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Atomic force microscopy (AFM), X-ray magnetic circular dichroism (XMCD), magnetic force microscopy (MFM) and vibrating sample magnetometry (VSM) have been used to measure the magnetic and geometrical characteristics of triangular-shaped Co structures of lateral size 730 nm and thickness 32 nm, prepared by nanosphere lithography (NSL). Evidence of in-plane six-fold magnetic anisotropy induced by the symmetry of the structure has been found. By means of XMCD measurements, performed at remanence after applying a pulsed field, a structure rotation angle-dependent oscillation of about 15% with a periodicity of 60° has been observed for both the orbital and spin moments. Furthermore, the system exhibits the angular hysteresis effect. The magnetic measurements performed by MFM show a reduction of the magnetic configurations to only two states, one quasi-single domain Y state and second, a combination of vortex and Y state.

An almost linear relation between the Pt amount and the orbital to spin moment ratio (μl/μs) has been found in Cox Pt100–x nanoparticles prepared by vapour deposition and possessing a well defined chemically disordered crystallographic structure. X-ray magnetic circular dichroism (XMCD) measurements done at the L2,3Co edges show that a 4.0 nm, x = 100 Co reference sample has a ratio μl/μs = 0.097, comparable to values found for Co thin layers. An increase of the amount of Pt increases the μl/μs value: μl/μs = 0.11 for x = 78, μl/μs = 0.12 when x = 74 and finally, for x = 38 it becomes μl/μs = 0.140. Nanoparticles prepared by chemical synthesis with size proportional to changes of the stoichiometric ratio, from Co25Pt75 to Co50Pt50, show values of the orbital to spin moment ratio compatible with the trend found for the former ones. The samples prepared by chemical synthesis in a diameter range from 4 nm to 8 nm are characterized by a sharp size distribution of 10%. In contrast to the in situ prepared nanoparticles that are oxide free, however, they show a varying degree of surface oxidation depending on the ambient conditions they are exposed to. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Magnetization reversal of thin Fe, Co and Ni layers deposited on single-crystalline layers of V2O3 is shown to be governed by various contributions to the magnetic anisotropy of the ferromagnetic layers, which reflect the sensitivity of the systems to the crystallographic and magnetic moment orientation or the structural morphology of the interfaces. The complex behaviour observed is related to the rich phase diagram of the oxide, which undergoes a transition from a paramagnetic metallic to an insulating antiferromagnetic phase as the temperature is decreased to below ∼150 K. Focussing on the anisotropy contributions that result from the exchange interaction across the interface with antiferromagnetic V2O3, we have found a remarkable behaviour of the Fe overlayers which grow in a textured BCC(1 1 0) structure. The exchange anisotropy, probed as a bias of the ferromagnetic hysteresis loops for field-cooled samples, is absent for the interface with the ( 1 1 2 ¯ 0 ) surface of antiferromagnetic V2O3, but exists for the interface with the V2O3 (0 0 0 1) surface. This points to a very different electronic interaction at the two interfaces. The behaviour is at variance with what is observed for the Co and Ni overlayers which show an exchange anisotropy for both surfaces of the oxide. Measurements of X-ray magnetic circular dichroism reveal the presence of a magnetically dead layer in Fe at the interface with V2O3 ( 1 1 2 ¯ 0 ) . It is likely to disrupt the exchange coupling, and thus may be at the origin of the missing exchange anisotropy of Fe on this surface. The different evolution of the spin magnetic moment of Fe with the overlayer thickness suggests a different growth morphology on the two surfaces of the oxide. As a result of the interaction with Fe, the V2O3 layers acquire a small magnetic polarization at the interface at room temperature for both orientations.

The magnetization of ultrathin Fe deposited in situ onto a V(1 1 0) single crystal has been studied by means of X-ray magnetic circular dichroism measurements at the L 2,3 absorption edges of Fe and V as a function of the Fe coverage. The Fe magnetization sets on at an Fe thickness of 2 ML in accordance with an earlier Polarized Neutron Reflectometry investigation. It increases rapidly with Fe thickness and asymptotically reaches the bulk magnetization at 5 ML. In the critical Fe thickness regime between 2 and 5 ML it significantly increases by cooling from room temperature to 110 K due to the reduced Curie temperature of the ultrathin Fe film. In contrast, the onset of the Fe magnetization at 2 ML is barely influenced by cooling which may be due to the coalescence of the Fe islands observed earlier with low-energy electron diffraction. Residual O contamination on the V surface reduces the Fe magnetization due to the formation of Fe oxide.

The properties of the interface between a thin layer of native SiO 2 and a low molecular compound were studied by means of ultraviolet photoelectron spectroscopy (UPS). The investigated material, an amphiphilic derivative of 2,5-diphenyl-1,3,4-oxadiazole (NADPO), was prepared by using the Langmuir–Blodgett (LB) technique. It is of interest since its electronic properties do not depend on the preparation conditions and it is also being candidate for use in optoelectronic devices. The UPS measurements were performed at the beam line Seya F2.2 at HASYLAB (Hamburg, Germany). This beamline is of a 1 m Seya–Namioka monochromator type, equipped with two gratings, which cover the photon energy range from 4 to 40 eV. The photoelectrons were collected with a Vacuum Generators ADES 400 angle resolving spectrometer system at room temperature. The base pressure during the experiment was 1×10 −10 mbar. We investigated the interface properties depending on the number of monolayers (2, 4, 6 and 8) of well oriented LB films, monitoring the highest occupied molecular orbital (HOMO) energy position.

Thermally stimulated luminescence (TSL), and ultraviolet photoelectron spectroscopy (UPS) measurements have been carried out on Langmuir–Blodgett films of a substituted 1,3,4-oxadiazole. UPS provided the experimental electronic band structure of the investigated material and also parameters like ionisation potential, 7.9 eV, and electron affinity, 4.8 eV. These high values indicate that it can be used as hole blocking/electron transporting layer in emitting devices. Thermally stimulated processes on oxadiazoles thin films have been studied in order to get information about their electronic properties. The fractional glow technique, applied on the thermal stimulated luminescence curves, highlights different peaks. They originate from carrier relaxation processes. We focused our attention on the complex structure between 120 and 160 K. By the analysis of the curves performed both with the initial rise method and with a fitting procedure, we estimated the activation energy, between 3.5 (initial rise method) and 4 meV (numerical simulation), the frequency factors, 104 s−1, and a Gaussian trap distribution shape.

The family of substituted 1,3,4-oxadiazole can be used in organic light-emitting diodes (OLEDs) acting both as hole blocking and as improved emissive layers. Thin films of aromatic 1,3,4-oxadiazoles, prepared in situ by evaporation from pinhole sources onto fresh deposited gold films on flat silicon wafers, were studied by means of ultraviolet photoelectron spectroscopy (UPS) in order to get detailed information about valence electronic band structure. The UPS measurements were performed at the beam line TGM-2 at BESSY (Berlin) in the energy range between 8 and 80 eV. Furthermore, we determined the vacuum level shift at the organic/metal interface. We used semi-empirical quantum-chemical calculations to evaluate the valence electronic structure of the isolated molecules. This was also the starting point used as input to simulate the UPS spectra obtained from the films. The good agreement between experimental and theoretical curves allows us to assign the different peaks to different molecular eigenstates and it also provides a more quantitative description of the experimental findings.

The dynamical phase transition mechanism of $({\mathrm{Na}}_{1/2}{\mathrm{Bi}}_{1/2}){\mathrm{TiO}}_{3}$ (NBT) was studied using inelastic neutron scattering. Softening was observed of multiple phonon modes in the phase transition sequence of NBT. As usual, the softening of the zone center transverse optical modes ${\ensuremath{\Delta}}_{5}$ and ${\ensuremath{\Sigma}}_{3}$ was observed in the (200) and (220) zones, showing the Ti vibration instabilities in ${\mathrm{TiO}}_{6}$ octahedra for both cubic-tetragonal (C-T) and tetragonal-rhombohedral (T-R) phase transitions. In these two phase transitions, however, ${\mathrm{Ti}}^{4+}$ has different preferential displacement directions. Surprisingly, the longitudinal optic mode also softens significantly toward the zone center in the range of the transition temperature, indicating the ${\mathrm{Na}}^{+}/{\mathrm{Bi}}^{3+}$ vibration instability against ${\mathrm{TiO}}_{6}$ octahedra during the T-R phase transition. Strong inelastic diffuse scattering shows up near $M$(1.5, 0.5, 0) and $R$(1.5, 1.5, 0.5) in the tetragonal and rhombohedral phases, respectively, indicating the condensations of the ${M}_{3}$ and ${R}_{25}$ optic modes for the corresponding transitions. This reveals the different rotation instabilities of ${\mathrm{TiO}}_{6}$ in the corresponding transition temperature range. Bottleneck or waterfall features were observed in the dispersion curves at certain temperatures but did not show close correlations to the formation of polar nanoregions. Additional instabilities could be the origin of the complexity of phase transitions and crystallographic structures in NBT.

Electronic band structure of three heterocyclic model compounds: 5,12-dihydro-quinoxalino-2,3-b-phenazine; 7,18-dihydro-5,7,11,16,18,20,22-octa-aza-nonacene; and 1,3,5-Tris[(3-phenyl-6-tri-fluoro-methyl)-quinoxaline-2-yl]benzene was studied by ultraviolet photoelectron spectroscopy. We also determined their ionisation potential, between 5.3 and 6.0 eV, suggesting that these materials can efficiently act as electron transporting layer in organic light emitting devices (OLEDs).

We studied ultraviolet photoelectron spectroscopy (UPS) and thermally stimulated processes in two types of phenylquinoxalines, and one type of starburst trisphenylquinoxaline in order to characterize the valence band structure and the localized states. An ionization potential of about 6.0 eV was determined for each material using UPS. Thermally stimulated discharge currents (TSDC) gave evidence of dipolar and/or charge transport mechanisms. We assigned each TSDC peak to the different elementary processes using in addition dielectric spectroscopy (DES), and from thermally stimulated luminescence (TSL) curves, performed for the first time on these materials, we estimated the mean trap depths.

Using the spectroscopies based on x-ray absorption, we have studied the structural and magnetic properties of ${\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}\mathrm{O}$ films ($x=0.1$ and 0.25) produced by reactive magnetron sputtering. These films show ferromagnetism with a Curie temperature ${T}_{C}$ above room temperature in bulk magnetization measurements. Our results show that the Co atoms are in a divalent state and in tetrahedral coordination, thus substituting Zn in the wurtzite-type structure of ZnO. However, x-ray magnetic circular dichroism at the $\mathrm{Co}\phantom{\rule{0.2em}{0ex}}{L}_{2,3}$ edges reveals that the $\mathrm{Co}\phantom{\rule{0.2em}{0ex}}3d$ sublattice is paramagnetic at all temperatures down to $2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, both at the surface and in the bulk of the films. The $\mathrm{Co}\phantom{\rule{0.2em}{0ex}}3d$ magnetic moment at room temperature is considerably smaller than that inferred from bulk magnetization measurements, suggesting that the $\mathrm{Co}\phantom{\rule{0.2em}{0ex}}3d$ electrons are not directly at the origin of the observed ferromagnetism.

Co–Pt and Co nanoparticles (∼2 nm diameter) of different composition and crystal structure have been studied by X-ray magnetic circular dichroism (XMCD) at the Co L 2,3 edges. The magnetic properties of the samples, covered with a thin protecting layer of Al 2 O 3 , depend upon the composition, order and presence of Co oxide contamination. Pure Co nanoparticles reveal an orbital-to-spin moment ratio, μ l / μ s =0.13 at room temperature (RT) and μ l / μ s =0.11 at 3.4 K, Co/Pt mixed nanoparticles show a temperature independent μ l / μ s ratio: 0.13 both at RT and at 3.4 K. In contrast, Pt-shell Co-core samples have a ratio μ l / μ s =0.14 at RT and a strongly enhanced ratio μ l / μ s =0.20 at 3.4 K. The same samples with a small CoO contamination show an enhanced μ l / μ ss =0.25 at RT and μ l / μ ss =0.28 at 3.4 K. In the reversed configuration (Pt-core and a Co-shell), where the CoO is probably not at the Co–Pt interface but at the Co surface, the enhancement of μ l / μ s caused by the simultaneous increases of the orbital moment and decreases of the spin moment reaches a record value of μ l / μ ss =0.33 at 3.4 K and μ l / μ ss =0.21 at RT.

Extended X ray absorption fine structure EXAFS data and magnetic EXAFS MEXAFS data were measured at two temperatures 180 K and 296 K in the energy region of the overlapping L edges of bcc Fe grown on a V 110 crystal surface. In combination with a Bayes Turchin data analysis procedure these measurements enable the exploration of local crystallographic and magnetic structures. The analysis determined the atomic like background together with the EXAFS parameters which consisted of 10 shell radii, the Debye Waller parameters, separated into structural and vibrational components, and the third cumulant of the first scattering path. The vibrational components for 97 different scattering paths were determined by a two parameter force field model using a priori values adjusted to Born von Karman parameters of inelastic neutron scattering data. The investigations of the system Fe V 110 demonstrate that the simultaneous fitting of atomic background parameters and EXAFS parameters can be performed reliably. Using the L2 and L3 components extracted from the EXAFS analysis and the rigid band model, the MEXAFS oscillations can only be described when the sign of the exchange energy is changed compared to the predictions of the Hedin Lundquist exchange and correlation functional

The ex situ preparation of chemically synthesized cobalt nanoparticles leads unavoidably to particles coated with organic surfactants and a thin CoO shell. Argon ion $({\mathrm{Ar}}^{+})$ etching is a method often used to remove the surfactant and the oxide shell. Ex situ prepared nanoparticles of $9.7\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ diameters were investigated as a function of the sputtering time, by means of soft x-ray absorption spectroscopy at the C and O K edges and at the Co ${\mathrm{L}}_{2,3}$ edges. Low-energy $(500\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ ${\mathrm{Ar}}^{+}$ ion etching was effective in removing the organic coating layer of the nanoparticles, while much higher energy and longer sputtering times were required to affect the CoO shell. On the other hand, only a short time using low-energy hydrogen ion $({\mathrm{H}}^{+})$ etching was necessary to completely reduce the oxide shell. Magnetic circular dichroism of soft x-rays was used to measure the magnetic properties of the etched samples. We found that the orbital to spin magnetic moment ratio, ${\ensuremath{\mu}}_{l}∕\ensuremath{\mu}_{s}{}^{\mathrm{eff}}$, increases as a function of the ${\mathrm{Ar}}^{+}$ etching time. Such an increase may be ascribed to the incomplete removal of the CoO shell as well as to surface strain, increased roughness, and an effective reduction of the particle size induced by the high-energy ion etching. A much smaller effect was observed for the low-energy $(500\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ and short time ${\mathrm{H}}^{+}$ etched samples, where the magnetic moment ratio after the reducing procedure ${\ensuremath{\mu}}_{l}∕\ensuremath{\mu}_{s}{}^{\mathrm{eff}}=0.12$ is comparable to values obtained for bulk cobalt.

Thermally stimulated processes on films of polyphenylquinoxalines (PPQs) and trisphenylquinoxalines (TPQs), materials suitable for electron transport/hole blocking layers on organic light-emitting devices (OLEDs), have been studied in order to get information about their electronic properties. By using thermally stimulated luminescence (TSL) curves, we estimated the mean trap depths.

Thin films of oxadiazoles and quinoxalines model compounds have been investigated by means of Ultraviolet Photoelectron Spectroscopy (UPS) in order to get a detailed description of the valence electronic band structure. Furthermore, we used semi-empirical quantum-chemical calculations to evaluate the valence electronic structure of the isolated molecules. UPS spectra have been calculated by use of molecular eigenvalues and we assigned the different peaks to the different molecular eigenstates.

One approach to increase the overall performance of organic light emitting devices is to separate the light-emitting volume from the ones which are assigned to charge injection or transport. The electronic properties of model heterocyclic compounds, namely low molecular phenylquinoxalines and related derivatives, for electron transport layers have been investigated by Ultraviolet Photoelectron Spectroscopy. The ionization potentials of these materials have been determined.