From colloidal Co/CoO core/shell nanoparticles to arrays of metallic nanomagnets: surface modification and magnetic properties.

The magnetic properties of nanoparticles can be subject to strong variations as the chemical composition of the particle surface is modified. To study this interrelation of surface chemistry and magnetism, self-assembled layers of colloidal 9.5 nm Co/CoO core/shell nanoparticles were exposed to mild reactive hydrogen and oxygen plasmas. The consecutive oxygen/hydrogen plasma treatment transforms the particle layer into an array of metallic nanomagnets with complete reduction of the oxide and removal of the organic surfactants. The original arrangement of the particle array and the number of Co atoms per particle remains unchanged within the experimental error, and thus this is a possible route for the fabrication of ultrahigh-density magnetic bit structures from colloidal dispersions. The magnetic properties can be tuned by controlling the thickness of the surface oxide layer, which magnetically hardens the particles, as evidenced by element-specific magnetic hysteresis loops.     

Low temperature self-ion irradiation of pure and granular aluminium-films

Al-films, evaporated at room temperature under different oxygen partial pressures, were irradiated with self-ions (500 keV, Al⁺⁺) at low temperature (<7 K). The observed increase of the resistivity and of the superconducting transition temperatureT _c depends strongly on the oxygen contentc ₀ present in the layers. A qualitative different behaviour of the above quantities was found for different fluence ranges of the bombarding ions. For high fluences in all cases an oxygen stabilized disorder state was obtained with correspondingT _c-increases betweenT _c=0.2 K for the purest films (c ₀0.5 at %) andT _c =1.2 K for films with c ₀=40 at %. The annealing behaviour of the irradiated films is also dependent on the oxygen contentc ₀.     

Kinetics, products, and mechanisms of secondary organic aerosol formation

Secondary organic aerosol (SOA) is formed in the atmosphere when volatile organic compounds (VOCs) emitted from anthropogenic and biogenic sources are oxidized by reactions with OH radicals, O(3), NO(3) radicals, or Cl atoms to form less volatile products that subsequently partition into aerosol particles. Once in particles, these organic compounds can undergo heterogenous/multiphase reactions to form more highly oxidized or oligomeric products. SOA comprises a large fraction of atmospheric aerosol mass and can have significant effects on atmospheric chemistry, visibility, human health, and climate. Previous articles have reviewed the kinetics, products, and mechanisms of atmospheric VOC reactions and the general chemistry and physics involved in SOA formation. In this article we present a detailed review of VOC and heterogeneous/multiphase chemistry as they apply to SOA formation, with a focus on the effects of VOC molecular structure on the kinetics of initial reactions with the major atmospheric oxidants, the subsequent reactions of alkyl, alkyl peroxy, and alkoxy radical intermediates, and the composition of the resulting products. Structural features of reactants and products discussed include compound carbon number; linear, branched, and cyclic configurations; the presence of C[double bond, length as m-dash]C bonds and aromatic rings; and functional groups such as carbonyl, hydroxyl, ester, hydroxperoxy, carboxyl, peroxycarboxyl, nitrate, and peroxynitrate. The intention of this review is to provide atmospheric chemists with sufficient information to understand the dominant pathways by which the major classes of atmospheric VOCs react to form SOA products, and the further reactions of these products in particles. This will allow reasonable predictions to be made, based on molecular structure, about the kinetics, products, and mechanisms of VOC and heterogeneous/multiphase reactions, including the effects of important variables such as VOC, oxidant, and NO(x) concentrations as well as temperature, humidity, and particle acidity. Such knowledge should be useful for interpreting the results of laboratory and field studies and for developing atmospheric chemistry models. A number of recommendations for future research are also presented.     

Anisotropy of a vortex instability observed at high current densities in YBa2Cu3O7δ superconducting films

Experimental data are provided for YBaCuO films that an instability of the vortex system, which manifests itself by a voltage jump at a critical current I*, exhibits strong anisotropy if the magnetic field is tilted from parallel to perpendicular to the c-axis. The angular dependence of I* can be well described by a model emphasizing the component of the magnetic field parallel to the c-axis. If the current range is restricted to values close to I*, the current-voltage characteristics below the instability show a satisfactory agreement with the prediction of the theory of ‘Self-Organized Criticality’ (SOC). In terms of this theory it is possible to relate the critical vortex velocity v* to the temperature and field dependent characteristic size of the underlying vortex avalanches. If, however, standard Larkin-Ovchinnikov theory is applied to describe the instability, this critical velocity is related to the scattering rate of quasiparticles. Analyzed in this way and assuming an isotropic diffusion constant of the quasiparticles, an anisotropic scattering rate and its temperature dependence can be extracted.     

Tomographic reconstruction of atmospheric turbulence with the use of time-dependent stochastic inversion

Acoustic travel-time tomography allows one to reconstruct temperature and wind velocity fields in the atmosphere. In a recently published paper [S. Vecherin et al., J. Acoust. Soc. Am. 119, 2579 (2006)], a time-dependent stochastic inversion (TDSI) was developed for the reconstruction of these fields from travel times of sound propagation between sources and receivers in a tomography array. TDSI accounts for the correlation of temperature and wind velocity fluctuations both in space and time and therefore yields more accurate reconstruction of these fields in comparison with algebraic techniques and regular stochastic inversion. To use TDSI, one needs to estimate spatial-temporal covariance functions of temperature and wind velocity fluctuations. In this paper, these spatial-temporal covariance functions are derived for locally frozen turbulence which is a more general concept than a widely used hypothesis of frozen turbulence. The developed theory is applied to reconstruction of temperature and wind velocity fields in the acoustic tomography experiment carried out by University of Leipzig, Germany. The reconstructed temperature and velocity fields are presented and errors in reconstruction of these fields are studied.