The electronic structure of the central Fe ion of iron phthalocyanine (FePc) and perfluorinated iron phthalocyanine (FePcF16) in thin films is investigated by X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD), supported by photoemission. Both molecules grow in a flat-lying adsorption geometry in thin films. Fe L-edge X-ray absorption spectra of FePc and FePcF16 exhibit minor, but distinct, differences of the shape, indicating a different electronic structure. A significantly stronger XMCD signal and thus larger magnetic moments were observed for FePc compared to FePcF16 at low temperatures (15 K). Multiplet calculations have been used to simulate the XA and XMCD spectra and give detailed insight into the electronic structure of Fe in FePc and FePcF16. We suppose that the electronic structure crucially depends on the detailed arrangement of FePc and FePcF16 molecules in thin films.

2D oxide quasicrystals (OQCs) are recently discovered aperiodic, but well‐ordered oxide interfaces. In this topical review, an introduction to these new thin‐film systems is given. The concept of quasicrystals and their approximants is explained for and derived OQCs and related periodic structures in these 2D oxides. In situ microscopy unravels the high‐temperature formation process of OQCs on Pt(111). The dodecagonal structure is discussed regarding tiling statistics and tiling decoration based on the results of atomically resolved scanning tunneling microscopy and various diffraction techniques. In addition, angle‐resolved ultraviolet photoemission spectroscopy and X‐ray photoelectron spectroscopy results prove a metallic character of the 2D oxide.

This work focuses on the generation of ferromagnetism at the surface of anatase TiO2 films by low-energy ion irradiation. Controlled Ar+-ion irradiation resulted in a thin (similar to 10) nm ferromagnetic surface layer. The intrinsic origin and robustness of the magnetic order has been characterized by x-ray magnetic circular dichroism at room temperature revealing that a Ti band is spin-polarized. These results, together with density functional theory calculations, indicate that Ti vacancy-interstitial pairs are responsible for the magnetic order. Superconducting quantum interference device measurements show the existence of a perpendicular magnetic anisotropy and a low remanent magnetization. Magnetic force microscopy reveals that this low remanence is due to oppositely aligned magnetic domains with magnetization vectors normal to the main surface. The weak domain-wall pinning, the magnetic anisotropy, together with the simplicity of the preparation method, open up interesting possibilities for future applications. As an example, single domain patterns of similar to 1 mu m width and several pan length can be easily prepared.

We have carried out a surface X-ray diffraction (SXRD) analysis of the approximant structure related to the recently discovered two-dimensional dodecagonal oxidic quasicrystal. The structure is characterized by the 3(2).4.3.4 Archimedean tiling, first described by Kepler in 1619. The tiling network is related to titanium atoms observed as protrusions in scanning tunneling microscopy images. All four titanium atoms within one two-dimensional unit cell (a(0) = 13.1 angstrom, b(0) = 12.9 angstrom, gamma = 90.5 degrees) are surrounded by three oxygen atoms. The TiO3 units are separated by barium atoms. The total stoichiometry is given by Ba-4 Ti-4 O-10.

We report on the first observation of an approximant structure to the recently discovered two-dimensional oxide quasicrystal. Using scanning tunneling microscopy, low-energy electron diffraction, and surface x-ray diffraction in combination with ab initio calculations, the atomic structure and the bonding scheme are determined. The oxide approximant follows a 3^{2}.4.3.4 Archimedean tiling. Ti atoms reside at the corners of each tiling element and are threefold coordinated to oxygen atoms. Ba atoms separate the TiO_{3} clusters, leading to a fundamental edge length of the tiling 6.7 Å.

The film growth of Mn$_3$O$_4$(001) films on Ag(001) up to film thicknesses of almost seven unit cells of Mn$_3$O$_4$ has been monitored using a complementary combination of near-edge X-ray absorption fine structure spectroscopy (NEXAFS), scanning tunneling microscopy (STM), and low-energy electron diffraction (LEED). The oxide films have been prepared by molecular beam epitaxy. Using NEXAFS, the identity of the Mn oxide has clearly been determined as Mn$_3$O$_4$. For the initial stages of growth, oxide islands with p(2$\times$1) and p(2$\times$2) structures are formed, which are embedded into the substrate. For Mn$_3$O$_4$ coverages up to 1.5 unit cells a p(2$\times$1) structure of the films is visible in STM and LEED. Further increase of the thickness leads to a phase transition of the oxide films resulting in an additional c(2$\times$2) structure with a 45$^\circ$ rotated atomic pattern. The emerging film structures are discussed on the basis of a sublayer model of the Mn$_3$O$_4$ spinel unit cell. While the polarity of the island edges determines the structure of initial islands, the surface energy of thicker layers is remarkably reduced by a film restructuring.

Different layer thicknesses of cobalt ranging from 2.6 Å (1.5 ML) up to 55 Å (30.5 ML) deposited on ferroelectric BaTiO3 have been studied regarding their magnetic behavior. The layers have been characterized using XMCD spectroscopy at remanent magnetization. After careful data analysis the magnetic moments of the cobalt could be determined using the sum rule formalism. There is a sudden and abrupt onset in magnetism starting at thicknesses of 9 Å (5 ML) of cobalt for measurements at 120 K and of 10 Å (5.5 ML) if measured at room temperature. Initial island growth and subsequent coalescence of Co on BaTiO3 is suggested to explain the sudden onset. In that context, no magnetically dead layers are observed.

Perovskite oxides represent a versatile class of materials with a simple cubic or pseudo-cubic crystal structure. The family of perovskite oxides contains insulators, metals, semiconductors, and superconductors with nearly identical lattice parameters. This structural equivalence additionally allows to combine perovskites with different properties in multilayer systems to produce functional materials with unique properties. We report here on the formation of a quasicrystal (QC) thin film on a threefold Pt(111) surface. This QC film is derived from the classical perovskite oxide BaTiO3 which is the most intensely studied ferroelectric perovskite oxide. An easily accessible ferroelectric to paraelectric phase transition at 400 K makes the material so interesting for basic and applied research. Due to matching lattice conditions BaTiO3 can be grown epitaxially on selected metal substrates. Periodic thin films of either BaTiO3(100) or BaTiO3(111) have been grown depending on substrate orientation and preparation conditions on Pt(001) and on Pt(111) [1, 2]. As we demonstrate here, astonishingly also a two-dimensional dodecagonal quasicrystalline structure can be formed by annealing an initially 1.4 nm thick BaTiO3 film on Pt(111) [3]. It develops at a temperature of 1250 K from a wetting layer spreading between a few thicker BaTiO3(111) islands. Surface sensitive electron diffraction (LEED) shows a bright and sharp pattern with dodecagonal symmetry. High-resolution scanning tunneling microscopy (STM) images reveal an arrangement of quadratic, triangular, and rhombic elements which compares well to a Gähler tiling. The development of higher-order self-similar structures is widely suppressed by a linear phason strain. This is supported by the fine structure of the diffraction data.

The unusual ordering of quasicrystals can be induced in thin films of a regular crystalline material; here a two-dimensional quasicrystal has been achieved by growing thin films of the perovskite barium titanate on an appropriately oriented crystalline platinum substrate.

High resolution x-ray absorption spectroscopy (XAS) affords new insight into the microscopic properties of perovskite transition metal oxides. Interpretation of XAS spectra in transition metal oxides requires theoretical tools capable of describing relativistic and many-body effects. In this work, full relativistic (SPR-KKR) and multiplet calculations (CTM4XAS) are carried out and compared to experimental multiedge XAS spectra of BaTiO${}_{3}$ single crystals. The impact of relativistic and many-body effects on the calculated density of states and x-ray absorption near edge structure spectra are individually considered.