Evidence of interface exchange magnetism in self-assembled cobalt-fullerene nanocomposites exposed to air

We report on the establishing of an exclusive magnetic effect in air-exposed Co_xC₆₀ nanocomposites (x > 2) created through self-assembling in the depositing mixture. In order to verify the influence of ambient air on the Co_xC₆₀ mixture film, we have studied in detail the film magnetization at rather low temperatures, which provides their ferromagnetic behavior. Tracing the possible exchange bias effect, we distinguished a clear vertical shift of the hysteresis loops recorded for the air-exposed Co_xC₆₀ films in the field cooling (FC) regime. The detected vertical shift of the FC loops is caused by an uncompensated magnetic moment M _u induced by exchange coupling of the Co spins at the Co/CoO interface. This interface arises due to the oxidation of small Co clusters distributed in a C₆₀-based matrix of self-assembled composite films, which occurs during air exposure. The core-shell structure of the Co/CoO magnetic clusters (about 2-3 nm in size) consisting of a ε-Co core and fcc-CoO shell was confirmed by means of transmission electron microscopy. Established interface magnetism testifies to a composite nanostructure in the Co_xC₆₀ mixture film with x > 2 and explains the influence of air exposure on the film structure. The discovered magnetic effect implies a new application potential for cobalt-fullerene composites in sensors and catalysis.

Effect of screw threading dislocations and inverse domain boundaries in GaN on the shape of reciprocal-space maps

Polar GaN layers containing domains with inverse polarities are studied by means of high-resolution X-ray diffraction and transmission electron microscopy. It is shown how the presence of inversion domain boundaries can be recognized directly from reciprocal-space maps measured by X-ray diffraction.

The microstructure of polar GaN layers, grown by upgraded high-temperature vapour phase epitaxy on [001]-oriented sapphire substrates, was studied by means of high-resolution X-ray diffraction and transmission electron microscopy. Systematic differences between reciprocal-space maps measured by X-ray diffraction and those which were simulated for different densities of threading dislocations revealed that threading dislocations are not the only microstructure defect in these GaN layers. Conventional dark-field transmission electron microscopy and convergent-beam electron diffraction detected vertical inversion domains as an additional microstructure feature. On a series of polar GaN layers with different proportions of threading dislocations and inversion domain boundaries, this contribution illustrates the capability and limitations of coplanar reciprocal-space mapping by X-ray diffraction to distinguish between these microstructure features.

Capability of X-ray diffraction for the study of microstructure of metastable thin films

Metastable phases are often used to design materials with outstanding properties, which cannot be achieved with thermodynamically stable compounds. In many cases, the metastable phases are employed as precursors for controlled formation of nanocomposites. This contribution shows how the microstructure of crystalline metastable phases and the formation of nanocomposites can be concluded from X-ray diffraction experiments by taking advantage of the high sensitivity of X-ray diffraction to macroscopic and microscopic lattice deformations and to the dependence of the lattice deformations on the crystallographic direction. The lattice deformations were determined from the positions and from the widths of the diffraction lines, the dependence of the lattice deformations on the crystallographic direction from the anisotropy of the line shift and the line broadening. As an example of the metastable system, the supersaturated solid solution of titanium nitride and aluminium nitride was investigated, which was prepared in the form of thin films by using cathodic arc evaporation of titanium and aluminium in a nitrogen atmosphere. The microstructure of the (Ti,Al)N samples under study was tailored by modifying the [Al]/[Ti] ratio in the thin films and the surface mobility of the deposited species.

Composition profiles across MIMs for resistive switching studied by EDS and EELS

Resistive switching in MIM (metal-insulator-metal) stacks is an effect that enables a promising data storage technology which is able to overcome the size limitations of conventional non-volatile memories. The resistive switching effect was already demonstrated for several binary as well as ternary transition metal oxides (TiO2, NiO, SrTiO3, Nb2O5) [1,2]. The current models of the switching mechanisms suggest the important role of defects like oxygen vacancies [3]. Here, we report on the local structural and electronic properties of transition metal oxides embedded in MIM stacks that were obtained by using transmission electron microscopy and electron spectroscopy. We focus on the development of the stoichiometry across the MIM stack for amorphous and partial crystalline niobium oxides. Therefore, electron energy loss spectra (EELS) as well as the energy dispersive X-ray spectra (EDS) were collected on the atomic scale utilizing a nanometer probe in the scanning transmission electron microscope (STEM). The differences in the oxygen content among the electrodes and the concentration profiles at the metal/oxide interfaces in particular were investigated in dependence on the preparation method and on the electrode material. Besides, focusing on the electron loss near edge structure (ELNES) of the oxygen K edge we employed simulations using FEFF9 to describe the modifications of the electronic structure with variations in the oxygen content.

Stacking fault energy in austenitic steels determined by usingin situX-ray diffraction during bending

A method is presented which determines the stacking fault energy in face-centred cubic materials from the critical stress that is inducedviasample bending in the early stages of plastic deformation. The critical stress is gauged byin situX-ray diffraction. This method utilizes the results of Byun's consideration of the stress dependence of the partial dislocation separation [Byun (2003).Acta Mater.51, 3063–3071]. Byun showed that the separation distance of the partial dislocations increases rapidly when the critical stress is reached and that the critical stress needed for the rapid separation of the partial dislocations is directly proportional to the stacking fault energy. In the approach presented here, the partial dislocation separation and the corresponding triggering stress are monitored by usingin situX-ray diffraction during sample bending. Furthermore, thein situX-ray diffraction measurement checks the possible interactions between stacking faults present on equivalent lattice planes and the interactions of the stacking faults with other microstructure defects. The capability of the proposed method was tested on highly alloyed austenitic steels containing chromium (∼16 wt%), manganese (∼7 wt%) and nickel as the main alloying elements. For the steels containing 5.9 and 3.7 wt% Ni, stacking fault energies of 17.5 ± 1.4 and 8.1 ± 0.9 mJ m−2were obtained, respectively.

Defects in 6 MeV H+ irradiated hydrothermal ZnO single crystal

The effect of 6 MeV H(+) irradiation on hydrothermally grown ZnO single crystal has been investigated using high resolution x-ray diffraction (HRXRD) and optical absorption (ultraviolet-visible) spectroscopy. The increase of the diffuse scattering in the reciprocal space maps measured using HRXRD indicates an increase of the point defect density upon irradiation. Within the penetration depth of x-rays of several micrometres, the defect density increased with increasing distance from the sample surface. On the other hand, the near band gap optical absorption became sharper for the irradiated crystal. This reflects enhanced band to band absorption and reduced sub-band gap absorption due to defects. Temperature dependent photoluminescence spectra of the pristine sample show negative thermal quenching (NTQ) of the luminescence which is due to the presence of two or more donor related defects. Upon irradiation, a single dominant donor bound transition can be found without any temperature induced NTQ. Enhancement of the band edge luminescence and reduction of the defect related luminescence is observed at 10 K. Such changes have been discussed in the light of the hydrogen present in the as-grown state of hydrothermal ZnO.

Pitfalls of local and quantitative phase analysis in partially stabilized zirconia

The addition of selected elements into the host structure of ZrO2stabilizes the tetragonal and cubic phases of zirconia, which are, in their undoped binary form, only stable at high temperatures. From the crystallographic point of view, the increasing amount of the stabilizer causes a continuous transition of the tetragonal zirconia to its cubic modification. In partially stabilized zirconia, local concentration gradients of the stabilizer are frequently present as a consequence of the production process, which results in a coexistence of zirconia domains having different degrees of tetragonality. The presence of the local concentration gradients in such samples and the continuous nature of the phase transformation are features important for many technological applications, but their analysis is not straightforward. Furthermore, these features complicate the quantitative phase analysis in partially stabilized zirconia. For the example of zirconia partially stabilized by magnesium, this contribution illustrates the capabilities and limitations of X-ray and electron backscatter diffraction. In particular, the ability of these experimental methods to reveal the gradual lattice distortion that is associated with the cubic to tetragonal phase transformation in zirconia and the reliability of the quantitative phase analysis are discussed. In this context, it is shown to what extent the choice of the microstructure model influences the result of the phase analysis.

Stacking fault model of ∊-martensite and itsDIFFaXimplementation

Plastic deformation of highly alloyed austenitic transformation-induced plasticity (TRIP) steels with low stacking fault energy leads typically to the formation of ∊-martensite within the original austenite. The ∊-martensite is often described as a phase having a hexagonal close-packed crystal structure. In this contribution, an alternative structure model is presented that describes ∊-martensite embedded in the austenitic matrixviaclustering of stacking faults in austenite. The applicability of the model was tested on experimental X-ray diffraction data measured on a CrMnNi TRIP steel after 15% compression. The model of clustered stacking faults was implemented in theDIFFaXroutine; the faulted austenite and ∊-martensite were represented by different stacking fault arrangements. The probabilities of the respective stacking fault arrangements were obtained from fitting the simulated X-ray diffraction patterns to the experimental data. The reliability of the model was proven by scanning and transmission electron microscopy. For visualization of the clusters of stacking faults, the scanning electron microscopy employed electron channelling contrast imaging and electron backscatter diffraction.

Silver nanoparticles/montmorillonite composites prepared using nitrating reagent at water and glycerol

Three procedures (P) were applied to prepare silver nanoparticles on natural Ca-montmorillonite (MT). The intercalation of the montmorillonite with silver nitrate in aqueous solution (P1), the intercalation of the montmorillonite with silver nitrate in glycerol (P2) and the successive combination of both P1 and P2 methods resulted to P3 method. X-ray powder diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) and Fourier Transform Infrared (FTIR) spectroscopy and the molecular modeling were employed to characterize silver nanoparticles and montmorillonite nanocomposite. The P1 produced MT-1 composite with 2.3 wt% Ag and the partially collapsed layered structure. Nanoparticles of silver larger than 20 nm with a lot of planar defects were randomly distributed on the MT-1 surface; nanoparticles smaller than 20 nm were oriented to the montmorillonite substrate. The MT-2 composite from P2 contained only 1 wt% of Ag. The molecular simulation model of MT-2 showed the interlayer space with the exchangeable cations and metallic silver atoms arrangement within the glycerol bilayer. The P3 produced composite MT-3 that contained 2.4 wt% Ag. The nanoparticles > 20 nm size had a well-defined geometry, very small nanoparticles were amorphous. The modeled structure showed the exchangeable cations, Ag+ and Ag0 located close to the silicate layers and monolayer of glycerol molecules in the interlayer space.