Publications

Interstitial Y(2)Fe(17-x)M(x)C(y) compounds, with M=A1 or Si and x less than or equal to 2, y less than or equal to 1, were investigated by (57)Fe Mossbauer spectroscopy at room temperature. In the analysis of the spectra, the preferential occupation by Al or Si of the Fe (12k) sites in the hexagonal R(2)Fe(17) phase was taken into account. The hyperfine parameters variations are discussed on the grounds of local crystallographic details at the inequivalent iron sites, altered by the composition modifications. The low values of the isomer shifts and quadrupole interactions suggest small electronic charge redistribution in Y(2)Fe(17-x)M(x)A(y) as compared to Y(2)Fe(17). However, the isomer shift variations upon Wigner-Seitz cell volume changes evidence for screening and electron transfer effects. In order to provide a more consistent picture of the local effects of interstitial versus substitutional atoms in R(2)(Fe,M)(17)A(y) compounds, the discussion of the present results is extended to (57)Fe Mossbauer data obtained on the parent compounds and their nitrides. (C) 1997 American Institute of Physics.

The growth of thin platinum silicide layers on Si(111) and (001) surfaces was studied with in situ grazing incidence diffraction and X-ray reflectivity measurements during the annealing process. The interface roughnesses, layer thicknesses, structures and orientations of the three phases, Pt, Pt2Si and PtSi, were identified and their temperature dependencies observed. In the case of PtSi on Si(111), a plane reaction front was found, growing from the Si substrate towards the sample surface and forming an epitactic PtSi layer. In contrast, PtSi on Si(001) grows upwards locally and forms 'islands' of polycrystalline material on the Si surface.

This paper presents an accurate design method for the monolithic filters composed from capacitively coupled two-pole resonators structures, with quartz and langasite as crystal substrates. Because the coupling coefficient of langasite is much larger than that of quartz the design method in the case of langasite includes specific adjustments. The main relations for the electrical and geometrical design of the monolithic filter are presented.

An EPR study of SrCl2:Fe2+ crystals grown in chlorine reveals the presence after X-ray irradiation, besides the trapped-hole Fe-cub(3+) center with cubic symmetry previously observed in crystals grown under argon, of a new Fe-trig(3+) center with axial [111] symmetry. The quantitative analysis of the strongly anisotropic X-and Q-band spectra resulted in a g = 2.0141 value and zero-held-splitting parameters B-2(0) = 800.6, B-4(0) = -0.44, B-4(-3) = -10.5 and B-4(3) = 0.33 (at T = 12 K, in 10(-4) cm(-1) units). The very large B-2(0) value is attributed to the presence of a charged defect replacing one of the eight nearest neighbor Cl- ligands accompanied by an off-center displacement of the Fe3+ ion towards it. (C) 1997 Elsevier Science Ltd.

In the biological effect of ionizing radiation, a very important process is the production of free radicals and their reactions with one another and with other biomolecules. Electron Spin Resonance (ESR) spectroscopy is an appropriate method to detect the free radicals produced by irradiation and also to estimate their concentration. The samples were irradiated at a linear electron accelerator, The ESR measurements were performed using X-band spectrometers. The resonance spectra of irradiated complex biological materials are presented and discussed. Interesting applications of the ESR method in the field of radiation treatment of biological material are outlined.

An alternative technique for the synthesis and crystal growth of langasite (La3Ga5SiO14) is presented. In order to keep constant the compound stoichiometry during these processes, we have used the raw material charge (mixed oxides) in a pressed pellet shape and also we have built a thermal screening system surrounded by separately controlled RF coils. In this way, a suitable solid/liquid interface shape has been obtained.

Bulk samples obtained from two wafers of a silicon monocrystal material produced by float-zone refinement have been analysed using the four-point probe method. One of the two wafers comes from an oxygenated ingot; two sets of pure and oxygenated samples have been irradiated with 24 GeV/c protons in the fluence range from 10(13) to 1.74 x 10(14) p/cm(2). Van der Pauw resistivity and Hall coefficient have been measured before and after irradiation as a function of the temperature. A thermal treatment (30 min at 100 degrees C) has been performed to accelerate the reverse annealing effect in the irradiated silicon. The irradiated samples show the same exponential dependence of the resistivity and of the Hall coefficient on the temperature corresponding to the presence of radiation-induced deep energy levels around 0.6-0.7 eV in the silicon gap. The free carrier concentrations (n, p) have been evaluated in the investigated fluence range. The inversion of the conductivity type from n to p occurred, respectively, at 7 x 10(13) and at 4 x 10(13) p/cm(2) before and after the annealing treatment for both the two sets. Only slight differences have been detected between the pure and the oxygenated samples.

In the Gd3Co11B4-xAlx alloys the ternary 3:11:4 phase with Ce3Co11B4 structure is formed with an Al content x approximate to 0.5 substituting for boron. The fraction of this phase decreases as the boron content decreases in the samples. The Al, Co and Gd in excess form Gd(Co,Al)(5) with CaCu5-type structure, and the fraction of this phase increases with increasing x. X-ray diffraction results show that these phases are not at equilibrium. A strong preference of aluminium for the 3g site in the CaCu5-type structure is determined by Rietveld refinement of the X-ray diffraction pattern of the end-series compound (x = 4). (C) 1997 Elsevier Science S.A.

The influence of Gd addition on the structural properties of Fe-Cu-Nb-Si-B nanocrystallized and amorphous alloys is studied. The crystallization temperature increases and the microstructure of the annealed samples changes. Gd addition induces the formation of Gd-Fe-B phases. In fully crystallized Fe-Gd-Cu-Nb-Si-B alloys the alpha-Fe(Si), Fe-Nb-B, Gd3Fe62B14 and Gd1.1Fe4B4 phases are observed. The evolution of the microstructure is followed as a function of the cumulative effects of annealing time and temperature. The results suggest the transformation of the metastable Gd3Fe62B14 phase into Gd1.1Fe4B4 and alpha-Fe. The hyperfine parameters of the Gd3Fe62B14 Mossbauer contribution are reported. (C) 1997 Elsevier Science S.A.

The magnetoresistivity tensor of an interacting two-dimensional electron system with a lateral and unidirectional electric or magnetic modulation, in a perpendicular quantizing magnetic field, is calculated within the Kubo formalism. The influence of the spin splitting of the Landau bands and of the density of states (DOS) on the internal structure of the Shubnikov-de Haas oscillations is analyzed. The Coulomb electron-electron interaction is responsible for strong screening and exchange effects and is taken into account in a screened Hartree-Fock approximation, in which the exchange contribution is calculated self-consistently with the DOS at the Fermi level. This approximation describes both the exchange enhancement of the spin splitting and the formation of compressible edge strips, unlike the simpler Hartree and Hartree-Fock approximations, which yield either one or the other.