Diffusion in amorphous NiZrAl alloys

Tracer diffusion coefficients for large and small sized impurities were measured in the bulk metallic glass Ni_xZr_yAl_z for several compositions. Molecular beam deposition was employed to grow thin films and secondary ion mass spectrometry was used to determine the concentration-depth profiles of several tracers. A dependence of the atomic mobility of the tracer on its size was found, as observed in the binary Ni_xZr_y amorphous alloy system. The presence of the Al reduces the diffusion coefficients in general, with a stronger decrease for the small sized tracer. The results support the existence of two different diffusion mechanisms proposed for amorphous metallic alloys.     

Synthesis and characterization of catalytic iridium nanoparticles in imidazolium ionic liquids

The reduction of [Ir(cod)Cl](2) (cod=1,5-cyclooctadiene) dissolved in 1-n-butyl-3-methyl tetrafluoroborate, hexafluorophosphate and trifluoromethane sulphonate ionic liquids in the presence of 1-decene by molecular hydrogen produces Ir(0) nanoparticles. The formation of these nanoparticles follows the two-step [A-->B, A+B-->2B (k(1),k(2))] autocatalytic mechanism. The same mean diameter values of around 2-3 nm were estimated from in situ TEM and SAXS analyses of the Ir(0) nanoparticles dispersed in the ionic liquids and by XRD of the isolated material. XPS and EXAFS analyses clearly show the interactions of the ionic liquid with the metal surface demonstrating the formation of an ionic liquid protective layer surrounding the iridium nanoparticles. SAXS analysis indicated the formation of an ionic liquid layer surrounding the metal particles with an extended molecular length of around 2.8-4.0 nm depending on the type of the anion.     

Preparation of thin films of the ternary heavy fermion system CeNi2Ge2

2 Ge₂ layers on W(110). In order to produce well-ordered and atomically clean surfaces of the Ce-based intermetallic system the growth was performed under UHV conditions (p<2×10^-11 mbar). Both the polycrystalline CeNi₂Ge₂ compound and the individual elements Ce, Ni, and Ge were used as evaporants. The characterisation of the layers was made with LEED, SEM, and XPS. We find a significant influence of the substrate temperature and the evaporation power on the growth characteristics. The compound material CeNi₂Ge₂ exhibits complicated behaviour when evaporated. Under carefully selected growth conditions we obtain well-ordered films with a stoichiometry of Ce:Ni:Ge=1:2:2 and a (001) oriented surface of the body-centered tetragonal ThCr₂Si₂-type structure. The k_∥ dispersion and binding energies of the valence bands of these layers were determined with ARUPS. Received: 26 October 1997/Accepted: 27 October 1997     

Characterisation of structured thin films made from complex materials by photoabsorption spectromicroscopy

2 Al₃ and YBa₂Cu₃O₇/PrBa₂Cu₃O₇. To investigate devices built from these complex materials we applied element-sensitive photoemission electron microscopy (PEEM). Information about the chemical composition of the imaged sample can be obtained by PEEM via tuning the photon energy to X-ray absorption edges. To apply spectromicroscopy we acquired microscopic images using photon energies near and at the edges. Such images give the lateral distribution of a specific element. Microspectroscopy is performed by recording the intensity of the true secondary electrons in selected spots during a sweep of the photon energy. The main aim of our work was to observe oxygen-related defects and changes in the composition affecting the physical properties of the materials. Therefore, we applied both methods to micro-patterned devices using soft-X-ray synchrotron radiation and found that small local defects and chemical differences can be easily detected. Such defects and chemical differences are quite critical to the physical properties of the devices, since they simulate spurious effects thus influencing the reliability of the devices. Received: 13 March 1998/Accepted: 22 April 1998     

Clean and ordered surfaces of CeNi2Ge2 layers on W(110)

Investigations of the geometric and electronic properties of ternary Ce-based heavy fermion systems CeT₂X₂ (T : Ni,Pd,Rh; X : Ge,Si) were carried out by means of electron spectroscopic methods. The main problem for these surface-sensitive techniques is the preparation of well-ordered and atomically clean surfaces. The ternary substance CeNi₂Ge₂ was grown on a W(110) substrate by MBE with subsequent annealing. A nearly layer-by-layer growth mode was detected using MEED. The annealed layers are ordered, but show small Ni₂Ge crystalline islands. The composition was characterised by means of AES in dependence of the substrate as well as the annealing temperature. Electronic properties are investigated by angle resolved photoelectron spectroscopy. Received: 24 June 1996 / Revised: 23 December 1996 / Accepted: 31 December 1996     

Influence of absorbed pump profile on the temperature distribution within a diode side-pumped laser rod

In this paper, an analytical model for temperature distribution of the side-pumped laser rod is extracted. This model can be used for side-pumped laser rods whose absorbed pump profile is a Gaussian profile. Then, it is validated by numerical results which exhibit a good agreement with the analytical results. Afterwards, by considering a general expression for super-Gaussian and top-hat profiles, and solving the heat equation, the influence of profile width and super-Gaussian exponent of the profile on temperature distribution are investigated. Consequently, the profile width turns out to have a greater influence on the temperature compared to the type of the profile.     

A novel approach in drug discovery: synthesis of estrone--talaromycin natural product hybrids

Hetero-Diels-Alder reaction of the steroidal exocyclic enol ethers 14 and 15, obtained from the secoestrones 8 and 9 by reduction, iodoetherification, and elimination, with ethyl O-benzoyldiformylacetate (16) leads to the spiroacetals 17 and 18 as a mixture of four diastereomers. Reduction of the major diastereomers 17a and 18a with DIBAH and subsequent hydrogenation yields the novel natural product hybrids 21, 23, 24, and 25, which possess the structural features of the steroid estrone (7) and the mycotoxin talaromycin 6.     

Field dependence of orbital magnetic moments in the Heusler compounds Co<Subscript>2</Subscript>FeAl and Co<Subscript>2</Subscript>Cr<Subscript>0.6</Subscript>Fe<Subscript>0.4</Subscript>Al

Orbital and spin magnetic moments of the Heusler compounds Co₂FeAl and Co₂Cr_0.6Fe_0.4Al were measured by magnetic circular dichroism in X-ray absorption (XMCD). The orbital magnetic moments per spin are quite large (0.1–0.2) compared to bulk values of Fe and Co metals, indicating a considerable spin–orbit coupling in these Heusler compounds. A strong localization of the 3d electron states might be responsible for this observation. The Co and Fe orbital to spin moment ratio shows a distinct decrease of r(Fe)=0.04±0.02 and r(Co)=0.06±0.02 with increasing external field for the ternary compound Co₂FeAl, while the ratio is within error limits independent of the field for Co₂Cr_0.6Fe_0.4Al. This is discussed in terms of a relation to magnetocrystalline anisotropies. PACS 75.50.Cc; 71.20.Lp; 78.40.Kc     

Structure and magnetic properties of iron–platinum particles with <Emphasis Type="Italic">γ</Emphasis>-ferric-oxide shell

Nanoparticles of solid solution Fe _x Pt_1?x , where 0.25≥x≥0 (?fcc lattice) with γ-Fe₂O₃ shell (lattice of the spinel type) were synthesised and characterised by high-resolution transmission electron microscopy, energy dispersive X-ray analysis, electron energy loss spectroscopy, Mössbauer spectroscopy and magnetometry. From the point of view of magnetic properties, such two-phase particles are interesting because their core is antiferromagnetic or paramagnetic (at very small values of x) whereas the shell is ferrimagnetic. The size of the particles was in the range of several nanometers. The Mössbauer measurements revealed a blocking temperature of about 100 K above which the particles are superparamagnetic. Towards lower temperatures, the magnetic characteristics of an ensemble of such particles show an increase of magnetic rigidity.