Dynamics of vortices in type-II superconductors with periodic square columnar pins

The dynamics of a two-dimensional vortex system with strong periodic square columnar pins is investigated. For the case vortex number matching pinning number, we find that the vortex liquid is frozen into square lattice via a continuous transition, and the freezing (melting) temperature T_m is the same as the thermal depinning temperature of vortices, which are different from the first-order phase transition at weak pinning. The zero-temperature critical depinning force F_c0 is exactly the same as the maximum pinning force, and the depinning property at T = 0 can be expressed by scaling v ～ (F ? F_c0)^β with the exponent β close to 0.5. The v–F curves at temperatures below T_m show that vortices are pinned at small driving force.     

Unique depinning fields at symmetric double notches in a ferromagnetic permalloy nanowire

Ferromagnetic domain-wall pinning and depinning mechanisms were investigated at permalloy nanowire notches using a micromagnetic calculation. A unique depinning field originated from the symmetric double notches irrespective of the wall polarity or the propagation direction, whereas several distinct pinning mechanisms appeared from single or asymmetric notches. The depinning field was principally determined by the exiting notch slope due to the dynamic narrowing of the domain wall thickness.     

Magnetic behaviour of arrays of Ni nanowires by electrodeposition into self-aligned titania nanotubes

Arrays of Ni nanowires electrodeposited into self-aligned and randomly disordered titania nanotube arrays grown by anodization process are investigated by X-ray diffraction, SEM, rf-GDOES and VSM magnetometry. The titania nanotube outer diameter is about 160 nm, wall thickness ranging from 60 to 70 nm and 300 nm in depth. The so-obtained Ni nanowires reach above 100 nm diameter and 240 nm length, giving rise to coercive fields of 98 and 200 Oe in the perpendicular or parallel to the nanowires axis hysteresis loops, respectively. The formation of magnetic vortex domain states is also discussed.     

Magnetic domain wall motion by current injection in CoPt nanowires consisting of notches

Magnetic domain wall behaviors in CoPt nanowires consisting of multiple pairs of notches were investigated by experimental measurements as well as by micromagnetic modeling. The nanowires were fabricated by ion-beam sputter deposition and e-beam lithography where one to three triangular shaped notches were installed at an interval of 1 μm. Based on the evaluated I–V characteristics, we observed that differential resistance curves showed two peaks with a local minimum at around zero current; the domain wall was trapped between the current ranges within these two peaks. As the number of notch was increased, the resistance of the nanowire became larger.     

Synthesis and photoluminescence properties of silver nanowires

Silver nanowires of 50–190 nm in diameters along with silver nanoparticles in the size range of 60–200 nm in prismatic and hexagonal shapes are synthesized through chemical process. The lengths of the silver nanowires lie between 40 and 1000 μm. The characterizations of the synthesized samples are done by X-ray diffraction (XRD), scanning electron microscopy (SEM) and UV–visible absorption spectroscopy. The syntheses have been done by using two processes. In the first process, relatively thicker and longer silver nanowires are synthesized by a soft template liquid phase method at a reaction temperature of 70 °C with methanol as solvent. In the second process, thinner silver nanowires along with silver nanoparticles are prepared through a polymer mediated polyol process at a reaction temperature of 210 °C with ethylene glycol as solvent. The variations of photoluminescence (PL) emission from the silver nanocluster dispersed in methanol as well as in ethylene glycol are recorded at room temperature under excitation wavelengths lying in between 300 and 414 nm. The blue–green PL emission is observed from the prepared samples and these emissions are assigned to radiative recombination of Fermi level electrons and sp- or d-band holes.     

XPS investigation of vacuum annealed vertically aligned ultralong ZnO nanowires

The surface properties of vertically aligned ZnO nanowires grown by chemical vapour deposition on GaN using a gold layer as a catalyst are investigated by X-ray Photoelectron Spectroscopy as a function of annealing temperature in ultra high vacuum (UHV). The nanowires are 8.5 μm long and 60 nm wide. 87% of the surface carbon content was removed after annealing at 500 °C in UHV. Analysis of the gold intensity suggests diffusion into the nanowires after annealing at 600 °C. Annealing at 300 °C removes surface water contamination and induces a 0.2 eV upward band bending, indicating that adsorbed water molecules act as surface electron donors. The contaminants re-adsorbed after 10 days in UHV and the surface band bending caused by the water removal was reversed. The UHV experiment also affected the nanowires arrangement causing them to bunch together. These results have clear implications for gas sensing applications with ZnO NWs.     

In vitro infrared thermography assessment of temperature peaks during the intra-oral welding of titanium abutments

Control of heat dissipation and transmission to the peri-implant area during intra-oral welding is very important to limit potential damage to the surrounding tissue. The aim of this in vitro study was to assess, by means of thermal infrared imaging, the tissue temperature peaks associated with the thermal propagation pathway through the implants, the abutments and the walls of the slot of the scaffold, generated during the welding process, in three different implant systems. An in vitro polyurethane mandible model was prepared with a 7.0 mm v-shape slot. Effects on the maximum temperature by a single welding procedure were studied using different power supplies and abutments. A total of 36 welding procedures were tested on three different implant systems. The lowest peak temperature along the walls of the 7.0 mm v-shaped groove (31.6 ± 2 °C) was assessed in the specimens irrigated with sterile saline solution. The highest peak temperature (42.8 ± 2 °C) was assessed in the samples with a contemporaneous power overflow and premature pincers removal. The results of our study suggest that the procedures used until now appear to be effective to avoid thermal bone injuries. The peak tissue temperature of the in vitro model did not surpass the threshold limits above which tissue injury could occur.     

Generation of dual-wavelength domain-wall rectangular-shape pulses in HNLF-based fiber ring laser

The generation of dual-wavelength domain-wall rectangular-shape pulses in a highly nonlinear fiber (HNLF)-based fiber ring laser is experimentally demonstrated. The dual-wavelength lasing operation is realized by employing the intracavity birefringence-induced spectral filtering effect. An 85 m long HNLF is introduced into the fiber ring laser to enhance the nonlinear effect, which is favorable for the cross coupling between the two lasing beams. Experimentally, it was found that the interval of two domain walls in the time domain could be adjusted by simply tuning the linear cavity phase delay, which results in the achievement of different output pulse shapes. By properly rotating the polarization controller (PC), the dual-wavelength rectangular-shape pulses could be efficiently obtained. The proposed fiber laser provides a simple and efficient way to generate rectangular-shape pulse.     

Enhanced depolarization temperature in 0.90NBT–0.05KBT–0.05BT ceramics induced by BT nanowires

The depolarization temperature (T_d) of piezoelectric materials is an important figure of merit for their application at elevated temperatures. This study focuses on the effect of BaTiO₃ (BT) nanowires on T_d and piezoelectric properties of morphotropic-phase-boundary 0.90NBT–0.05KBT–0.05BT ceramics. The results reveal that BaTiO₃ nanowires can pin the domain wall, leading to the increase of coercive field (E_c) from 21.06 kV/cm to 34.99 kV/cm. The T_d value of 0.90NBT–0.05KBT–0.05BT ceramics can be enhanced approximately 20 °C when using BT nanowires instead of BT solution as the raw material. Meanwhile, at the same polarization conditions, the piezoelectric constant of the ceramic added BT nanowires (172 pC/N) is decreased but still remains a larger value compared with those of other lead-free ceramics. The results imply that the addition of BT nanowires into NBT–KBT is a very effective route to improve T_d.     

Modification of domain-wall propagation in Co nanowires via Ga<Superscript>+</Superscript> irradiation

The propagation of domain walls in polycrystalline Co nanowires grown by focused-electron-beam-induced deposition is explored. We have found that Ga⁺ irradiation via focused ion beam is a suitable method to modify the propagation field of domain walls in magnetic conduits. Magneto-optical Kerr effect measurements show that global Ga⁺ irradiation of the nanowires increases the domain-wall propagation field. Additionally, we have observed by means of scanning transmission X-ray microscopy that it is possible to produce substantial domain-wall pinning via local Ga⁺ irradiation of a narrow region of the nanowire. In both cases, Ga⁺ doses of the order of 10¹⁶ ions/cm² are required to produce such effects. These results pave the way for the controlled manipulation of domain walls in Co nanowires via Ga⁺ irradiation.     

High resolution measurement and modelling of magnetic domain structures in epitaxial FePd (0 0 1) L10 films with perpendicular magnetisation

Magnetic domain structures in two 50 nm thick chemically-ordered FePd (0 0 1) epitaxial films with different perpendicular anisotropies have been studied using Lorentz microscopy. Domain and domain wall structures vary significantly according to the magnitude of the anisotropy. For lower anisotropy films, a stripe domain structure with a period of ≈100 nm is formed in which there is a near-continuous variation in orientation of the magnetisation vector. By contrast, in the film with higher anisotropy, a maze-like domain structure is supported. The magnetisation within domains is perpendicular to the film plane and adjacent domains are separated by narrow walls, less than 20 nm wide. Micromagnetic modelling is generally in good quantitative agreement with experimental observations and provides additional information on the domain wall structure.     

Surface modification of Ni and Co metal nanowires through MeV high energy ion irradiation

Nickel (Ni) and cobalt (Co) metal nanowires were fabricated by using an electrochemical deposition method based on an anodic alumina oxide (Al₂O₃) nanoporous template. The electrolyte consisted of NiSO₄ · 6H₂O and H₃BO₃ in distilled water for the fabrication of Ni nanowires, and of CoSO₄ · 7H₂O with H₃BO₃ in distilled water for the fabrication of the Co ones. From SEM and TEM images, the diameter and length of both the Ni and Co nanowires were measured to be ∼ 200 nm and 5–10 μm, respectively. We observed the oxidation layers in nanometer scale on the surface of the Ni and Co nanowires through HR–TEM images. The 3 MeV Cl²⁺ ions were irradiated onto the Ni and Co nanowires with a dose of 1 × 10¹⁵ ions/cm². The surface morphologies of the pristine and the 3 MeV Cl²⁺ ion-irradiated Ni and Co nanowires were compared by means of SEM, AFM, and HR–TEM experiments. The atomic concentrations of the pristine and the 3 MeV Cl²⁺ ion-irradiated Ni and Co nanowires were investigated through XPS experiments. From the results of the HR–TEM and XPS experiments, we observed that the oxidation layers on the surface of the Ni and Co nanowires were reduced through 3 MeV Cl²⁺ ion irradiation.     

Vapor–solid–solid growth of Ge nanowires from GeMn solid cluster seeds

We describe the fabrication of Ge nanowires during a single co-deposition step of Ge and Mn at high temperature. In these experimental conditions, a phase separation occurs and two different phases Ge and Ge_1 ? xMn_x are formed with Ge_1 ? xMn_x in the shape of small clusters distributed randomly in the Ge matrix. Because of the high deposition temperature, a new Ge_1 ? xMn_x phase with low eutectic point is stabilized; this phase is different from the one (commonly Ge₃Mn₅) stabilized at lower temperature. During the growth process at 350 °C, the crystalline clusters remain solid but they are highly mobile and can float at the surface, serving as seeds to direct the growth of crystalline Ge nanowires from the vapor. The sketch steps of NWs formation are first the phase separation with formation of specific Ge_1 ? xMn_x critical nuclei with low eutectic point and second the growth of Ge NWs directed by the Ge_1 ? xMn_x solid cluster seeds. Ge NWs growth is forced along particular crystalline axis by the cluster seeds that lower the interfacial energy Ge/Ge_1 ? xMn_x and the energy formation of the germanium crystal stabilizes the cluster position at the tip of the NWs. The density of NWs can be tuned by varying the nominal Mn concentration since this density is related to the number of clusters with the specific Ge_1 ? xMn_x phase (with low eutectic point). The single step MBE process presented here has the main advantage to fully avoid any incorporation of unintentional impurity into Ge nanowires (apart from Mn atoms) and could be applied to several other systems. This work also provides new insights into the vapor–solid–solid growth mechanisms of Ge NWs.     

Self-limited growth of the CaF nanowire on the Si(5 5 12)-2 × 1 template

The atomic structure and interfacial bonding of the ordered-and-isolated CaF nanowires on Si(5 5 12)-2 × 1 have been disclosed by scanning tunneling microscopy and synchrotron photoemission spectroscopy. Initially, CaF molecules dissociated from thermally deposited CaF₂ molecules are adsorbed preferentially on the chain structures of Si(5 5 12)-2 × 1 held at 500 °C. With increasing CaF₂ deposition amount, one-dimensional (1D) CaF nanowires composed of (113) and (111) facets are formed. The line density of these CaF nanowires increases as a function of deposition amount. Finally, at a submonolayer coverage, the surface is saturated with these 1D nanowires except for the (225) subunit, while the original period of Si(5 5 12)-2 × 1, 5.35 nm, is preserved. It has been deduced by the present studies that, owing to these preferential adsorption of CaF and facet-dependent growth of a CaF layer within a unit periodic length of Si(5 5 12)-2 × 1, such a self-limited growth of the CaF nanowire with a high aspect ratio becomes possible.     

Molecular dynamic simulation of the CsCl–Al2O3 interface

The interface between solid cesium chloride and α-aluminum oxide was simulated by molecular dynamics technique. It was shown that due to a misfit between lattices of the components the interfacial contact may be presented as a small-angle boundary saturated with dislocations. Also a domain structure is formed. The dislocations and interdomain boundaries act as a source of defects and give rise to the total ionic mobility along the interface and boundaries. According to the calculation at a temperature of nearly 70% of the melting point, the diffusion coefficients of ions along misfit dislocation cores and domain walls, ∼ 10^– 6 cm²/s, are only an order of magnitude lower than the corresponding values for molten salts.     

Low-temperature study of the magnetization reversal and magnetic anisotropy of Fe,Ni, and Co nanowires

We present an extensive study of the magnetic reversal mechanism of Fe and Ni nanowires with diameters down to 6 nm, i.e. smaller than the domain wall width. The coercive field at 5 K is a factor of 3 lower than the prediction for rotation in unison. We also observe that the activation energy associated with the reversal process is proportional to the cross-section of the wires and nearly independent of the wire length. From the temperature dependence of the coercive field and the magnetic viscosity we can conclude that magnetization reversal takes place via a nucleation of a small magnetic domain, probably at the end of the wire, followed by the movement of the domain wall. For Co wires, we observe a different behavior that is dominated by the competition between the shape anisotropy and the temperature-dependent magnetocrystalline anisotropy.     

Luminescence and physical properties of copper doped CdO derived nanostructures

In this paper, we studied the photoluminescence (PL), the morphological, electrical and optical properties of pure and copper-doped cadmium oxide. CdO films were grown by a facile sol–gel spin coating process at 1200 rpm, and doped with copper at 2 and 3%. A (1 1 1)-oriented cubic structure with a lattice parameter of a=4.69 Å was confirmed by X-ray diffraction. Copper was shown to improve the optical transmittance in the short wavelength range of the visible spectrum. The optical band gap of CdO ranged between 2.49 and 2.62 eV as a result of Cu content. At room temperature, resistance fell drastically with Cu doping levels. AFM analysis of samples exhibited nano-mounts and nanowires. Finally, PL results showed a strong blue–violet emission peak at 2.80 eV.     

Size effects on characteristic magnetic fields of a spin-valve multilayer by computer simulation

The change in characteristic magnetic fields of a spin-valve multilayer is investigated as a function of the size by computer simulation. The spin-valve modeled in this work is IrMn (9 nm)/CoFe (4 nm)/Cu (2.6 nm)/CoFe (2 nm)/NiFe (6 nm). The spin-valve dimensions are varied widely from 20 mm×10 mm to 0.5 μm×0.25 μm, but the aspect ratio defined by the ratio of the length to the width is fixed at 2.0. The magnetostatic interactions begin to affect the magnetic properties substantially at a spin-valve length of 5 μm, and, at a length of 1 μm, they become even more dominant. The main consequences of the magnetostatic interactions are a significant increase of the coercivity and a very large shift of the bias field in both the pinned and free layers. It is shown that these changes can be explained by two separate contributions to the total magnetostatic interactions: the coercivity change by the self-demagnetizing field and the change of the bias field by the interlayer magnetostatic interaction field.