The surface morphology of CeO(2)(111) single crystals and silicon supported ceria films is investigated by non-contact atomic force microscopy (NC-AFM) and Kelvin probe force microscopy (KPFM) for various annealing conditions. Annealing bulk samples at 1100 K results in small terraces with rounded ledges and steps with predominantly one O-Ce-O triple layer height while annealing at 1200 K produces well-ordered straight step edges in a hexagonal motif and step bunching. The morphology and topographic details of films are similar, however, films are destroyed upon heating them above 1100 K. KPFM images exhibit uniform terraces on a single crystal surface when the crystal is slowly cooled down, whereas rapid cooling results in a significant inhomogeneity of the surface potential. For films exhibiting large terraces, significant inhomogeneity in the KPFM signal is found even for best possible preparation conditions. Applying X-ray photoelectron spectroscopy (XPS), we find a significant contamination of the bulk ceria sample with fluorine while a possible fluorine contamination of the ceria film is below the XPS detection threshold. Time-of-flight secondary ion mass spectroscopy (TOF-SIMS) reveals an accumulation of fluorine within the first 5 nm below the surface of the bulk sample and a small concentration throughout the crystal.

Electronic and magnetic properties of the charge ordered phase of LuFe 2 O 4 are investigated by means of x-ray spectroscopic and theoretical electronic structure approaches. LuFe 2 O 4 is a compound showing fascinating magnetoelectric coupling via charge ordering. Here, we identify the spin ground state of LuFe 2 O 4 in the charge ordered phase to be a 2:1 ferrimagnetic configuration, ruling out a frustrated magnetic state. An enhanced orbital moment may enhance the magnetoelectric coupling. Furthermore, we determine the densities of states and the corresponding correlation potentials by means of x-ray photoelectron and emission spectroscopies, as well as electronic structure calculations. DOI: 10.1103/PhysRevB.80.220409 PACS number͑s͒: 75.80.ϩq, 71.20.Ϫb, 78.70.Dm, 78.70.En Multiferroic transition metal oxides, i.e., compounds in which more than one ferroic phase coexist, have gained enormous attention during the last few years. 1-4 Besides a number of perovskites and related compounds, 2,5,6 the charge frustrated layered compound LuFe 2 O 4 has attracted intense interest due to its fascinating ferroelectric and magnetoelectric properties. 7,8 LuFe 2 O 4 has a rhombohedral crystal structure ͑space group R3m͒. The underlying layered structure consists of W-like hexagonal Fe 2 O 2.5 and U-like LuO 1.5 layers. 9 The W layers comprise two triangular nets of Fe ions; the resulting electric polarization is induced via a frustrated charge ordering of Fe 2+ and Fe 3+ ions on the resulting honeycomb lattice below 330 K. [10][11][12] Below 240 K a longrange ferrimagnetic order sets in. 7 The fact that the ferroelectricity is caused by correlated electrons from the Fe ions leads to unusual properties and unique capabilities of LuFe 2 O 4 . A large response of the dielectric constant by applying small magnetic fields has been found, opening a possible route for future devices. 8 Phase transitions from the charge ordered ͑CO͒ phase have been very recently associated with a nonlinear current-voltage behavior and an electric-field-induced phase transition, which might be of interest for potential electric-pulse-induced resistive switching applications. 13,14 The large magnetoelectric coupling has been attributed to an intricate interplay between charge and spin degrees of freedom with the crystal lattice and external electrical and magnetic fields to some extent on a short-range order. [15][16][17][18][19] However, there is still some confusion about the nature of spin-charge coupling in LuFe 2 O 4 . In particular a model finding a ͱ 3 ϫ ͱ 3 CO ground state 20,21 is challenged by simulations implying that the electrical polarization in LuFe 2 O 4 is due to spin-charge coupling and a spin frustrated magnetic ground state in a chain CO state. 22,23 On the other hand the first model finds a ferrimagnetic spin ground state where Fe 2+ and 1/3 of Fe 3+ make up the majority spin, and 2/3 of Fe 3+ make up the minority spin.X-ray magnetic circular dichroism ͑XMCD͒ is a very powerful tool to investigate the internal magnetic stru...

Giant Keplerate-type molecules with a {Mo72Fe30} core show a number of very interesting properties, making them particularly promising for various applications. So far, only limited data on the electronic structure of these molecules from X-ray spectra and electronic structure calculations have been available. Here we present a combined electronic and magnetic structure study of three Keplerate-type nanospheres--two with a {Mo72Fe30} core and one with a {W72Fe30} core by means of X-ray absorption spectroscopy, X-ray magnetic circular dichroism (XMCD), SQUID magnetometry, and complementary theoretical approaches. Furthermore, we present detailed studies of the Fe(3+)-to-Fe(2+) photoreduction process, which is induced under soft X-ray radiation in these molecules. We observe that the photoreduction rate greatly depends on the ligand structure surrounding the Fe ions, with negatively charged ligands leading to a dramatically reduced photoreduction rate. This opens the possibility of tailoring such polyoxometalates by X-ray spectroscopic studies and also for potential applications in the field of X-ray induced photochemistry.

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We report on the characterization of various salts of [MnIII6CrIII]3+ complexes prepared on substrates such as highly oriented pyrolytic graphite (HOPG), mica, SiO2, and Si3N4. [MnIII6CrIII]3+ is a single-molecule magnet, i.e., a superparamagnetic molecule, with a blocking temperature around 2 K. The three positive charges of [MnIII6CrIII]3+ were electrically neutralized by use of various anions such as tetraphenylborate (BPh4-), lactate (C3H5O3-), or perchlorate (ClO4-). The molecule was prepared on the substrates out of solution using the droplet technique. The main subject of investigation was how the anions and substrates influence the emerging surface topology during and after the preparation. Regarding HOPG and SiO2, flat island-like and hemispheric-shaped structures were created. We observed a strong correlation between the electronic properties of the substrate and the analyzed structures, especially in the case of mica where we observed a gradient in the analyzed structures across the surface.