin stamps enables multiple successive stamp-substrate contacts for lithographic ink deposition under ambient conditions. No deterioration of the quality of the deposited pattern occurs, and no interruptions for ink replenishment are required. Under a high contact pressure, porous phenolic resin stamps are used for decal transfer printing. In this way, the tips of the stamps' contact elements are lithographically transferred to counterpart substrates. The granular nature of the phenolic resin facilitates the rupture of the contact elements upon stamp retraction. The deposited phenolic resin micropatterns characterized by abundance of exposed hydroxyl groups are used as generic anchoring sites for further application-specific functionalizations. As an example, we deposited phenolic resin micropatterns on quartz crystal microbalance resonators and further functionalized them with polyethylenimine for preconcentration sensing of humidity and gaseous formic acid. We envision that also preconcentration coatings for other sensing methods, such as attenuated total reflection infrared spectroscopy and surface plasmon resonance spectroscopy, are accessible by this functionalization algorithm.

Reactive molecular beam epitaxy of NiO on Ag(001) has been characterized by high-resolution low energy electron diffraction performed during deposition of Ni on Ag(001) in molecular oxygen atmosphere for deposition temperatures of 300–420 K. The initial stage of film growth at high deposition temperature is determined by competing processes of Ni oxidation and Ni interdiffusion into Ag(001) with Ni-Ag alloying. Ni oxidation is favored at low deposition temperatures. Thus, NiO bilayers are immediately formed. At high deposition temperatures, however, alloying with Ag is enhanced and the formation of the initial NiO bilayer is delayed. After formation of a closed NiO bilayer, alloying is prevented and all deposited Ni is oxidized and NiO growth proceeds in the layer-by-layer growth mode. Later stages of epitaxy are governed by relaxation processes induced by the formation of mosaics due to interface dislocations. The tilt angle of the mosaic decreases with increasing film thickness due to elastic deformation of the NiO lattice. Dislocation related mosaics are paired and form grooves. Postdeposition characterization shows periodic arrangement of grooves related to the formation of a regular dislocation network.

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

The magnetooptic Kerr effect (MOKE) is a well known and handy tool to characterize ferro-, ferri- and antiferromagnetic materials. Many of the MOKE techniques employ effects solely linear in magnetization $\bm{M}$. Nevertheless, a higher-order term being proportional to $\bm{M}^2$ and called quadratic MOKE (QMOKE) can additionally contribute to the experimental data. Here, we present detailed QMOKE spectroscopy measurements in the range of 0.8 -- 5.5\,eV based on a modified 8-directional method applied on ferromagnetic bcc Fe thin films grown on MgO substrates. From the measured QMOKE spectra, two further complex spectra of the QMOKE parameters $G_s$ and $2G_{44}$ are yielded. The difference between those two parameters, known as $\Delta G$, denotes the strength of the QMOKE anisotropy. Those QMOKE parameters give rise to the QMOKE tensor $\bm{G}$, fully describing the perturbation of the permittivity tensor in the second order in $\bm{M}$ for cubic crystal structures. We further present experimental measurements of ellipsometry and linear MOKE spectra, wherefrom permittivity in the zeroth and the first order in $\bm{M}$ are obtained, respectively. Finally, all those spectra are described by ab-initio calculations., Comment: 15 pages + references, , 15 Figures

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

The initial growth stages of MgO on Fe 3 O 4 films are studied by means of X-ray photoelectron spectroscopy and low energy electron diffraction to clarify stoichiometric and structural properties of these layered structures. This bilayer structure is important to fabricate high quality magnetic tunnel junctions based on Fe 3 O 4 electrodes and MgO tunneling barriers. For this purpose, the deposition temperature of MgO has been varied between 100 °C and 250 °C. Initially, MgO grows layer-by-layer on Fe 3 O 4 /MgO(0 0 1) forming a wetting layer. Depending on the growth temperature, after growth of a 2–3 nm thick laminar wetting layer, the MgO films finally start to roughen during growth. Thus the growth of MgO on Fe 3 O 4 /MgO(0 0 1) is described by a Stranski–Krastanov growth mode. Diffraction experiments show that the magnetite ( 2 × 2 )R45° superstructure is removed already during the initial stages of MgO deposition. Furthermore, these experiments show that MgO films are rougher for growth at low deposition temperatures.

The properties of beta-NaEuF4/NaGdF4 core-shell nanocrystals have been thoroughly investigated. Nano-particles with narrow size distribution and an overall diameter of similar to 22 nm have been produced with either small beta-NaEuF4 cores (similar to 3 nm diameter) or large beta-NaEuF4 cores (similar to 18 nm diameter). The structural properties and core-shell formation are investigated by X-ray diffraction, transmission electron microscopy and electron paramagnetic resonance, respectively. Optical luminescence measurements and X-ray photoelectron spectroscopy are employed to gain information about the optical emission bands and valence states of the rare earth constituents. Magnetic characterization is performed by SQUID and X-ray magnetic circular dichroism measurements at the rare earth M-4.5 edges. The characterization of the core-shell nanoparticles by means of these complementary techniques demonstrates that partial intermixing of core and shell materials takes place, and a significant fraction of europium is present in the divalent state which has significant influence on the magnetic properties. Hence, we obtained a combination of red emitting Eu3+ ions and paramagnetic Gd3+ ions, which may be highly valuable for potential future applications.

In present investigation, the effect of phase change, induced via sintering, from hematite (Fe2O3) to pure spinel structure on structural and optical properties in coprecipitated Mn0.4Zn0.6Fe2O4 nanoferrites has been studied. The Mn0.4Zn0.6Fe2O4 nanoparticles have been sintered at three temperatures 700 °C, 900 °C, and 1100 °C for 3 h. The presence of hematite (Fe2O3) phase in samples sintered at temperatures 700 °C and 900 °C has been observed from XRD. The crystallite size of the sintered samples has been found to increase on increasing the sintering temperature from 700 °C to 900 °C and then decrease for 1100 °C. Pure cubic spinel structure having single-phase has been obtained at 1100 °C. A cation distribution has also been proposed. Two absorption bands in the range 445 cm−1-473 cm−1, as well as 556 cm−1-582 cm−1, have been recognized in FTIR spectra and that indicates the formation of metal ion oxygen bond in spinel structure. With the rising sintering temperature, the peak position in the absorption spectra has been noticed to shift toward lesser wavelength, i.e., from 266 nm to 261.18 nm. The optical band gap shows shrinkage with the augmentation in the crystallite size on elevating the sintering temperature from 700 °C to 900 °C, while the optical band gap shows a broadening on raising the sintering temperature to 1100 °C. Photoluminescence study on the sintered samples has also been performed to examine the impurity levels and defects in samples. Stokes shift has been observed in all three samples.

We investigate the ${\mathrm{CaF}}_{1}/\mathrm{Si}(111)$ interface using a combination of high-resolution scanning tunneling and noncontact atomic force microscopy operated at cryogenic temperature as well as x-ray photoelectron spectroscopy. Submonolayer ${\mathrm{CaF}}_{1}$ films grown at substrate temperatures between 550 and 600 ${}^{\ensuremath{\circ}}\mathrm{C}$ on $\mathrm{Si}(111)$ surfaces reveal the existence of two island types that are distinguished by their edge topology, nucleation position, measured height, and inner defect structure. Our data suggest a growth model where the two island types are the result of two reaction pathways during ${\mathrm{CaF}}_{1}$ interface formation. A key difference between these two pathways is identified to arise from the excess species during the growth process, which can be either fluorine or silicon. Structural details as a result of this difference are identified by means of high-resolution noncontact atomic force microscopy and add insights into the growth mode of this heteroepitaxial insulator-on-semiconductor system.

High overpotentials, particularly an issue of common anode materials, hamper the process of water electrolysis for clean energy generation. Thanks to immense research efforts up to date oxygen evolution electrocatalysts based on earth-abundant elements work efficiently and stably in neutral and alkaline regimes. However, non-noble metal-based anode materials that can withstand low pH regimes are considered to be an indispensable prerequisite for the water splitting to succeed in the future. All oxygen evolving electrodes working durably and actively in acids contain Ir at least as an additive. Due to its scarcity and high acquisition costs noble elements like Pt, Ru and Ir need to be replaced by earth abundant elements. We have evaluated a Ni containing stainless steel for use as an oxygen-forming electrode in diluted H2SO4. Unmodified Ni42 steel showed a significant weight loss after long term OER polarization experiments. Moreover, a substantial loss of the OER performance of the untreated steel specimen seen in linear sweep voltammetry measurements turned out to be a serious issue. However, upon anodization in LiOH, Ni42 alloy was rendered in OER electrocatalysts that exhibit under optimized synthesis conditions stable overpotentials down to 445 mV for 10 mA cm-2 current density at pH 0. Even more important: The resulting material has proven to be robust upon long-term usage (weight loss: 20 mug/mm2 after 50 ks of chronopotentiometry at pH 1) towards OER in H2SO4. Our results suggest that electrochemical oxidation of Ni42 steel in LiOH (sample Ni42Li205) results in the formation of a metal oxide containing outer zone that supports solution route-based oxygen evolution in acidic regime accompanied by a good stability of the catalyst., arXiv admin note: text overlap with arXiv:1712.01100

The surface of steel S235 was oxidized by Cl2 gas and checked for its electrocatalytic efficiency regarding oxygen formation in aqueous solution. If exposed to humid Cl2 gas for 110 min, steel S235 became an electrocatalyst that exhibits an overpotential for the oxygen evolution reaction (OER) of 462 mV at 1 mA cm(-2) at pH 7. The OER activity of the same sample at pH 13 was moderate (347 mV overpotential at 2.0 mA cm(-2) current density) in comparison with OER electrocatalysts developed recently. Potential versus time plots measured at a constant current demonstrate the sufficient stability of all samples under catalysis conditions at pH 7 and 13 for tens of hours. High-resolution X-ray photoelectron spectra could be reasonably resolved with the proviso that Fe2 O3 , FeO(OH), MnO(OH), and Mn2 O3 are the predominant Fe and Mn species on the surface of the oxidized steel S235.

The cheap stainless commodity steel AISI 304, which basically consists of Fe, Ni, and Cr, was surface-oxidized by exposure to Cl2 gas. This treatment turned AISI 304 steel into an efficient electrocatalyst for water splitting at pH 7 and pH 13. The overpotential of the anodic oxygen evolution reaction (OER), which typically limits the efficiency of the overall water-splitting process, could be reduced to 260 mV at 1.5 mA/cm2 in 0.1 M KOH. At pH 7, overpotentials of 500–550 mV at current densities of 0.65 mA/cm2 were achieved. These values represent a surprisingly good activity taking into account the simplicity of the procedure and the fact that the starting material is virtually omnipresent. Surface-oxidized AISI 304 steel exhibited outstanding long-term stability of its electrocatalytic properties in the alkaline as well as in the neutral regime, which did not deteriorate even after chronopoteniometry for 150 000 s. XPS analysis revealed that surface oxidation resulted in the formation of Fe oxide and C...

Au-induced wire structures grown on Ge(100) were shown to exhibit spectroscopic signatures of strong electronic correlation. However, the atomistic structure of these wires is still controversially discussed. In this study the morphology has been investigated by means of low energy electron diffraction (SPA-LEED). The detailed analysis of diffraction profiles favors strongly to the model of an Au-induced giant missing row (GMR) structure rather than the heterodimer model. Systematic variation of the initial Au-coverage has revealed further that approximately 0.25 ML of the deposited Au coverage penetrates into subsurface positions stabilizing the adsorbate-induced surface structure.

The oxygen evolution reaction (OER) is known as the efficiency-limiting step for the electrochemical cleavage of water mainly due to the large overpotentials commonly used materials on the anode side cause. Since Ni–Fe oxides reduce overpotentials occurring in the OER dramatically they are regarded as anode materials of choice for the electrocatalytically driven water-splitting reaction. We herewith show that a straightforward surface modification carried out with AISI 304, a general purpose austenitic stainless steel, very likely, based upon a dissolution mechanism, to result in the formation of an ultra-thin layer consisting of Ni, Fe oxide with a purity >99%. The Ni enriched thin layer firmly attached to the steel substrate is responsible for the unusual highly efficient anodic conversion of water into oxygen as demonstrated by the low overpotential of 212 mV at 12 mA cm−2 current density in 1 M KOH, 269.2 mV at 10 mA cm−2 current density in 0.1 M KOH respectively. The Ni, Fe-oxide layer formed on the steel creates a stable outer sphere, and the surface oxidized steel samples proved to be inert against longer operating times (>150 ks) in alkaline medium. In addition Faradaic efficiency measurements performed through chronopotentiometry revealed a charge to oxygen conversion close to 100%, thus underpinning the conclusion that no “inner oxidation” based on further oxidation of the metal matrix below the oxide layer occurs. These key figures achieved with an almost unrivalled-inexpensive and unrivalled-accessible material, are among the best ever presented activity characteristics for the anodic water-splitting reaction at pH 13.

We identify and investigate thermal spin transport phenomena in sputter-deposited $\mathrm{Pt}/{\mathrm{NiFe}}_{2}{\mathrm{O}}_{x}$ ($4\ensuremath{\ge}x\ensuremath{\ge}0$) bilayers. We separate the voltage generated by the spin Seebeck effect from the anomalous Nernst effect (ANE) contributions and even disentangle the ANE in the ferromagnet (FM) from the ANE produced by the Pt that is spin polarized due to its proximity to the FM. Further, we probe the dependence of these effects on the electrical conductivity and the band gap energy of the FM film varying from nearly insulating ${\mathrm{NiFe}}_{2}{\mathrm{O}}_{4}$ to metallic ${\mathrm{Ni}}_{33}{\mathrm{Fe}}_{67}$. A proximity-induced ANE could only be identified in the metallic $\mathrm{Pt}/{\mathrm{Ni}}_{33}{\mathrm{Fe}}_{67}$ bilayer in contrast to $\mathrm{Pt}/{\mathrm{NiFe}}_{2}{\mathrm{O}}_{x}$ ($xg0$) samples. This is verified by the investigation of static magnetic proximity effects via x-ray resonant magnetic reflectivity.

Motivated by the complex diffraction pattern observed for bundled rare-earth silicide nanowires on the Si(0 0 1) surface, we investigate the influence of the width and the spacing distribution of the nanowires on the diffraction pattern. The diffraction pattern of the bundled rare-earth silicide nanowires is analyzed by the binary surface technique applying a kinematic approach to diffraction. Assuming a categorical distribution for the (individual) nanowire size and a Poisson distribution for the size of the spacing between adjacent nanowire-bundles, we are able to determine the parameters of these distributions and derive an expression for the distribution of the nanowire-bundle size. Additionally, the comparison of our simulations to the experimental diffraction pattern reveal that a (1 × 1)-periodicity on top of the nanowires has to be assumed for a good match.

The journal of physical chemistry / C 118(6), 3056 - 3061(2014). doi:10.1021/jp4082917, The structural changes of ultrathin PrO$_2$(111) films grown on Si(111) due to thermal reduction in ultra high vacuum are investigated. Therefore, praseodymia films are treated with oxygen plasma to obtain completely oxidized PrO$_2$ films. The reduction of these films is studied with thermal desorption spectroscopy combined with ex situ X-ray diffraction measurements. It is shown that the distinct desorption peaks of molecular oxygen are correlated with stabilization of several intermediate phases of praseodymia. The most intense signal of desorbing oxygen is observed at ∼650 K. It is coupled to the strongest impact on the crystalline structure since we observe the transition PrO$_2$ → Pr$_5$O$_9$. Further oxygen desorption leads to additional phase transitions with laterally coexisting phases. Grazing incidence X-ray diffraction measurements reveal that the initial pinning to the lateral lattice constant of hexagonal Pr$_2$O$_3$(0001) is not breached due to the plasma treatment. This hexagonal pinning is released during the oxygen desorption so that the crystalline structure of the praseodymia films does not only expand vertically but also laterally., Published by Soc., Washington, DC

After deposition of rare-earth elements (Dy, Tb) on Si(111) at elevated temperatures, a formerly unknown $\left(2\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}\right)R{30}^{\ensuremath{\circ}}$ reconstruction is observed by low-energy electron diffraction, while scanning tunneling microscopy measurements exhibit a $\left(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}\right)R{30}^{\ensuremath{\circ}}$ reconstruction. On the basis of density-functional theory calculations, the structure of the larger unit cell is explained by periodically arranged subsurface Si vacancies. The vacancy network in the first subsurface layer has a $\left(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}\right)R{30}^{\ensuremath{\circ}}$ periodicity, while strain is released by a $\left(2\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}\right)R{30}^{\ensuremath{\circ}}$ Si vacancy network in the second subsurface layer. In addition, this vacancy network forms quasi-one-dimensional structures (striped domains) separated by periodically arranged antiphase domain boundaries. The diffraction spot profiles are explained in detail by kinematic diffraction theory calculations, and average domain widths are deduced.

Motivated by diffraction experiments on the 23×3R30∘ reconstructed Si(111) due to deposition of rare earth elements (Dy, Tb) and silicide formation we analyse the splitting and non-splitting of superstructure spots. For this purpose, we model diffraction patterns for one dimensional structures generated by the binary surface technique and use supercell models to keep the analysis simple. Diffraction pattern are calculated in the framework of the kinematical diffraction theory and they are analyzed as a function of the domains and domain boundaries. Basic properties of the diffraction pattern are analyzed for model systems of a two-fold and a three-fold periodicity. The rules derived from these calculations are applied to the "real-world" system of Si(111)-(23×3R30∘)-RESi2 (RE = Dy or Tb). Depending on the combination of domains and domain boundaries of different types a plethora of different features are observed in the diffraction patterns. These are analyzed to determine the sizes of both domain boundaries and domains from experimentally observed splitting of specific superstructure spots.

Ferrites with (inverse) spinel structure display a large variety of electronic and magnetic properties, making some of them interesting for potential applications in spintronics. We investigate the thermally induced interdiffusion of ${\mathrm{Ni}}^{2+}$ ions out of NiO into ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ ultrathin films, resulting in off-stoichiometric nickel ferrite--like thin layers. We synthesized epitaxial ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$/NiO bilayers on Nb-doped ${\mathrm{SrTiO}}_{3}$(001) substrates by means of reactive molecular beam epitaxy. Subsequently, we performed an annealing cycle comprising three steps at temperatures of 400 ${}^{\ensuremath{\circ}}\mathrm{C}$, 600 ${}^{\ensuremath{\circ}}\mathrm{C}$, and 800 ${}^{\ensuremath{\circ}}\mathrm{C}$ under an oxygen background atmosphere. We studied the changes of the chemical and electronic properties as result of each annealing step with help of hard x-ray photoelectron spectroscopy and found a rather homogeneous distribution of Ni and Fe cations throughout the entire film after the overall annealing cycle. For one sample we observed a cationic distribution close to that of the spinel ferrite ${\mathrm{NiFe}}_{2}{\mathrm{O}}_{4}$. Further evidence comes from low-energy electron diffraction patterns indicating a spinel-type structure at the surface after annealing. Site- and element-specific hysteresis loops performed by x-ray magnetic circular dichroism uncovered the antiferrimagnetic alignment between the octahedral coordinated ${\mathrm{Ni}}^{2+}$ and ${\mathrm{Fe}}^{3+}$ ions and the ${\mathrm{Fe}}^{3+}$ ion in tetrahedral coordination. We find a quite low coercive field of 0.02 T, indicating a rather low defect concentration within the thin ferrite films.

We study the underlying chemical, electronic, and magnetic properties of a number of magnetite-based thin films. The main focus is placed onto ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$(001)/NiO bilayers grown on MgO(001) and Nb-${\mathrm{SrTiO}}_{3}$(001) substrates. We compare the results with those obtained on pure ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$(001) thin films. It is found that the magnetite layers are oxidized and ${\mathrm{Fe}}^{3+}$ dominates at the surfaces due to maghemite ($\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$) formation, which decreases with increasing magnetite layer thickness. For layer thicknesses of around 20 nm and above, the cationic distribution is close to that of stoichiometric ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$. At the interface between NiO and ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ we find the Ni to be in a divalent valence state, with unambiguous spectral features in the Ni $2p$ core level x-ray photoelectron spectra typical for NiO. The formation of a significant ${\mathrm{NiFe}}_{2}{\mathrm{O}}_{4}$ interlayer can be excluded by means of x-ray magnetic circular dichroism. Magneto-optical Kerr effect measurements reveal significant higher coercive fields compared to magnetite thin films grown on MgO(001), and an altered in-plane easy axis pointing in the $\ensuremath{\langle}100\ensuremath{\rangle}$ direction. We discuss the spin magnetic moments of the magnetite layers and find that a thickness of 20 nm or above leads to spin magnetic moments close to that of bulk magnetite.

We present x-ray resonant magnetic reflectivity (XRMR) as a very sensitive tool to detect proximity induced interface spin polarization in Pt/FM heterostructures. Different XRMR experiments are carried out and the results are evaluated for their dependence on the magneto-optical depth profile, the photon energy, the optical parameters, and the ferromagnetic material. We demonstrate that a detailed analysis of the reflected x-ray intensity gives insight into the spatial distribution of the spin polarization of a nonmagnetic metal across the interface to a ferromagnetic layer. The evaluation of the experimental results with simulations based on optical data from ab initio calculations provides the induced magnetic moment per Pt atom in the spin-polarized volume adjacent to the ferromagnet. For a series with different ferromagnetic materials consisting of Pt/Fe, $\text{Pt}/{\mathrm{Ni}}_{33}{\mathrm{Fe}}_{67}, \text{Pt}/{\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$ (permalloy), and Pt/Ni bilayers we find the largest spin polarization in Pt/Fe and a much smaller magnetic proximity effect in Pt/Ni. Additional XRMR experiments with varying photon energy are in good agreement with the theoretical predictions for the energy dependence of the magneto-optical parameters and allow identifying the optical dispersion $\ensuremath{\delta}$ and absorption $\ensuremath{\beta}$ across the $\mathrm{Pt}\phantom{\rule{0.28em}{0ex}}{\mathrm{L}}_{3}$-absorption edge.

Rare-earth induced layered structures on the Si(111) surface are investigated by a combined approach consisting of ab initio thermodynamics, electron and x-ray diffraction experiments, angle-resolved photoelectron spectroscopy, and scanning tunneling microscopy. Our density functional theory calculations predict the occurrence of structures with different periodicity, depending on the rare-earth availability. Microscopic structural models are assigned to the different silicide phases on the basis of stability criteria. The thermodynamically stable theoretical models are then employed to interpret the experimental results. The agreement between the simulated and measured scanning tunneling microscopy images validates the proposed structural models. The electronic properties of the surfaces are discussed on the basis of the calculated electronic band structure and photoelectron spectroscopy data.

SPIE OPTO, San Francisco, California ; Proceedings of SPIE 9749, 974917(2016). doi:10.1117/12.2219627, Ultrathin magnetite ($Fe_{3}O_{4}$) films are attractive for applications in the field of spintronics due to their ferrimagnetic behavior with assumed high degree of spin polarized electrons at the Fermi energy. For these applications, it is necessary to form epitactical bilayer structure combining ferrimagnetic magnetite with an antiferromagnetic layer. Therefore, here we study $Fe_{3}O_{4}$/NiO bilayers on MgO(001) substrates. Bilayers grown by reactive molecular beam epitaxy are stoichiometric and have well-developed surface and interface structures. The NiO layers are laterally pinned to the structure of the MgO(001) substrate while the magnetite films gradually relax. The interfaces show smooth morphologies and the films have very homogeneous film thickness necessary for spintronical applications. The magnetic and magneto optical properties of the $Fe_{3}O_{4}$/NiO bilayers were probed by the magneto optical Kerr effect. Compared to single $Fe_{3}O_{4}$ layers on MgO(001), the bilayers show complicated ferrimagnetic behavior depending on the azimuthal direction of the external applied field. The coercive field of the bilayers, however, is increased with the coercive field of single layer $Fe_{3}O_{4}$/MgO(001) structures making the Fe3O4/NiO bilayers attractive for spintronic applications., Published by SPIE, Bellingham, Wash.

Janus type Water-Splitting Catalysts have attracted highest attention as a tool of choice for solar to fuel conversion. AISI Ni 42 steel was upon harsh anodization converted in a bifunctional electrocatalyst. Oxygen evolution reaction- (OER) and hydrogen evolution reaction (HER) are highly efficiently and steadfast catalyzed at pH 7, 13, 14, 14.6 (OER) respectively at pH 0, 1, 13, 14, 14.6 (HER). The current density taken from long-term OER measurements in pH 7 buffer solution upon the electro activated steel at 491 mV overpotential was around 4 times higher (4 mA/cm2) in comparison with recently developed OER electrocatalysts. The very strong voltage-current behavior of the catalyst shown in OER polarization experiments at both pH 7 and at pH 13 were even superior to those known for IrO2-RuO2. No degradation of the catalyst was detected even when conditions close to standard industrial operations were applied to the catalyst. A stable Ni-, Fe- oxide based passivating layer sufficiently protected the bare metal for further oxidation. Quantitative charge to oxygen- (OER) and charge to hydrogen (HER) conversion was confirmed. High resolution XPS spectra showed that most likely gamma-NiO(OH) and FeO(OH) are the catalytic active OER and NiO is the catalytic active HER species.

Epitaxial oxide thin film layers are of interest for model catalytic studies. We report the growth of Ce 1− x Pr x O 2− δ mixed oxide layers of different stoichiometries ( x =0–1) and oxygen deficiency ( δ >0) on Si(111) by co-evaporating molecular beam epitaxy. The main objective is to identify the crystal phases and to investigate the correlation between compositions and crystal structures. X-ray photoemission spectroscopy was performed to quantify the stoichiometries. An extensive laboratory and synchrotron based X-ray diffraction analysis was carried out to determine the vertical and lateral lattice orientations and the strain status of the layers. The study revealed that single crystalline Ce 1− x Pr x O 2− δ /Si(111) heterostructures can be epitaxially grown on Si(111) for model catalytic studies. In addition to the structure–stoichiometry relationship typical to mixed oxide bulk powders, we identified a hexagonal mixed Ce–Pr oxide thin film phase not yet reported in bulk studies.

Degenerately boron-doped vicinal Si(111) substrates were annealed at high temperatures to achieve complete saturation of the ‘dangling bonds’ by surface segregating boron atoms under reconstruction to the Si(111) (3×3)R30∘ phase. Different cooling procedures were applied to obtain well ordered superstructures. The surface morphology of the prepared substrates was studied by spot profile analysis of low energy electron diffraction (SPA-LEED). The splitting of the diffraction peaks into multiple satellites points to the formation of step bunches of different inclination (equivalent to different terrace widths) during the segregation process due to changes in the surface free energy. A model with step bunches consisting of steps with monoatomic or biatomic heights and large (111)-oriented terraces is developed to explain our results. Steep step bunches with short terraces and biatomic step heights are formed independently of the preparation process while flat step bunches with large terraces are dissolved with increasing annealing time.

Anatase phase titanium dioxide thin films have been deposited at various substrate temperatures by chemical spray pyrolysis of an aerosol of titanyl acetylacetonate. Deposited TiO2 films were nanocrystalline and preferentially oriented along [101] direction, uniform and adherent to the glass substrate. Best films processed at 450 °C were characterized to analyze its phase composition, texture, roughness, optical and electrical properties. X-ray photoelectron spectroscopy revealed that the surface of the film has only the Ti4+ cations to form perfect TiO2 stoichiometry with less amount of hydration. Atomic force microscopy image demonstrated the existence of homogeneous and rough surface, suitable for electrocatalytic applications. The film has an optical transmittance more than 90% and the refractive index of 2.07 was recorded at the wavelength 633 nm. Due to nano-sized grains, obtained optical band gap (3.65 eV) of the TiO2 thin film was larger than that of the bulk TiO2 (3.2 eV). Calculated porosity of the films 0.44, revealed the porous nature of the films. Hall measurements indicated that these materials are p-type and yield a carrier density of the order 8.8 × 1020 cm−3 and a carrier mobility of 0.48 × 10−6 cm2/Vs. The dc electrical conductivity was therefore very low (8.91 × 10−6 S/cm) because of lower value of mean free path of the charge carriers (4.36 × 10−11 cm). It gives an impression that the process of spray pyrolysis provides an easy way to tailor make thin films possessing superior properties.

Spray pyrolysis technique has been employed successfully for the synthesis of single phase mixed valence spinel hausmannite (Mn3O4) thin films using alcoholic start solution of manganese acetate (Mn(CH3COO)2·4H2O) on pyrex glass substrates at atmospheric pressure using air as a carrier gas. Thermal decomposition of the precursor in the temperature range 320–490 °C led to the formation of Mn3O4 phase as revealed from the thermogravimetry analysis. Prepared samples are characterized by X-ray diffraction that shows spinel structure with space group I41/amd. Pure and well crystallized specimen is subjected to X-ray photoelectron spectroscopy for the surface chemistry investigation of these systems at a molecular level. Surface Mn/O ratio is compared to the bulk composition of the sample. Atomic force micrographs revealed that the morphology and the surface grains of the films largely influenced by the substrate temperature.

Ternary semiconducting oxide compound magnesium indium oxide films (MgIn2O4), manifesting high transparency were prepared by metal organic chemical spray pyrolysis technique. Precursors prepared for various cationic ratios of Mg/In = 0.35, 0.40, 0.45 and 0.50 were thermally sprayed onto quartz substrates, decomposed at 450 °C and the spinel phase evolution was studied. X-ray diffraction, Rutherford backscattering and X-ray photoelectron spectroscopy studies have been conducted to confirm the formation of single-phase MgIn2O4 films with Mg/In ratio 0.50. From optical transmission studies, the observed optical band gaps varied from 3.18 to 3.86 eV (0.35

Praseodymia films have been exposed to oxygen plasma at room temperature after deposition on Si(111) via molecular beam epitaxy. Different parameters as film thickness, exposure time and flux during plasma treatment have been varied to study their influence on the oxygen plasma oxidation process. The surface near regions have been investigated by means of X-ray photoelectron spectroscopy showing that the plasma treatment transforms the stoichiometry of the films from Pr2O3 to PrO2. Closer inspection of the bulk properties of the films by means of synchrotron radiation based X-ray reflectometry and diffraction confirms this transformation if the films are thicker than some critical thickness of 6 nm. The layer distance of these films is extremely small verifying the completeness of the plasma oxidation process. Thinner films, however, cannot be transformed completely. For all films, less oxidized very thin interlayers are detected by these experimental techniques.

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.