Table of contents

We have studied how the switching field of permalloy elements with sub-micron widths varies as a function of the orientation of the applied field. Switching fields for individual elements were determined using Lorentz microscopy, the studies being facilitated greatly by the use of a newly designed magnetizing stage. Brief details of the stage are given. For elements with widths ∼400 nm, switching fields increased modestly as the angle the applied field made with the long axis of the element increased. In contrast, as the element width was reduced to 160 nm, little variation with field orientation was observed. The results suggest that the residual domain structures close to the ends of the elements played a major role in determining their switching characteristics, even for the smallest elements investigated here.

Through the magneto-optical Kerr effect, we determined the magnetic phase diagrams of Pt/Co(t)/Pt ultrathin film structures uniformly irradiated by He ions in the (T, F) and (t, F) planes, where F is the ion fluence, T the temperature and t the Co film thickness. Three ferromagnetic states with out-of-plane, in-plane and oblique spin configurations were identified. For a fixed Co thickness, an out-of-plane spin state is favoured at low fluence, while in-plane and oblique states are stabilized at higher fluence. Thus, He ion irradiation controls the magnetic system's gradual evolution from Ising-type to XY-type. We show that a similar evolution also occurs at room temperature when increasing the Co thickness for a given fluence value.

Nanopores with a wide range of aspect ratios were fabricated in an anodized aluminium oxide layer on bulk metallic aluminium. The aspect ratios (L/D) were around 20–1000 (pore length, L, 1000–60 000 nm; pore diameter, D, 50–100 nm). For comparison, nanopores in polymer films were also prepared using heavy-ion-track etched polyimide films (L, 70 000 nm; D, 70 nm; L/D = 1000) and polycarbonate films (L, 30 000 nm; D, 100 nm; L/D = 300). The pore diameter of the anodized aluminium oxide layer was controlled by the anodization voltage, while the pore diameter of the heavy-ion-tracked polyimide and polycarbonate films was controlled by the etching time in a sodium hydroxide alkaline solution. Ni and Co homogeneous single nanowires were fabricated using the electrodeposition and in situ contacted techniques in the nanoporous templates. The Ni and Co nanowires with the largest aspect ratios (L/D = 1000) showed around 2.3% and 1.6% of the typical anisotropic magnetoresistance (AMR), and the effects of aspect ratio on the resistance and AMR were investigated.

Fast and cost-effective precessional switching of magnetization is possible in magnetically hard, perpendicularly magnetized nanostructures. It requires partial breaking of the rotational invariance around the anisotropy axis and implementing a precessional switching. We propose to assist this by submitting the media to an exchange bias field oriented perpendicular to the anisotropy axis. This lowers the reversal field by half the exchange field, provided that the field rise-time is sufficiently short. This also accelerates the reversal events, down to less than 100 ps. Promising applications in magnetic storage are anticipated.

This paper reports results of a study of magnetic and acoustic properties of a ferrofluid, EMG-605. The measurements were performed for three samples of the same ferrofluid differing in concentration. The magnetic susceptibility was measured as a function of an external magnetic field for each sample, which allowed us to determine the magnetization curves and saturation magnetization. The results provide information on the mean magnetic moment and the mean radius of the magnetite grain. In the samples subjected to an external magnetic field, the anisotropy of the ultrasonic wave absorption coefficient was determined. The mechanisms of ultrasonic wave energy dissipation through the translational and rotational degrees of freedom were established for some ferrofluid concentrations.

The 'two-term' approximation (representation of the electron distribution by the first two terms of an expansion in spherical harmonics in velocity space) continues to occupy a central role in the low-temperature plasma physics literature, in spite of the mass of evidence illustrating its inadequacy in the swarm (free diffusion) limit for many molecular gases. Part of the problem lies in the failure of many authors to specify quantitatively what they mean when they say that the two-term approximation is 'acceptable'. Thus for example, an error of 10% in transport coefficients may well be acceptable in many plasma applications, but for analysis of highly accurate swarm experiments to compare with ab initio and beam-derived cross-sections, 0.1% or less is required, making 'multi-term' analysis mandatory. While reconciliation of the swarm and plasma literature along the lines of two different accuracy regimes may thus be possible, we dispute claims that the two-term approximation is generally satisfactory for inversion of swarm experiment data to obtain electron impact cross-sections. The unsatisfactory nature of other assumptions implicit in much of the modern plasma kinetic theory literature is also discussed.

The electron drift velocity, the longitudinal diffusion coefficient, the effective ionization coefficient and the limiting field strength for mixtures of SF₆ with CHF₃ and CF₄ have been measured with a pulsed Townsend technique. The overall density-reduced electric field strength, E/N, could be varied between 60 and 520 Td (1 Td = 10⁻¹⁷ V cm²), while the SF₆ content in the gas mixtures was varied over the range 1–50%. We have found that the electron drift velocity and the density-normalized longitudinal diffusion coefficient vary weakly with the amount of SF₆ in the mixture. In contrast, the effective ionization coefficient shows a strong dependence with the SF₆ content, becoming more electronegative as the amount of SF₆ is increased. The measured limiting field strength for these two mixtures is lower than those for SF₆–N₂. To our knowledge, no previous ionization or electron transport data for these mixtures have been published.

Cross-sections of erosion craters formed by vacuum arc discharges on graphite cathodes are examined using electron microscopy. These reveal erosion craters that are covered with a growth layer forming a continuous film on the arced surface. The film surface is composed of sphere-like structures similar to cauliflower diamond and to the emitted macro-particles (MPs) observed in coating experiments using graphite vacuum arc sources. The film thickness within the cathode erosion craters evolves with the mean values of the MP diameter and mean crater depth for four graphite material morphologies. These two parameters increase with a decrease in arc spot velocity. The growth layer shows a relatively porous columnar structure, with micro-Raman spectroscopy indicating an evolution from amorphous carbon (cathode base), to graphitic (middle), to diamond dominating the top layer.

In order to quantitatively describe the electrical working principles of dielectric barrier discharges (DBDs), a dynamic electrical model for homogeneous DBDs has been put forward that is composed of a new equivalent circuit for homogeneous DBDs and the equations derived from it. This model is a global and self-consistent model, valid for an arbitrary external excitation voltage. This model reveals instantaneous relations of internal electrical quantities in the gap (gap voltage, internal discharge current and internal power consumption process) to external electrical quantities (external voltage and external total current) and provides the theoretical fundamentals to calculate the temporal development processes of all internal electrical quantities in the discharge gap from the measured external voltage and external total current. The knowledge obtained of dynamic processes of DBDs in the discharge gap explains quantitatively the mechanisms that result in ignition, development and extinction of DBDs and provide physical interpretation of the measured external total current and other phenomena such as memory effect and multiple current pulses in one half period. In this model, several current terms (external total current, external displacement current, external discharge current, internal discharge current and internal displacement current) are introduced to distinguish the different currents involved in DBDs. Moreover, the equations for charge and energy deposition by one discharge and in one half period are derived. Applications of this model to studying a bipolar sine wave excited DBD and a unipolar pulse excited DBD are also included. This model has been proved to be a useful tool to understand DBDs better.

Investigation of defects created in optical grade fused silica due to 200 MeV silver ion irradiation is reported. Paramagnetically positively charged oxygen vacancies or neutral dangling Si bonds (E' centres), non-bridging oxygen hole centres (NBOHC) and non-paramagnetic defects like B₂ bands are observed. The fluence dependent optical and paramagnetic behaviours of these defects are studied using UV–visible absorption spectroscopy, photoluminescence spectroscopy, infrared (IR) absorption and electron paramagnetic resonance. It is observed that generation of E' centres, NBOHC and B₂ bands gets saturated beyond a fluence of 1 × 10¹² ions cm⁻². IR spectra showing saturation in transmission at this fluence also support this observation. At this fluence samples get fully covered with latent tracks containing these defects.

A similarity parameter for quasi-steady fluid flows advancing into horizontal capillary channels is presented. This parameter can be interpreted as the ratio of the average fluid velocity in the capillary channel to a characteristic velocity of quasi-steady capillary flows. It allows collapsing a large data set of previously published and recent measurements spanning five orders of magnitude in the fluid velocity, 14 different fluids, and four different geometries onto a single curve and indicates the existence of a universal prescription for such flows. On timescales longer than the characteristic time it takes for the flow to become quasi-steady, the one-dimensional momentum equation leads to a non-dimensional relationship between the similarity parameter and the penetration depth that agrees well with most measurements. Departures from that prescription can be attributed to effects that are not accounted for in the one-dimensional theory.

Pyroelectric properties of phosphoric acid (H₃PO₄)-doped triglycine sulfate (TGSP) single crystals grown from solutions containing 0.1–0.5 mol of H₃PO₄ have been studied. Incorporation of H₃PO₄ into the crystal lattice is found to induce an internal bias field (E_b) and is observed through the presence of a sustained polarization and pyroelectricity beyond the transition temperature. The internal bias field has been estimated theoretically by fitting the experimentally measured data on temperature dependence of the pyroelectric coefficient (λ), dielectric constant (ε') and polarization (P). A high E_b value in the range 9 × 10³–15.5 × 10⁴ V m⁻¹ is obtained for crystals grown with 0.1–0.5 mol of H₃PO₄ in the solution, and a specific concentration of 0.2–0.25 mol of H₃PO₄ in the solution during crystal growth is found to be optimum for a high figure of merit for detectivity, F_d = 428 µC m⁻² K⁻¹.

The absorption of a liquid into a rectangular bar of an initially dry porous material that is sealed on all surfaces except the inflow face is analysed in terms of Sharp Front theory. Sharp Front models are developed for both complete and incomplete displacement of air ahead of the advancing wetting front. Experiments are described from which a characteristic capillary potential of the material is obtained by measuring the equilibrium pressure of the air displaced and compressed ahead of the advancing wetting front. Results for the absorption of water and n-heptane by a fired clay brick ceramic suggest that this wetting front capillary pressure (or capillary potential) scales approximately with the surface tension and also that the permeability scales inversely with the liquid viscosity. The pressure of the air trapped in the wetted region is found to be the same as the pressure of the displaced air. For this material the wetting front capillary pressure for water at 20°C is 0.113 MPa, equivalent to a hydraulic tension head of 11.5 m and to a Young–Laplace pore diameter of 2.6 µm. The capillary pressure so measured is apparently a fundamental percolation property of the material that can be interpreted as the air pressure at which liquid phase continuity and unsaturated conductivity both vanish. The method described can be applied generally to porous materials.

A dual direct current and radio frequency (DC-RF) plasma system was used to deposit hydrogenated diamond-like carbon (DLC) films from methane plasma. It has the advantages of separately controlling ion density and ion energy by RF power and DC bias, respectively, over conventional simply capacitive-coupled radio frequency plasma enhanced chemical vapour deposition system. Thus, the hydrogenated DLC films were obtained at different RF powers and DC biases, using CH₄ plus Ar as the feedstock. The effects of RF power and DC bias on the structure and properties of the films were investigated by means of Fourier transformation infrared spectroscopy, Raman spectroscopy, x-ray photoelectron spectroscopy, and nano-indentation. The results were as follows: the sp³ content, hardness, and Young's modulus of the DLC films increased with increasing RF power at a constant DC bias of −200 V and reached the maximum values at an RF power of 300 W, after which they decreased with further increase of the RF power. The DC bias had a similar but greater effect on the structure and properties of the films, owing to a greater influence of the ion energy on the characteristics of the films than the ion current density. The film deposited at an RF power of 300 W and DC bias of −200 V has the most diamond-like characteristics with maximum hardness, Young's modulus, and sp³ content. Since both the ion current density and ion energy greatly affect the structure and characteristics of the DLC films, it is imperative to select proper processing parameters to obtain high quality DLC films.

The introduction of cable joints and optical repeaters into submarine telecommunication cable inevitably results in amalgamation regions in the polyethylene insulation. These amalgamation regions are known to increase the probability of high voltage breakdown, when compared with the bulk material. This increased risk has been attributed to the accumulation of space charge in the amalgamation regions. Measurement of the space charge accumulation (using the pulsed electro-acoustic technique), and calculation of the resultant electric field distributions of both press moulded LDPE and injection moulded LDPE containing an amalgamation zone were conducted. The space charge dynamics are discussed, and the roles of interfacial stress and Schottky injection on the space charge accumulation are detailed.

The introduction of cable joints and optical repeaters into submarine telecommunication cable inevitably results in amalgamation regions in the polyethylene insulation. These amalgamation regions are known to increase the probability of high voltage breakdown, when compared with the bulk material. This increased risk has been attributed to the accumulation of space charge in the amalgamation regions. Progressive dc testing on press moulded, and amalgamated injection moulded samples, which were subjected to varying amounts of electrical ageing were conducted. The morphology of the two sample types was investigated, and the variation in breakdown distributions and characteristic values over time are discussed with reference to the time varying space charge distribution.

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