[en] The biomimetic, deterministic aperiodic golden angle Vogel spiral lacks both translational and rotational symmetry and exhibits circularly symmetric scattering rings in Fourier space. We present a versatile and low cost method for maskless fabrication of a Vogel spiral relief grating by illumination of a photoresist with laser light complexly structured using a spatial light modulator and subsequent transfer of its negative into a polymer layer via replica molding. We investigate its diffractive properties and present as one application its implementation as a surface structure, leading to an efficiency enhancement of 18% in an organic solar cell. (paper)

[en] Magnetite ultrathin films were grown using different deposition rates and substrate temperatures. The structure of these films was studied using (grazing incidence) x-ray diffraction, while their surface structure was characterized by low energy electron diffraction. In addition to that, we performed x-ray photoelectron spectroscopy and magneto optic Kerr effect measurements to probe the stoichiometry of the films as well as their magnetic properties. The diffraction peaks of the inverse spinel structure, which originate exclusively from Fe ions on tetrahedral sites are strongly affected by the preparation conditions, while the octahedral sites remain almost unchanged. With both decreasing deposition rate as well as decreasing substrate temperature, the integrated intensity of the diffraction peaks originating exclusively from Fe on tetrahedral sites is decreasing. We propose that the ions usually occupying tetrahedral sites in magnetite are relocated to octahedral vacancies. Ferrimagnetic behaviour is only observed for well ordered magnetite films.

[en] Epitaxial ultrathin iron oxide films of different thicknesses were grown by reactive molecular beam epitaxy in 10^-6 mbar oxygen atmosphere on MgO(001) single crystal substrates at room temperature. Afterwards, the films were studied by x-ray diffraction, x-ray reflectivity and x-ray photoelectron spectroscopy to provide information regarding film structure as well as chemical composition of the films. Except for a very thin interface layer of subnanometre thickness, the iron oxide films have magnetite stoichiometry and structure and Mg does not diffuse from the substrate into the iron oxide film. The interface layer has a wuestite structure as determined by kinematic diffraction analysis. The magnetite films exhibit very homogeneous thickness while the vertical lattice constant decreases gradually towards its bulk value.

[en] Magnetite (Fe_3O_4) is an eligible candidate for magnetic tunnel junctions (MTJs) since it shows a high spin polarization at the Fermi level as well as a high Curie temperature of 585°C. In this study, Fe_3O_4/MgO/Co-Fe-B MTJs were manufactured. A sign change in the TMR is observed after annealing the MTJs at temperatures between 200°C and 280°C. Our findings suggest an Mg interdiffusion from the MgO barrier into the Fe_3O_4 as the reason for the change of the TMR. Additionally, different treatments of the magnetite interface (argon bombardment, annealing at 200°C in oxygen atmosphere) during the preparation of the MTJs have been studied regarding their effect on the performance of the MTJs. A maximum TMR of up to -12% could be observed using both argon bombardment and annealing in oxygen atmosphere, despite exposing the magnetite surface to atmospheric conditions before the deposition of the MgO barrier

[en] 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 diffraction 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)

[en] Iron oxide monolayers are grown on Ag(0 0 1) via reactive molecular beam epitaxy (metal deposition in oxygen atmosphere). The monolayer shows FeO stoichiometry as concluded from x-ray photoemission spectra. Both low energy electron diffraction as well as scanning tunneling microscopy demonstrate that the FeO layer has a quasi-hexagonal (1 1 1) structure although deposited on a surface with square symmetry. Compared to bulk values, the FeO(1 1 1) monolayer is unidirectionally expanded by 3.4% in 〈11 2-bar 〉 directions while bulk values are maintained in 〈1 1-bar 0〉 directions. In 〈11 2-bar 〉 directions, this lattice mismatch between FeO(1 1 1) monolayer and Ag(0 0 1) causes a commensurate undulation of the FeO monolayer where 18 atomic rows of the FeO(1 1 1) monolayer match 17 atomic rows of the Ag(0 0 1) substrate. In 〈1 1-bar 0〉 directions, however, the FeO(1 1 1) monolayer has an incommensurate structure. (paper)

[en] 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 (Fe_3O_4). 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 Fe_3O_4 〈110〉 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. (paper)

[en] Crystalline Fe_3O_4/NiO bilayers were grown on MgO(001) substrates using reactive molecular beam epitaxy to investigate their structural properties and their morphology. The film thickness either of the Fe_3O_4 film or of the NiO film has been varied to shed light on the relaxation of the bilayer system. The surface properties as studied by X-ray photoelectron spectroscopy and low energy electron diffraction show clear evidence of stoichiometric well-ordered film surfaces. Based on the kinematic approach X-ray diffraction experiments were completely analyzed. As a result the NiO films grow pseudomorphic in the investigated thickness range (up to 34 nm) while the Fe_3O_4 films relax continuously up to the thickness of 50 nm. Although all diffraction data show well developed Laue fringes pointing to oxide films of very homogeneous thickness, the Fe_3O_4/NiO interface roughens continuously up to 1 nm root-mean-square roughness with increasing NiO film thickness while the Fe_3O_4 surface is very smooth independent on the Fe_3O_4 film thickness. Finally, the Fe_3O_4/NiO interface spacing is similar to the interlayer spacing of the oxide films while the NiO/MgO interface is expanded. - Highlights: • Oxide films prepared by reactive molecular beam epitaxy • Increasing relaxation of Fe_3O_4 film with film thickness • Suppressed relaxation for NiO film • Increasing Fe_3O_4–NiO interface roughness with NiO film thickness • Smooth Fe_3O_4 film surface