Table of contents

Owing to their reduced dimensions, the magnetic properties of ultrathin magnetic films and multilayers, e.g. magnetic anisotropies and exchange coupling, often depend strongly on the surface and interface structure. In addition, chemical composition, crystallinity, grain sizes and their distribution govern the magnetic behaviour. All these structural properties can be modified by light-ion irradiation in an energy range of 5–150 keV due to the energy loss of the ions in the solid along their trajectory. Consequently the magnetic properties can be tailored by ion irradiation. Similar effects can also be observed using Ga⁺ ion irradiation, which is the common ion source in focused ion beam lithography.

Examples of ion-induced modifications of magnetic anisotropies and exchange coupling are presented. This review is limited to radiation-induced structural changes giving rise to a modification of magnetic parameters. Ion implantation is discussed only in special cases.

Due to the local nature of the interaction, magnetic patterning without affecting the surface topography becomes feasible, which may be of interest in applications. The main patterning technique is homogeneous ion irradiation through masks. Focused ion beam and ion projection lithography are usually only relevant for larger ion masses. The creation of magnetic feature sizes below 50 nm is shown. In contrast to topographic nanostructures the surrounding area of these nanostructures can be left ferromagnetic, leading to new phenomena at their mutual interface.

Most of the material systems discussed here are important for technological applications. The main areas are magnetic data storage applications, such as hard magnetic media with a large perpendicular magnetic anisotropy or patterned media with an improved signal to noise ratio and magnetic sensor elements. It will be shown that light-ion irradiation has many advantages in the design of new material properties and in the fabrication technology of actual devices.

Planar waveguides have been formed by the electric field assisted diffusion of copper ions from a metallic copper film into a borosilicate glass (Corning 7740). The waveguides have been analysed using conventional prism coupling techniques and have been compared with the electric field assisted diffusion of silver into the same glass type. The measured refractive index profiles for copper guides are shown to have nearly Fermi function shapes, which are consistent with the theory of field assisted ion diffusion in glass. It is shown, however, that the guide depth formed by copper or silver ions is larger than that predicted by calculations based on the nominal composition, possibly indicating that only a fraction of the sodium ions within the glass are mobile. Annealing experiments have been performed on the guides in order to determine the copper self-diffusion coefficient in the glass. The coefficients are extracted from the data by fitting a non-linear diffusion equation model of ion motion in glass to the experimental profiles.

We examine the question of whether the excited states in argon contribute significantly to ionization in a capacitively coupled plasma through metastable pooling and step-ionization processes. We look at this issue by means of a one-dimensional particle-in-cell (PIC) code, with collisions treated by a Monte Carlo collision package. In the range explored, 50–1000 mTorr, the main source of ionization, in the absence of secondary emission, is direct ionization from the ground state with a contribution from excited states that is negligible at lower pressures, but increases in importance at higher pressures. When secondary electrons are included, their interaction with ground state neutrals dominates the ionization. At higher pressures the metastable profiles can reveal useful information on the non-uniform mean electron energy across the discharge, even though these states do not necessarily play a significant role in ionization.

In order to develop charging-free plasma processing, we investigated the function of negative charge injection to a SiO₂ wafer, during etching in a pulsed two-frequency capacitively coupled plasma under various external conditions in CF₄/Ar at low pressures, by using emission-selected computerized tomography. The formation of a negatively-charged layer by an excess negative charge in the interface close to the wafer and the degree of acceleration of negative charges were experimentally estimated through the spatiotemporal characteristics of the net excitation rate of the short-lived Ar(2p₁) used as a probe. A bias pulse operation with a single positive component at the wafer during the off-phase of very high frequency (100 MHz) plasma production resulted in the most efficient formation of the negatively-charged layer close to the wafer. The influence of the external plasma conditions on the negatively-charged layer is observed in detail.

This paper is devoted to a study of the effect of the non-equilibrium parameter θ = T_e/T_h on the plasma composition and thermodynamic and transport properties of ternary mixtures Ar–H₂–He, which are commonly used in dc plasma spraying. Calculations are performed in the temperature range 300–30 000 K and at atmospheric pressure.

First, to calculate the non-equilibrium composition, two techniques are used: the equilibrium constant (van de Sanden) and pseudo-kinetic methods.

Second, the specific heat at constant pressure is obtained by a five-point numerical differentiation of the specific mass enthalpy.

Third, the most recent data of potential interactions or elastic differential cross sections are carefully examined in order to choose the most appropriate ones to determine the collision integrals.

Finally a convergence study of transport coefficients (thermal conductivity, viscosity and electrical conductivity) is conducted to evaluate the influence of the non-equilibrium parameter θ on this convergence. It has to be pointed out that the latter has a non-negligible influence on transport coefficients.

Deformation in shear and associated tribological behaviours of ultra-thin lubricants are of significant importance for the lubrication of magnetic hard disks and for other applications such as micro-electromechanical systems, nano-fluidics and nanotechnology. This paper presents the characteristics of the perfluoropolyether ultra-thin lubricant, in terms of its surface profiles when subjected to a contact sliding test. The results indicate that for a several-monolayers thick (∼4.0–4.5 nm) lubricant film, sliding produces a considerable amount of surface roughness due to peaks of lubricant that persist during sliding; however, it can flow back or return to a smooth profile after a lapse of time when the sliding is stopped. For a monolayer-thin (∼1.4–1.57 nm) film, the lubricant flow is restricted, and the rough profile created due to sliding persists and almost becomes permanent on the wear track. During sliding, due to high shear stress, a characteristic feature of lubricant profile modulation is observed. This modulation, or waviness, is due to the accumulation of lubricant in piles or islands, giving certain amplitudes and frequencies, which themselves depend upon the percentage of lubricant molecules that are chemically bonded to the substrate and the lubricant thickness. The results indicate that ultra-thin lubricants (monolayer and thicker) behave more like a semi-solid (having some sliding characteristics similar to those of rubbers) than a liquid when subjected to a high shear rate during contact sliding.

Ba_1−xSr_xTiO₃ (BST) thin films with compositional bilayer structure are fabricated using rf magnetron sputtering by controlling the deposition conditions. A highly (100) preferred orientation is developed, and enhanced tunability is obtained with the introduction of the bilayer structure. The orientation of the films and the interface between the two layers are both responsible for the improvement in the performance. It is also demonstrated that the in situ annealing before the deposition of the upper layer plays an important role in determining the tunable properties. A long post-annealing could destroy the bilayer structure, and hence degrade the enhanced dielectric characteristics and tuning properties of the films.

Non-thermal plasmas under atmospheric pressure are of great interest in material surface processing because of their convenience, effectiveness and low cost. In this paper, the treatment of a glass surface for improving hydrophobicity using a non-thermal plasma generated by a dielectric barrier corona discharge (DBCD) with a needle array-to-plane electrode arrangement in atmospheric air is conducted, and the surface properties of the glass before and after the DBCD treatment are studied using contact angle measurement, surface resistance measurement and the wet flashover voltage test. The effects of the plasma dose (the product of average discharge power and treatment time) of DBCD on the surface modification are studied, and the mechanism of interaction between the plasma and glass surface is discussed. It is found that a layer of hydrophobic coating is formed on the glass surface through DBCD treatment, and the improvement of hydrophobicity depends on the plasma dose of the DBCD. It seems that there is an optimum plasma dose for the surface treatment. The test results of thermal ageing and chemical ageing show that the hydrophobic layer has quite stable characteristics.

Detailed Raman studies were conducted for the structural characterization of Cu_xGa_ySe₂ films grown using physical vapour deposition. Cu_xGa_ySe₂ gradient composition samples were mapped using Raman spectroscopy in combination with photoluminescence (PL) and energy dispersive x-ray spectroscopy. The Raman spectra recorded under excitation with near band gap laser light have a high signal-to-noise ratio and can be used to study the film structure. In particular, composition dependent changes result in significant changes in the A₁-mode frequency at 187 cm⁻¹, the line width of the E₁-mode at 277 cm⁻¹, and the normalized intensities of the E₁-mode at 277 cm⁻¹ and the B₂-mode at 199 cm⁻¹. It is shown that the intensity increase in Raman modes, with the [Ga]-content increasing in the range 0.9 < [Cu]/[Ga] < 1, is correlated to an increase in defect concentration and lowering of film crystallinity. Moreover, it is shown that the [Cu]/[Ga] fraction of Ga-rich films, used in solar cell device applications, can be calibrated with respect to the band width of the E₁-mode.

Sn doped ZnO (SZO) nanowires were fabricated by a vapour–liquid–solid growth process. The reaction temperature for the formation of the nanowires can be reduced to ∼100°C due to Sn doping. The growth direction and morphology of SZO nanowires depend on the amount of Sn, which is attributed to the difference in sizes between Zn and Sn atoms. The ultraviolet emission of SZO nanowires varies from 380 to 396 nm since Sn acts as a doubly ionized donor and introduces deep states in the band gap. In addition, the SZO nanowires exhibit significantly improved field emission characteristics with a turn-on electric field of 0.05 V µm⁻¹ under a current density of 0.5 mA cm⁻² in comparison with undoped ZnO nanowires. The work function of the SZO nanowire decreases for higher carrier concentrations and the field enhancement factor increases for smaller diameters. Also, the resistance of the SZO nanowire is decreased for the higher Sn mole fraction. Therefore, it is expected that SZO nanowires can be applied in nano-lasers and flat panel displays in the future.

A study of a cylindrical Cerenkov detector, with the aim of investigating a possible application of such a detector for distinguishing between pions and kaons, in the range of a few GeV/c, has been carried out. The detector design could, for example, be used for the experimental B-physics study with the upgraded collider detectors at the Fermilab Tevatron. Simulation methods were used to find the response of several configurations of such a detector as a function of the incident pion and kaon energies in the energy range of a few giga electron volts. It is shown that a straightforward configuration based on conventional materials could work.

We present details of a non-contacting technique that uses ultrasound to detect reliably surface defects with depths that are significantly smaller than the average ultrasonic wavelength to many times this depth. Two electro-magnetic acoustic transducers (EMATs) are used to generate and detect a broadband Rayleigh wave signal with a central frequency close to 200 kHz and extending to approximately 400 kHz. These transducers are moved with a fixed separation between them along a sample towards a defect. Enhancement of the ultrasonic signal is observed when the detector is very close to the defect, with the enhanced signal about 1.6 times the signal away from the defect. A similar enhancement is observed when using an EMAT for generation and a laser interferometer to measure the absolute surface displacement of the sample. We attribute the enhancement of the signal to straightforward interference between the ultrasonic signal that passes directly from the generating coil to the detector and that reflected from the defect. The enhancement can be used to pinpoint accurately a surface or subsurface defect, with the wide-band nature of the signal allowing us to gauge the depth of a wide range of surface-breaking defects.

Two procedures for deriving the optimal steady state operation of heat exchangers are presented. The first procedure, which is based on calculus of variations, can be used in case the optimization criterion is minimum entropy generation. The second procedure can be used in principle for any optimization criterion. Here the minimum entropy generation and the minimum lost available work are envisaged as examples. Both procedures show that, whatever the optimization criterion, parallel flow heat exchangers cannot be optimal but counter-flow devices can be. The empirical wisdom embodied in conventional counter-flow heat exchangers is examined. The usual 'equilibrated' operation is in many cases rather close to the optimum.

Dynamic neutron radiography was applied to obtain the experimental data concerning the kinetics of the wetting process in two different types (fired-clay and siliceous brick) of porous construction materials. The technique provides accurate experimental data concerning the first instants as well as the advanced stages of the imbibing process. A significant difference in water absorption parameters, contrasting the materials studied, is found. Both the advance of the wetting front and the time evolution of water content distribution within the sample are analysed. It is shown that for both materials the time dependence of the wetting front position d deviates from the classical square root of time t behaviour. The deviation is analysed in terms of the intercept b in the approximation and the exponent n in the power law d ∼ t^n/2. Both materials differ in the sign of the intercept parameter: b > 0 for siliceous brick and b < 0 for fired-clay brick. Two stages of the process with different values of the power law exponent were observed. The advanced period exponents are different for each type of brick: n < 1 for siliceous brick and n > 1 for fired-clay brick. At the initial stages (t < 1000 s) the exponents are substantially less than 1 and smaller than those describing the advanced process. The initial stage exponents are lower for the siliceous brick samples. It is shown that the experimental water content profiles collapse to a single curve when plotted against the variable xt^−n/2. The evolution of the distribution of liquid in the porous medium was analysed in terms of the anomalous diffusion equation of Küntz and Lavallée (2001 Experimental evidence and theoretical analysis of anomalous diffusion during water infiltration in porous building materials J. Phys. D: Appl. Phys.34 2547–54) based on the non-linear generalization of the Fick's law. This approach makes a detailed analysis of the generalized hydraulic diffusivity in the low water content region possible. Successful fitting of experimental data with the diffusivity vanishing at null water content proved the importance of the hypodiffusive component of the process.

The reflection of x-rays from a semi-infinite water target, for energies ranging from 10 to 60 keV, often used in x-ray diagnostics, is investigated by Monte Carlo simulation. The same process was also treated analytically as the classical albedo problem for isotropic scattering without energy loss. Good agreement of results for the angular distribution of reflected photons of the two approaches is obtained for higher photon energies from the energy range considered. Multiple collision scattering dominates at higher energies, leading to isotropization of the photon distribution. Discrepancies between the isotropic scattering model and Monte Carlo results appear at the lower part of the energy range. Monte Carlo results show that at these energies photon reflection is governed mainly by single collisions and that these discrepancies are caused by anisotropy of the distribution of single backscattered photons. It is shown that the inclusion of the anisotropy of single Compton backscattering in the analytical model greatly improves the agreement with Monte Carlo results.

The experimental results of a study of the tree-inception phenomenon for three-layer dielectrics in a divergent field are presented in this paper. It is shown that the tree-inception time depends on both the position of the high-permittivity barrier in the insulating gap and the ratio of the permittivities of the barrier material and main dielectric, and that it has a maximum at the optimal barrier position. It is found that the tree-inception length has a minimum value at this barrier position. Good agreement between the coefficient of the local field non-uniformity and the tree-inception time or the initial tree length was found.

Many microdevices for manipulating particles and cells use electric fields to produce a motive force on the particles. The movement of particles in non-uniform electric fields is called dielectrophoresis, and the usual method of applying this effect is to pass the particle suspension over a microelectrode structure. If the suspension has a noticeable conductivity, one important side effect is that the electric field drives a substantial conduction current through the fluid, causing localized Joule-heating. The resulting thermal gradient produces local conductivity and permittivity changes in the fluid. Dielectrophoretic forces acting upon these pockets of fluid will then produce motion of both the fluid and the particles.

This paper presents a numerical solution of the electrical force and the resulting electrothermal driven fluid flow on a travelling wave structure. This common electrode geometry consists of interdigitated electrodes laid down in a long array, with the phase of the applied potential shifted by 90° on each subsequent electrode. The resulting travelling electric field was simulated and the thermal field and electrical body force on the fluid calculated, for devices constructed from two typical materials: silicon and glass. The electrothermal fluid flow in the electrolyte over the electrode array was then numerically simulated.

The model predicts that the thermal field depends on the conductivity and applied voltage, but more importantly on the geometry of the system and the material used in the construction of the device. The velocity of the fluid flow depends critically on the same parameters, with slight differences in the thermal field for glass and silicon leading to diametrically opposite flow direction with respect to the travelling field for the two materials. In addition, the imposition of slight external temperature gradients is shown to have a large effect on the fluid flow in the device, under certain conditions leading to a reversal of the fluid flow direction.

A composite layer with an electrorheological (ER) fluid sandwiched between two flexible electrodes of conducting-glue-painted plastic film is devised and its sound transmission properties are investigated. Within 80–150 Hz the transmitted sound pressure level (SPL) dramatically increases with the applied electric field and the plot of SPL versus frequency exhibits a resonant peak within 80–150 Hz at each electric-field strength. It is also found that the phase angle of the transmitted sound varies manifestly with the electric field. With an increase in starch volume fraction, the responding frequency band in which the SPL can be electrically tuned, moves in the direction of high frequency, and the maximal increment of SPL also increases. It is thought that the variations in the layer vibration and the sound radiation from the layer surface due to the complex electric-field-induced viscoelasticity in the ER fluid are responsible for the observed results. The proposed thin ER layer is thought to be useful in designing tunable phononic crystals and other acoustic devices.

A samarium (Sm)-based superconducting melt-processed single-domain bulk magnet was applied to weakly ionized neon plasmas. The magnet, of diameter 39 mm, thickness 17 mm and through-hole diameter 11.8 mm at the centre, yielded a maximum field of ∼0.75 T in the hole bore at 77 K, with a configuration of magnetic mirrors. The dc plasma, which was generated in a glass tube at ∼ 0.2 Torr and magnetized by the magnetic mirror, was more than six times as dense as the unmagnetized plasma at identical discharge currents. The magnet will provide a clue to developing new compact effective plasma/charged particle beam devices.

Positron emission tomography (PET) is a powerful diagnostic/imaging technique requiring the production of the short-lived positron emitting isotopes ¹¹C, ¹³N, ¹⁵O and ¹⁸F by proton irradiation of natural/enriched targets using cyclotrons. The development of PET has been hampered due to the size and shielding requirements of nuclear installations. Recent results show that when an intense laser beam interacts with solid targets, megaelectronvolt (MeV) protons capable of producing PET isotopes are generated. This report describes how to generate intense PET sources of ¹¹C and ¹⁸F using a petawatt laser beam. The work describing the laser production of ¹⁸F through a (p,n) ¹⁸O reaction, and the subsequent synthesis of 2-[¹⁸F] is reported for the first time. The potential for developing compact laser technology for this purpose is discussed.