Magnetic properties of Fe-Cu-Nb-Si-B nanocrystaliine magnetic alloys

Several ribbons of composition Fe73.5Cu1Nb 3Si16.5B6 and Fe73.5Cu1 Nb3Si13.5B9 were prepared by annealing the as-quenched samples between 525��C and 700��C; which induced nucleation of nanocrystallites of Fe bcc-type composition. Mean grain sizes were obtained from X-ray diffraction. Static magnetic properties were measured with both a Magnet Physik Hysteresis-Graph (up to 200 Oe) and a SHE SQUID magnetometer (up to 50 kOe). Soft magnetic parameters (coercive field and initial permeability) were very sensitive to grain size. The ZFC magnetization at low field showed a broad peak at a temperature TM, thus signalling a certain distribution of nanocrystalline sizes, and TM strongly decreased when the mean grain size decreased. Isothermal magnetization curves at low temperature showed the expected asymptotic behavior of a random magnet material at low and high fields.

Influence of the (111) twinning on the formation of diamond cubic/diamond hexagonal heterostructures in Cu-catalyzed Si nanowires

The occurrence of heterostructures of cubic silicon/hexagonal silicon as disks defined along the nanowire (111) growth direction is reviewed in detail for Si nanowires obtained using Cu as catalyst. Detailed measurements on the structural properties of both semiconductor phases and their interface are presented. We observe that during growth, lamellar twinning on the cubic phase along the (111) direction is generated. Consecutive presence of twins along the (111) growth direction was found to be correlated with the origin of the local formation of the hexagonal Si segments along the nanowires, which define quantum wells of hexagonal Si diamond. Finally, we evaluate and comment on the consequences of the twins and wurtzite in the final electronic properties of the wires with the help of the predicted energy band diagram.

Chemisorption of group-III metals on the Si(111) and Ge(111) surfaces: An ab initio study

Chemisorption of group-III metal adatoms on Si(111) and Ge(111) has been studied through the ab initio Hartree-Fock method including nonempirical pseudopotentials and using cluster models to simulate the surface. Three different high-symmetry sites (atop, eclipsed, and open) have been considered by using X4H9, X4H7, and X6H9 (X=Si,Ge) cluster models. In a first step, ideal surface geometries have been used. Metal-induced reconstruction upon chemisorption has also been taken into account. Equilibrium distances, binding energies, and vibrational frequencies have been obtained and compared with available experimental data. From binding-energy considerations, the atop and eclipsed sites seem to be the most favorable ones and thus a coadsorption picture may be suggested. Group-III metals exhibit a similar behavior and the same is true for Si(111) and Ge(111) surfaces when chemisorption is considered.

Obtenci��n de l��minas delgadas de Y-Ba-Cu-O sobre Si (100)

Se ha utilizado la evaporaci��n secuencial de Cu, Y2O3 y BaF2 para la obtenci��n de l��minas delgadas superconductoras de YBa-CuO sobre substratos de Si monocristalino con orientaci��n [100], recubiertos con una l��mina barrera de Zr02. Se han estudiado los efectos de la variaci��n de los espesores relativos de las l��minas constituyentes y del espesor total de la l��mina resultante. Las l��minas se han caracterizado mediante medidas de la variaci��n de la resistencia con la temperatura, microscop��a electr��noca de barrido, difractometr��a de rayos X, microson-da electr��nica y espectometr��a de masas de iones secundarios. Las l��minas presentan un ligero car��cter semiconductor en el estado normal, con temperaturas de inicio de la transici��n su-perconductora alrededor de 90 K, y resistencia nula, en el mejor de los casos, a 45 K.

Fabricaci�� de Lents Zonals de Fresnel per aplicaci�� als raigs X tous

Estudi elaborat a partir d���una estada al Paul Scherrer Institut del Maig a l���Octubre del 2006 amb l���ajuda i supervisi�� dels Dr. Konstantins Jefimovs i Dr. Christian David. Focalitzar raigs X tous ��s una necessitat essencial per al microan��lisis, la microscopia, i fer imatges en moltes Instal��lacions de Radiaci�� Sincrotr��. Les Lents Zonals de Fresnel (FZP, de la denominaci�� anglesa ���Fresnel Zone Plates���) han demostrat donar uns punts focals amb una resoluci�� espacial destacada i una baixa il��luminaci�� de fons. Tanmateix, la fabricaci�� de FZP ��s complexa i no totalment reprodu��ble. A m��s a m��s, el temps de vida de les FZP ��s for��a curt, ja que estant situades sobre membranes de nitrur de silici molt fines i altament absorbents. Per tant, hem fet esfor��os per implementar FZP de silici, que s���espera que siguin m��s resistents. L���element est�� fet d���una oblia de cristall de silici poc absorbent, i no presenta cap interfase entre materials. Aix�� doncs, aquestes lents s��n especialment adequades per a aguantar les extremes c��rregues de radiaci�� de les fonts de raigs X m��s brillants. Particularment, aix�� ��s molt important per a les aplicacions a les pr��ximes generacions de fonts de raigs X, com els L��sers d���Electrons Lliures (FEL, de la denominaci�� anglesa ���Free Electron Laser���). El silici tamb�� garanteix que no hi hagi cap banda d���absorci�� en el rang d���energies de la finestra de l���aigua (200-520 eV), fent aquestes lents ideals per a fer imatges de mostres biol��giques. En aquest informe, hi ha una descripci�� detallada de tots els passos involucrats en la fabricaci�� de les Lents Zonals de Fresnel de silici. En resum, les estructures de FZP es modelen sobre una resina utilitzant litografia per feix d���electrons i llavors el patr�� es transmet al silici mitjan��ant un gravat d���ions reactius (RIE, de la denominaci�� anglesa ���Reactive Ion Etching���) utilitzant una fina (20 nm) m��scara de Crintermitja. Les membranes de silici es poden aprimar despr��s de la fabricaci�� de les estructures per a garantir una transmissi�� suficient fins i tot a baixes energies. Aquest informe tamb�� inclou l���an��lisi i la discussi�� d���alguns experiments preliminars per avaluar el rendiment de les Si FZPs fets a la l��nia de llum PolLux del Swiss Ligth Source amb l���ajuda dels Dr. J��rg Raabe i Dr. George Tzvetkov.

S��ntesis de nanopart��culas de ferritas tipo MFe2O4 (M=Fe, Co, Cu) para su aplicaci��n en superconductores

La tecnolog��a actual ha permitido que en los ��ltimos a��os la nanociencia y la nanotecnolog��a sean puntos cr��ticos en el desarrollo del conocimiento. En estos momentos se desarrollan sistemas de dimensiones nanom��tricas que son interesantes debido a sus potenciales aplicaciones en diferentes ��mbitos como en qu��mica, f��sica, biolog��a, materiales, medicina, cosm��tica... Dentro de estos sistemas nanosc��picos se encuentran las nanopart��culas, estructuras con un tama��o inferior a los 100nm de longitud. En esta clasificaci��n existen a su vez diferentes categor��as, como las nanopart��culas met��licas, semiconductoras, magn��ticas, etc. y es exactamente en esta ��ltima tipoog��a donde se centra este estudio. Este proyecto de investigaci��n desarrolla la s��ntesis de magnetita (Fe3O4), ferrita de cobalto (CoFe2O4) y ferrita de cobre (CuFe2O4) con la finalidad de utilizarlas como dopante en superconductores. El m��todo sint��tico utilizado es del tipo solvot��rmico y se lleva a cabo en trietilenglicol, el cual act��a a la vez como disolvente y como estabilizante de las nanopart��culas. Las part��culas as�� obtenidas son dispersables en medios polares como el etanol absoluto. Los precursores de este m��todo sint��tico son los respectivos acetilacetonatos met��licos debido a que el ligando org��nico descompone en productos vol��tiles. Existen diferentes factores que afectan a la s��ntesis, tales como la velocidad de ascenso de la temperatura, la agitaci��n, la presencia de agua, la temperatura de descomposici��n de los precursores, etc. Algunos de estos factores han sido estudiados con detalle y aplicados con tal de optimizar el m��todo experimental. Las nanopart��culas sintetizadas han sido analizadas mediante diversas t��cnicas f��sicas con tal de establecer diferentes par��metros, tales como su composici��n fnal, su pureza, su estructura, sus propiedades magn��ticas, etc. Estas t��cnicas son diversas: desde la espectroscopia infrarroja hasta medidas mediante SQUID, pasando por rayos X, microscop��a electr��nica y termogravimetr��a. Los resultados han sido favorables en la s��ntesis de la magnetita y tambi��n en la ferrita de cobalto, ya que las nanopart��culas obtenidas son homog��neas, f��cilmente dispersables en algoholes, estables por largos per��odos de teimpo, r��pidas de sintetizar, etc. El ��nico problema observado ha sido la s��ntesis de ferrita de cobre la cual se ha de optimizar, ya que el producto final ha resultado ser una mezcla de tres compuesto diferentes.

Low-loss rib waveguides containing Si nanocrystals embedded in SiO2

We report on the study and modeling of the structural and optical properties of rib-loaded waveguides working in the 600-900-nm spectral range. A Si nanocrystal (Si-nc) rich SiO2 layer with nominal Si excess ranging from 10% to 20% was produced by quadrupole ion implantation of Si into thermal SiO2 formed on a silicon substrate. Si-ncs were precipitated by annealing at 1100��C, forming a 0.4-um-thick core layer in the waveguide. The Si content, the Si-nc density and size, the Si-nc emission, and the active layer effective refractive index were determined by dedicated experiments using x-ray photoelectron spectroscopy, Raman spectroscopy, energy-filtered transmission electron microscopy, photoluminescence and m-lines spectroscopy. Rib-loaded waveguides were fabricated by photolithographic and reactive ion etching processes, with patterned rib widths ranging from 1��to��8����m. Light propagation in the waveguide was observed and losses of 11dB/cm at 633 and 780 nm were measured, modeled and interpreted.

Size dependence of refractive index of Si nanoclusters embedded in SiO2

he complex refractive index of SiO2 layers containing Si nanoclusters (Si-nc) has been measured by spectroscopic ellipsometry in the range from 1.5 to 5.0 eV. It has been correlated with the amount of Si excess accurately measured by x-ray photoelectron spectroscopy and the nanocluster size determined by energy-filtered transmission electron microscopy. The Si-nc embedded in SiO2 have been produced by a fourfold Si+ ion implantation, providing uniform Si excess aimed at a reliable ellipsometric modeling. The complex refractive index of the Si-nc phase has been calculated by the application of the Bruggeman effective-medium approximation to the composite media. The characteristic resonances of the refractive index and extinction coefficient of bulk Si vanish out in Si-nc. In agreement with theoretical simulations, a significant reduction of the refractive index of Si-nc is observed, in comparison with bulk and amorphous silicon. The knowledge of the optical properties of these composite layers is crucial for the realization of Si-based waveguides and light-emitting devices.

The effect of substrate on high-temperature annealing of GaN epilayers: Si versus sapphire

We have studied the effects of rapid thermal annealing at 1300����C on GaN epilayers grown on AlN buffered Si(111) and on sapphire substrates. After annealing, the epilayers grown on Si display visible alterations with craterlike morphology scattered over the surface. The annealed GaN/Si layers were characterized by a range of experimental techniques: scanning electron microscopy, optical confocal imaging, energy dispersive x-ray microanalysis, Raman scattering, and cathodoluminescence. A substantial Si migration to the GaN epilayer was observed in the crater regions, where decomposition of GaN and formation of Si3N4 crystallites as well as metallic Ga droplets and Si nanocrystals have occurred. The average diameter of the Si nanocrystals was estimated from Raman scattering to be around 3��nm. Such annealing effects, which are not observed in GaN grown on sapphire, are a significant issue for applications of GaN grown on Si(111) substrates when subsequent high-temperature processing is required.

Modifications in the Si valence band after ion-beam-induced oxidation

The changes undergone by the Si surface after oxygen bombardment have special interest for acquiring a good understanding of the Si+-ion emission during secondary ion mass spectrometry (SIMS) analysis. For this reason a detailed investigation on the stoichiometry of the builtup surface oxides has been carried out using in situ x-ray photoemission spectroscopy (XPS). The XPS analysis of the Si 2p core level indicates a strong presence of suboxide chemical states when bombarding at angles of incidence larger than 30��. In this work a special emphasis on the analysis and interpretation of the valence band region was made. Since the surface stoichiometry or degree of oxidation varies with the angle of incidence, the respective valence band structures also differ. A comparison with experimentally measured and theoretically derived Si valence band and SiO2 valence band suggests that the new valence bands are formed by a combination of these two. This arises from the fact that Si��Si bonds are present on the Si��suboxide molecules, and therefore the corresponding 3p-3p Si-like subband, which extends towards the Si Fermi level, forms the top of the respective new valence bands. Small variations in intensity and energy position for this subband have drastic implications on the intensity of the Si+-ion emission during sputtering in SIMS measurements. A model combining chemically enhanced emission and resonant tunneling effects is suggested for the variations observed in ion emission during O+2 bombardment for Si targets.

Model for the emission of Si+ ions during oxygen bombardment of Si(100) surfaces

The variation in the emission of Si+ ions from ion-beam-induced oxidized silicon surfaces has been studied. The stoichiometry and the electronic structure of the altered layer has been characterized using x-ray photoelectron spectroscopy (XPS). The XPS analysis of the Si 2p core level indicates the strong presence of suboxide chemical states when bombarding at angles of incidence larger than 30 ��. Since the surface stoichiometry or degree of oxidation varies with the angle of incidence, the corresponding valence-band structures also differ among each other. A comparison between experimental measurements and theoretically calculated Si and SiO2 valence bands indicates that the valence bands for the altered layers are formed by a combination of those two. Since Si-Si bonds are present in the suboxide molecules, the top of the respective new valence bands are formed by the corresponding 3p-3p Si-like subbands, which extend up to the Si Fermi level. The changes in stoichiometry and electronic structure have been correlated with the emission of Si+ ions from these surfaces. From the results a general model for the Si+ ion emission is proposed combining the resonant tunneling and local-bond-breaking models.

State equation for shape-memory alloys. Aplication to Cu-Zn-Al

We deal with the hysteretic behavior of partial cycles in the two��phase region associated with the martensitic transformation of shape��memory alloys. We consider the problem from a thermodynamic point of view and adopt a local equilibrium formalism, based on the idea of thermoelastic balance, from which a formal writing follows a state equation for the material in terms of its temperature T, external applied stress ��, and transformed volume fraction x. To describe the striking memory properties exhibited by partial transformation cycles, state variables (x,��,T) corresponding to the current state of the system have to be supplemented with variables (x,��,T) corresponding to points where the transformation control parameter (���� and/or T) had reached a maximum or a minimum in the previous thermodynamic history of the system. We restrict our study to simple partial cycles resulting from a single maximum or minimum of the control parameter. Several common features displayed by such partial cycles and repeatedly observed in experiments lead to a set of analytic restrictions, listed explicitly in the paper, to be verified by the dissipative term of the state equation, responsible for hysteresis. Finally, using calorimetric data of thermally induced partial cycles through the martensitic transformation in a Cu��Zn��Al alloy, we have fitted a given functional form of the dissipative term consistent with the analytic restrictions mentioned above.

Elastic constants of the beta-Cu-Zn alloy system: A Monte Carlo study

Monte Carlo calculations of the isothermal elastic constants of the beta-CuxZn1-x alloy system as a function of the composition have been carried out. We assume the atoms interact via a two-body Morse potential function and use numerical values for the potential parameters evaluated taking into account experimental data. We find a quite good agreement between our results and the expected experimental behavior.

Contribuci��n al conocimiento de la anapaita de la Cerdanya.

Se estudian mineral��gicamente los fosfatos mioc��nicos de Bellver de la Cerdanya (L��rida). Para ello, se emplean las t��cnicas siguientes: difracci��n de rayos X, espectroscopia de emisi��n y absorci��n de infrarrojos. Mediante el estudio roentgenol��gico determinamos que la especie mineral en estudio es la Anapaita (FeCaz(P04)2.4Hz0). La espectroscopia de emisi��n confirma, por una parte, la existencia de los elementos constituyentes de la f��rmula qu��mica: P, Ca y Fe con l��neas bien marcadas y, por otra parte, la presencia de Mn, Al, Si, Mg, Na y Cu como contaminantes. Finalmente, el estudio del espectro de absorci��n de rayos infrarrojos detecta el grupo fosfato, b��sico, y la presencia de agua de absorci��n.