Transport and vortex pinning in micron-size superconducting Nb films

We have carried out Hall measurements on thin films of Nb in the flux-flow regime. The Hall bars were several microns in scale. Oscillations with magnetic field in the transverse and longitudinal resistances between the depinning field Bd and the upper critical field Bc2 are observed below Tc. The Hall effect may even change sign. The transverse and longitudinal resistances are interpreted in terms of current-driven motion of vortices in the presence of a few impurities. Simulations from time-dependent Ginzburg-Landau equations confirm this argument

Giant exchange bias and the vertical shifts of hysteresis loops in gamma-Fe2O3-coated Fe nanoparticles

We fabricated core/shell-structured Fe nanoparticles, in which the alpha-Fe core is about 5 nm in diameter and the gamma-Fe2O3 shell is about 3 nm thick, and systematically studied their structural and magnetic properties. The magnetic hysteresis (M-H) loops, measured at low temperatures, after the particles were cooled from 350 K in a 50 kOe field, show significant shifts in both horizontal and vertical directions. It has been found that the exchange-bias field can be as large as 6.3 kOe at 2 K, and that the coercive field is also enhanced greatly in the field-cooled (FC) loops. The large exchange bias and vertical shifts of the FC loops at low temperatures may be ascribed to the frozen spins in the shells. A simple model is proposed to interpret the observations. (C) 2004 American Institute of Physics

Microstructure and growth of bamboo-shaped carbon nanotubes

Bamboo-shaped carbon nanotubes fabricated by the catalytic growth method with Ni/metallic oxide catalyst are systematically studied using transmission electron microscopy. It is found that except for the nodes the bamboo-shaped carbon nanotubes can be regarded as stacking of truncated conical graphene sheets. Depending on the size of a catalyst, the Ni particles can be encapsulated inside the tubes or at the growth front of the tubes with a plug-shape. A linear relationship between size of the nanotube and the catalyst particle is found. The results suggest that the nanometer-sized Ni catalyst particles are in the liquid state during nanotube growth. A simple model for the growth of bamboo-shaped nanotubes is proposed. (C) 2001 Elsevier Science B.V. All rights reserved

Synthesis of dense, single-crystalline CrO2 nanowire arrays using AAO template-assisted chemical vapor deposition

High-density, vertically aligned CrO2 nanowire arrays were obtained via atmospheric-pressure CVD assisted by AAO templates. The CrO 2 nanowire arrays show remarkably enhanced coercivity compared with CrO2 films or bulk. It was found that the length of the nanowires is greatly influenced by the pore diameter of the AAO template used. The growth mechanism and the pore size dependence of the CrO2 nanowire arrays are discussed. The present method provides a useful approach for the synthesis of CrO2 nanowire arrays. Such highly ordered nanowire arrays within an AAO template may have important applications in ultrahigh-density perpendicular magnetic recording devices and the mass production of spintronic nanodevices