National Institute Of Materials Physics - Romania

Gold layers were deposited at oblique incidence onto glass plates. A B30.2 Hochvakuum Bedampfungsanlage equipment was used in this aim. The deposition conditions were chosen in order to obtain nanostructured layers. Correlations were found among these conditions and the corresponding nomograms were then obtained. Examples of such depositions were given and the layers structure was characterized and additionally supported by observing the molecular alignment given in the liquid crystal cells obtained with these gold layers.

A simple and novel method for production of metal clusters as building blocks for nanoscale devices is presented. With this procedure, called the gas / cluster aggregation method, a wide range of clusters may be synthesised and, more important, these clusters may be subsequently modified and functionalized in-situ by adding atoms/molecules of different nature, on the surface of readily formed clusters. The cluster size is extremely well controlled by the vapour pressure of the picked-up species. Moreover, the method is versatile, since it allows multiple pick-up processes within the same rare gas cluster for producing, for example, core-shell nanoparticles with metal core and non-metallic shell or vice-versa, nano-onions, with different species successively attached to the surface of the initial picked-up cluster, and so on. Initial formation of Fe gas-stabilised clusters and core-shell nanoparticles with Fe core and Fe oxide shell, as well as their structure and morphology, are presented and discussed. The core-shell nanoparticles show incipient self-organization into hexagonal cluster superlattice. Structural, magnetic and Mossbauer spectroscopy investigations have been performed on the Fe cluster samples. The magnetic properties of supported Fe clusters show marked differences compared to the bulk. A small hysteresis is observed in the parallel applied field while in the perpendicular case, lack of saturation at the highest applied field is noticed. Such behaviour has been also observed in FeRh [1] and AgCo [2] bimetallic nanoparticles. This behaviour marks the occurrence of a strong planar magnetic anisotropy in the sample and may also be a consequence of increased surface spin disorder and finite size effects, which are typical for nanoparticles in the reported size range.

Samples of hematite were exposed to mechanochemical activation by high energy ball milling for 0-27 h. The milling-induced changes to the structural and magnetic properties of hematite were characterized by X-ray diffraction (XRD) and Mssbauer spectroscopy. The particle size was found to decrease from 80 to 16.5 nm after 8 h of ball milling time, followed by a small increase to 19.8 nm at the end of the milling period. An overall expansion of the crystalline lattice parameters a and c with the milling time was deduced. The magnetic hyperfine field decreased with the ball milling time, from 51.46 down to 50.68 T after 27 h of grinding. Magnetite and traces of iron were observed at the longest milling time employed. The recoilless fraction (f) was measured simultaneously using a dual Mssbauer absorber consisting of hematite and a stainless steel etalon. The f factor first decreased with the milling time due to occurrence of nanoparticles in the system, had a maximum at 12 h due to agglomerations of nanoparticles and exhibited a second maximum at 27 h, due to the appearance of magnetite in the system. More samples of hematite were subjected to magnetomechanical activation by magnetic ball milling for 52 and 134 h. A phase mixture of hematite and magnetite was observed.

The FePtNbB alloys are of interest as a new class of exchange spring magnets. In strictly defined compositional range and after proper conditions of annealing, they develop a microstructure made of small nanograins of hard and soft magnetic phases alternatively disposed and coupled through exchange interactions. Taking advantage of the high uniaxial aniosotropy of the tetragonal hard magnetic FePt phase and high saturation magnetization of the cubic soft magnetic FePt phases, an exchange coupled nanocomposite magnet can be derived. A compositional study of FePtNbB alloys is provided in order to understand the role of each additional element. The possibility of casting alloys where two or more magnetic phases can be obtained is discussed in terms of the eutectic point of the FePt phase diagram. Also, the formation and evolution of the magnetic phases is studied by differential scanning calorimetry (DSC) and detailed analysis of these DSC scans is provided. Activation energy determined from DSC scans provides essential informations about crystallization and disorder-order phase transformation in these alloys.

Mixed oxides nanocomposites of the type xCr(2)O(3)-(1-x)alpha-Fe2O3 (x = 0.0-1.0) were synthesized using hydrothermal technique and characterized by X-ray diffraction (XRD) and Mossbauer spectroscopy. The same characterization methods were applied to a set of samples annealed at 600 degrees C for 2 h. High chromium content correlated with occurrence of amorphization effects and onset of superparamagnetism in the as-obtained samples. The structure and properties of the thermally annealed samples, however, are dominated by the equilibrium of two phases, Cr:Fe2O3 and Fe:Cr2O3. The final (x = 1.0) particle size of the nanocomposite is close to 19 nm.

Au3Zr intermetallic phase was obtained as pure phase by melt spinning technique. The sample with the same composition obtained by powder metallurgy exhibits some Au4Zr impurity. Rietveld refinements of the X-Ray data were done to obtain the unit cell parameters and atom positions inside the unit cell. Geometry optimizations for the atom positions inside the unit cell were performed by using ab-initio band structure calculations. The calculated values agree well with the ones obtained by Rietveld refinements of the diffraction data. The electronic mechanisms are explained on the basis of the projected densities of states to atom sites.

One of the most important trend in electronic devices manufacturing is miniaturisation. Among other techniques used to decrease the physical dimensions of microwave devices one is to employ materials with high permitivity [1], providing that the dimension of the device is proportional to the wavelength in the material, which is square effective permittivity times less than the wavelengths in free space. The paper shortly presents the manufacturing process to obtain a high permittivity ZST ((Zr(0.8), Sn(0.2))TiO(4)) material used to build a dielectric resonator oscillator, which is used as a proximity Doppler sensor. Computed and experimental results as well as the procedure to measure the parameters of the Doppler sensor are presented. The sensor described in the paper may be considered as a short range radar device.

In the aim to improve the adherence of the nanostructured gold layers onto glass substrate, gold deposition has been first time performed onto a polystyrene (PS) layer. The PS layer of ca. 50nm was spin-coated onto glass plates from a toluene solution. Gold layers of 10-20nm were vacuum deposited at small incidence angle. We found that at variance from the plates with gold deposited directly on the glass, the plates with an intermediate PS layer do not peel under overnight treatment in ethanol solution. The layers were characterized by several methods. X-ray diffraction (XRD) measurements showed that gold peaks have the position corresponding to the face-centered cubic structure: However, the crystallites on the sample with PS layer seem to be a little bit smaller than those with gold deposited directly onto the glass. XR reflectometry measurements have given the thickness of the gold layer in good agreement with the value estimated from quartz monitor readings and with the ellipsometric data as well, Liquid crystal cells were obtained to observe the molecular alignment imposed by the nanostructured gold layer deposited onto PS film. A rather strong interaction of gold atoms with the substrate molecules can be considered on the basis of the XRD and ellipsometry measurements.

We performed magnetization relaxation experiments for optimally doped YBa(2)Cu(3)O(7-delta) (YBCO) films with the external magnetic field H = 20-30 kOe oriented along the c axis and at relatively low temperatures T = 10-32 K, where a crossover elastic (collective) vortex creep-plastic creep is expected. The creep crossover is generated in this case by the T dependent macroscopic currents induced in the specimen, and manifests itself through the occurrence of a maximum in the current density J (or T) variation of the normalized vortex-creep activation energy U*, at which the creep exponent changes sign. With the same magnetization relaxation data the widely used Maley technique to construct the J dependence of the vortex-creep activation energy U supplies an U(J) variation close to the logarithmic one. It appears that the Maley technique may lead to unreliable results, due to the creep crossover characteristic to standard zero-field-cooling magnetization measurements and the reduced overall relaxation (in a convenient relaxation time window) for highly disordered specimens. It is suggested that the results obtained with this scheme should be checked by determining the U*(T, J) variation.

The influence of the shape of silicon quantum dots embedded in an amorphous silica matrix on the quantum confinement energy levels, as well as that of the Si/SiO2 potential barrier, are studied. The energy levels are computed using both the infinite and finite rectangular quantum well models for spherical quantum dots and the infinite rectangular quantum well for prolate spheroidal quantum dots. The results are compared with each other and also with the experimental activation energies obtained from the temperature dependence of the dark current. These activation energies are identified with the differences between the quantum confinement energies, subject to the selection rules. The finite rectangular quantum well model takes into account the experimental value of the finite potential barrier and the matrix-to-dot electron mass ratio. The energy levels are smaller than those for the infinite rectangular quantum well case; they decrease when the potential barrier decreases and the mass ratio increases. Different aspects of the models are discussed. All the errors are less than about 4%. The spheroidal shape lifts the degeneracy on the magnetic quantum number. The energy levels can decrease or increase with eccentricity as a consequence of the different quantum confinement effects along the major and minor axes. The supplementary information on the magnetic quantum number is beneficial for optical applications.