Table of contents

The requirements for imaging magnetic vortices properly by magnetic force microscopy are discussed. Special attention is devoted to the influence of the tip on the sample magnetic configuration. It is shown that one can obtain a quantitative understanding of the experiments using micromagnetic simulations within a general framework for reversible tip-induced perturbations. Finally, preliminary results obtained with specially prepared 'nanowire tips' are presented.

A new rare-earth Nd-based bulk metallic glass (BMG) Nd₆₀Cu₂₀Ni₁₀Al₁₀ was prepared in the shape of a rod up to 5 mm in diameter by suction casting. In contrast to the previously reported hard magnetic Nd₆₀Fe₂₀Co₁₀Al₁₀ BMG, the present BMG exhibits a distinct glass transition (GT) with low transition temperature, a stable supercooled liquid region, obvious multi-crystallization, near eutectic melting, a high reduced GT temperature and paramagnetic property.

The optical field emitted by radiatively decaying plasmons is studied for the case when the surface supporting the plasma excitations is that of a ferromagnetic metal rather than that of one of the commonly studied noble metals. Comparison is made between the phenomenon observed for plasmons travelling freely on a continuous conducting surface and those localized around nano-scale particles on an insulating substrate or embedded in a dielectric host. In the first case, structure mediated optical excitation on a nickel grating produces plasmons that travel away from their source becoming subject to a variety of decay mechanisms including radiative emission. Similarly, localized plasmons created in a cobalt longitudinal recording medium composed of particles having near mono-disperse magnetic volumes ≈7 × 10⁻²⁵ m³ are likewise observed to re-emit optical radiation in a narrow beam at high non-specular angles consistent with radiative plasmon decay. Moreover, the plasmon mode structure supported by the particles, and which is reflected in the angular structure of the emitted radiation, appears indicative of the physical shape of the particles. The possible significance of plasmon behaviour in the development of ultra-high density recording media is highlighted.

Magnetic transitions and the magnetoresistance effect of the HfFe₆Ge₆-type Y_1−xLa_xMn₆Sn₆ (x = 0.1 and 0.2) compounds have been investigated in the temperature range 5–385 K. The samples display an antiferromagnetic behaviour in the whole magnetic ordering temperature range, and their Néel temperature is 309 K and 323 K for x = 0.1 and 0.2, respectively. The metamagnetic transition from antiferromagnetism to ferromagnetism can be induced by an applied field. The metamagnetic transition field varies from 20 kOe at 5 K to 4 kOe at 300 K. The giant magnetoresistance effect is observed with the metamagnetic behaviour, such as −10.4% at 245 K for x = 0.1 under 50 kOe. Interestingly, the positive magnetoresistance for x = 0.2 appears below 110 K.

Variants of fully spin-polarized electronic structures that may be found in ferromagnetic metals, semimetals and semiconductors are reviewed, with examples. A simple classification scheme is presented, with six broad categories. The attribution of a material to a particular category is based on chemical systematics, or density functional theory calculations. A simplified notation for the electronic structure of these ferromagnets is proposed.

(Some figures in this article are in colour only in the electronic version)

The electro-optic properties of a vertically aligned liquid crystal device with a three-electrode structure for fast response applications have been studied. The fast response is realized by a three-electrode structure that can produce an electric field horizontally and vertically, alternately to turn the liquid crystal cell on and off so that both the turn-on and turn-off processes are driven by the electric field. The response time, transmittance–voltage curve, and the bias effect were characterized for cells with various stripe widths/gaps. A simple theoretical model was used to explain the bias voltage dependence of the transmission with good agreement with the experimental findings. A total response time (rise-time plus fall-time) of 1.88 ms was obtained.

The process of second harmonic generation of an intense short pulse laser in a plasma is resonantly enhanced by the application of a magnetic wiggler. The wiggler of suitable wave number provides necessary momentum to second harmonic photons to make harmonic generation a resonant process. The laser imparts an oscillatory velocity to electrons and exerts a longitudinal ponderomotive force on them at , where ω₁ and are the frequency and the wave number of the laser, respectively. As the electrons acquire oscillatory velocity at the second harmonic, the wiggler magnetic field beats with it to produce a transverse second harmonic current at , driving the second harmonic electromagnetic radiation. However, the group velocity of the second harmonic wave is greater than that of the fundamental wave, hence, the generated pulse slips out of the main laser pulse and its amplitude saturates.

The photo-induced birefringence of three new azo polymer films containing different substitutions was studied. Under a pumping light with the same polarization and intensity, the samples showed different profiles of birefringence as a function of temperature. It was found that the smaller azo molecules possess higher photo-induced birefringence but less stability. These phenomena have been explained in terms of the molecules' thermal motion and structure.

We have investigated the transient electroluminescence (EL) onset of the double-layer light-emitting devices made from poly(N-vinylcarbozole) (PVK) doped with 4-(dicyanomethylene)-2-t-butyl-6(1, 1, 7, 7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) and tris(8-hydroxy-quinoline) aluminium (Alq₃). For the double-layered device in which PVK was doped with 0.1 wt% DCJTB, the EL onset of PVK lags that of DCJTB and Alq₃, while the EL onset of DCJTB and Alq₃ is simultaneous. However, the EL emission of the double-layered device of PVK/Alq₃ originates only from Alq₃. The results show that DCJTB dopants can not only help to tunnel electrons from Alq₃ zone to PVK but can also assist electrons transfer in PVK under high electric field by hopping between DCJTB molecules or from DCJTB to PVK sites at a low doping concentration of 0.1 wt%. When the DCJTB doping concentration is 4.0 wt%, the EL onset of Alq₃ lags that of DCJTB. The difference in the EL onsets of DCJTB, Alq₃ and PVK is attributed to the slow build-up of the internal space charge in the vicinity of the interface between PVK and Alq₃. The electron potential difference of the interface between Alq₃ and PVK doped by DCJTB can be adjusted by changing the DCJTB doping concentration in double-layer devices.

Self-assembled InAs quantum dots (QDs) with differing deposition thicknesses covered by In_xAl_1−xAs (x = 0.2, 0.3) and In_0.2Ga_0.8As combination strain-reducing layers (CSRLs) were grown by molecular beam epitaxy. Their structural and optical properties were investigated by atomic force microscopy and photoluminescence spectroscopy, respectively. The emission peak position of InAs QDs capped by CSRL can reach 1.34 µm at room temperature with a relatively larger energy splitting of 93 meV between the ground and first excited states.

A prototype micro-thermophotovoltaic (micro-TPV) power generator is described in this paper. The system comprises a SiC emitter, a simple nine-layer dielectric filter and a GaSb PV cell array. When the flow rate of hydrogen is 4.20 g h⁻¹ and the H₂/air ratio is 0.9, the micro-TPV system is able to deliver an electrical power output of 0.92 W in a micro-combustor of 0.113 cm³ in volume. The open-circuit electrical voltage and short-circuit current are 2.32 V and 0.52 A, respectively. If we replace the GaSb PV cells with GaInAsSb PV cells, then an electrical power output of 1.45 W can be expected.

The influence of the dielectric barrier on the discharge regime of a uniform atmospheric pressure glow discharge is studied through fast, time-resolved imaging of the discharge optical emission and by a one-dimensional fluid model. The experiments show that the discharge regime can be adjusted over a wide range from a glow-like regime with a pronounced Faraday dark space and positive column to a Townsend-like discharge regime in which those features are absent. The determining factor for the discharge regime is the current limitation through the dielectric. Results of the one-dimensional fluid model confirm this observation. The fluid model also indicates that metastable helium atoms generated during a discharge pulse contribute significantly to the pre-ionization of the gas before the next breakdown through Penning ionization of nitrogen impurities.

A demonstration plant for the thermal destruction of medical waste using dc plasma torches as the energy source has been developed and tested in several set-ups and under different conditions. Three-dimensional CFD modelling of the gaseous phase in the thermal plasma reactor has been carried out to investigate the experimentally observed phenomena, with the objective of improving the process with respect to conversion rate and power consumption per unit weight. Several models for energy release, and additional parameter studies required to approach the real process as closely as possible, will be discussed. Results for velocity, temperature, and residence time distribution are presented and qualitatively compared with images taken from the running process.

Nano powder of aluminium nitride (AlN) is one of the materials necessary for developing solid insulating materials with high thermal conductivities. This paper describes the results of chemical equilibrium composition calculations and the results of experiments in synthesizing AlN nano powder using transferred type arc plasma. In order to increase the AlN content in synthesized powder, ammonia was recommended as a reacting/quenching gas, to be blown into aluminium gas in a temperature range between 2000 and 2400 K. The AlN content increased by more than 99% on passing the synthesized powder through ethanol.

Calorimetric measurements of a refractory (pure and thoriated tungsten) cathode in a high-current (100–500 A) high-pressure (1–5 atm) arc in nitrogen were performed. The measurements confirmed the existence of a 'high-current' mode of cathode spot (CS) operation—the mode we observed by a footprint method in our work (2003 J. Phys. D: Appl. Phys. 36 3007). In this mode, the heat load of a cathode does not depend on the pressure and is directly proportional to the arc current as opposed to the 'low current' mode where it is inversely proportional to the square root of pressure and directly proportional to the square root of current.

The current density at the cathode surface calculated from the measured heat flux to the cathode is approximately half of the current density obtained by a footprint method. Both densities exhibit the same current and pressure dependences.

The radial distribution of the cathode temperature inside the CS is discussed. We hypothesize that the temperature has its maximum at the spot periphery. In the case of the thoriated tungsten cathode, this maximum may be so high that, at this location, the cathode is virtually thorium free.

Ozone production in a negative corona discharge has been studied experimentally at atmospheric pressure in mixtures of N₂O+O₂ at ambient temperature. Ozone formation was found to be dramatically reduced with increase in the content of N₂O in the mixture. The reaction of O(¹D) with nitrous oxide is the most likely process reducing the rate of ozone generation. A considerable decrease in the mean discharge current at a constant voltage was also observed with increasing content of nitrous oxide in the mixture and is attributed to the formation of anions in the discharge. The low values of the calculated mobility of negative charge carriers in the drift region of the discharge is caused by the formation of O⁻ · (N₂O)_n and NO⁻·(N₂O)_n cluster anions in the drift region of the negative corona discharge.

We investigated spatial pulsations in a cylindrical arc plasma column in the electric current range I = 100–200 A and gas flow rate G = 4–8 g s⁻¹. The distribution functions of these pulsations were obtained. On the basis of experimental results we proposed a model for the description of the pulsations. In this model, the axis of the arc O' participates at the same time on two movements: random pulsation and rotation of the column axis. The lateral and radial intensities and radial temperatures were calculated for different rotation radii and standard deviations. For this study, the spectral line of Ar I 415.8 nm was chosen. Experimental and theoretical results are in good agreement.

In reactive sputtering, the introduction of a reactive gas can lead to a hysteresis transition from metal to compounds in both the target and substrate. The hysteresis transition is characterized by a sudden change in partial pressure, sputtering rate, fraction of compound formation, etc. Therefore, stability is an important issue in the process control. In this paper, a mathematical model with variable sticking coefficients based on surface kinetics is used to study the stability of the process. The variable sticking coefficient represents different mechanisms for surface reactions from the Langmuir to precursor type. In order to facilitate the analysis, several nondimensional parameters are identified and used for formulation. Results show that an unsteady system converges to a steady state relatively fast at low inflow rates. With an eigenvalue analysis, the range of positive eigenvalues is consistent with the presence of a hysteresis loop. It is also found that when the chemical reaction on the substrate is moderate, a higher sputter yield of the compound leads to a more stable steady state at lower inflow rates. Regarding the sticking mechanism, for the type of precursors with the parameter k < 1, the compound is easier to form and saturate on the surface due to the higher default sticking coefficient and the lower operating conditions for the hysteresis transition.

This paper details an experimental investigation of the clamping effect associated with thick-film piezoelectric elements printed on a substrate. The clamping effect reduces the measured piezoelectric coefficient, d₃₃, of the film. This reduction is due to the influence of the d₃₁ component in the film when a deformation of the structure occurs, by either the direct or indirect piezoelectric effect. Theoretical analysis shows a reduction in the measured d₃₃ of 62%, i.e. a standard bulk lead zirconate titanate (PZT)-5H sample with a manufacturer specified d₃₃ of 593pC/N would fall to 227.8pC/N. To confirm this effect, the d₃₃ coefficients of five thin bulk PZT-5H samples of 220 µm thickness were measured before and after their attachment to a metallized 96% alumina substrate. The experimental results show a reduction in d₃₃ of 74% from 529pC/N to 139pC/N. The theoretical analysis was then applied to existing University of Southampton thick-film devices. It is estimated that the measured d₃₃ value of 131pC/N of the thick-film devices is the equivalent of an unconstrained d₃₃ of 345pC/N.

Using femtosecond time-resolved optical Kerr technique, the ultrafast third-order nonlinear optical response of two soluble multi-wall carbon nanotubes (MWNTs), MWNToda and MWNTols, was measured at wavelengths of 640 nm and 830 nm, respectively. The experimental results reveal that, the value of the average hyperpolarizability per each carbon atom γ_av can reach to 2.0 × 10⁻³² esu. Moreover, obvious aggregation effect on γ_av was demonstrated.

The electronic and structural properties of thin epitaxial Mn films on Si(111)-(7 × 7) and their silicide reaction are studied by means of low-energy electron diffraction, scanning tunnelling microscopy (STM) and photoemission spectroscopy (PES). The deposition of Mn at room temperature initially results in the growth of islands. The metal–silicon reaction already occurs at this temperature, which is further enhanced by annealing up to 400°C, leading to the formation of manganese silicide and turning islands into nearly closed films at higher coverage. A pseudo-(1 × 1) phase develops for Mn films of up to 1 monolayer (ML) thickness. For films of higher thicknesses of up to 5 ML, a ( )R30° phase is observed. STM images show that then the silicide film is almost closed and exhibits a strain relief network reflecting an incommensurate interface structure. PES reveals that the (1 × 1) phase is semiconducting while the ( )R30° phase is metallic. For both phases, Si 2p core level photoemission data indicate that the surface is probably terminated by Si atoms.

Pb(Zr_0.53Ti_0.47)O₃ (PZT) thin films have been successfully deposited on glass, silicon and ITO coated glass substrates by a 3.3 kJ Mather type dense plasma focus device. The x-ray diffraction spectra of the films deposited on glass substrates kept at a distance of 4.2 cm from the top of the anode with 10, 15 and 25 shots show peaks at 2θ = 31.3° corresponding to the perovskite phase of PZT. Transmission electron microscopy shows the presence of 0.5 nm grains of PZT. The leakage current density is found to be 10⁻⁶ A cm⁻² at a reverse voltage of 1 V, from current density–voltage (J–V) characteristics. The capacitance–voltage (C–V) characteristics show a counter-clockwise hysteresis loop with a memory window of 1.2 V. The ferroelectric characteristic has been confirmed using the polarization–field hysteresis loop. The resistance of the film is about 1 GΩ. The spontaneous polarization, remanent polarization and coercive field values are found to be 20.1 µC cm⁻², 8.6 µC cm⁻² and 79.9 kV cm⁻¹, respectively.

Cubic titanium nitride (TiN) films preferentially oriented to the (200) lattice plane were deposited onto (111) silicon wafers using an ion beam assisted deposition technique with an electron cyclotron resonance ion source for ionizing nitrogen gas and an electron beam evaporator for evaporating Ti metals. The resistivities of the TiN films were inversely proportional to the average size of the crystallites making up the TiN films and decreased with increasing substrate temperature and film thickness. TiN films thicker than 50 nm had resistivities around 30 µ Ω cm, slightly higher than the resistivities of TiN crystals.

An optical method is proposed for the calibration of pulsed magnetic fields using photoluminescence from synthetic diamond. Generally, the pulsed magnetic field profile is reconstructed by measuring the pick-up voltage in a small coil with an effective area that needs to be known accurately. A useful method to calibrate this area is presented using the 1.4040 eV optical transition at the 1.4 eV Ni-related centre in diamond. The field value is calculated from the Zeeman splitting of the optical lines using the g-factor previously characterized by electron spin resonance. Numerous advantages of the method presented are discussed.

In this paper, the authors analysed the solidification phenomenon in the laser supersonic heating technique used for producing metallic particles. A mathematical model was established to predict the velocity, temperature and solidification situation of metallic particles leaving a spray nozzle. The numerical analysis method was used to simulate the flow field structure of shock waves and to proceed with related experiment. In the experiment, a pulsed Nd–YAG laser was used as the heat source on a carbon steel target within the nozzle, and carbon steel particles were ejected by high pressure air. The solidification problem of carbon steel particles with radii of 1–50 µm in the compressible flow field was calculated and compared with experimental results. The result shows that the shock wave flow fields are generated at different entrance pressures (3–7 bar), and there is no significant difference in the radii of carbon steel particles produced by a fixed laser energy; however, in the flow field without the shock wave effect, the cooling effect is less evident in the solidification process.

We present a model for thickness-sensitive spectrally selective paints and use it to optimize their optical properties with respect to the particle size of the pigment. Pigments were chosen from different classes of materials such as metals, low band gap insulators and semiconductors and carbon. Silicone was chosen as the binder and the paint thickness was varied from 1 to 4 µm. Scattering and absorption cross sections were derived from Mie theory for spherical particles, and the particle radii ranged between 10 and 500 nm. The reflectance was derived from a radiative transfer formulation of a four-flux model, assuming a mono-disperse particle ensemble. The integrated values for near-normal solar absorptance and thermal emittance at 100°C were calculated from the total near-normal spectral reflectance in the wavelength range 0.3–30 µm. It was found that all the pigments investigated have an optimal particle radius of about 100 nm in the case of a 1.0 µm thick paint layer and a particle volume fraction of 0.20. The optimal particle size increases slightly for thicker films. It was also found that direct, low band gap semiconductors give the best spectral selectivity. A solar absorptance of 0.91 and a thermal emittance of 0.13 were computed for PbS particles of volume fraction 0.20 in a 2.0 µm thick paint layer on aluminium.

An inverse vibration problem based on the conjugate gradient method is presented for estimating the unknown tip–sample interaction force with an atomic force microscope (AFM) by using the measured cantilever displacement. No prior information on the functional form of the unknown quality is required beforehand. The accuracy of the inverse analysis is examined using simulated exact and inexact measurements of the displacement. Numerical results show that an excellent estimate of the tip–sample interaction force by an AFM can be obtained within 1 s of CPU time with a Pentium III-450 MHz PC.

The structural phase transitions of Sr₂Ta₂O₇ single crystals have been studied by the modulated temperature differential scanning calorimetry (MDSC), Brillouin scattering and dielectric spectroscopy. The specific heat (C_p) was measured over a wide temperature range from −150°C to 25°C and from 100°C to 210°C. The C_p curve showed an anomaly at T_o = 166.7°C, indicating the phase transition Cmcm → P2₁/m. The transition enthalpy ΔH, the transition entropy ΔS and specific heat jump ΔC_p at T_o were estimated to be 0.465 J g⁻¹, 1.01 mJ g⁻¹ K⁻¹ and 9.78 mJ g⁻¹ K⁻¹, respectively. The C_p anomaly associated with the ferroelectric phase transition at T_c = −107°C has not been detected. However, both Brillouin and dielectric data showed the anomalies corresponding to the ferroelectric phase transition from P2₁/m to P2₁.

Propagation of the shock wave generated during pulsed laser ablation in cavities was measured using laser shadowgraph imaging and compared with laser ablation on a flat surface. The cavities were fabricated in fused silica with the same diameter and variable depths to provide aspect ratios (depth/diameter) of 3 and 6. It was found that outside the cavity, after ∼30 ns the radius of the expanding shock wave was proportional to t^2/5, corresponding to a spherical blast wave as predicted by the Sedov–Taylor solution. The calculated pressures and temperatures of the shocked air outside of the cavities were higher than on the flat surface. The incident energy driving the shock wave outside the cavity was smaller than that on the flat surface; the greater the cavity aspect ratio, the smaller the energy supporting the shock wave. Plasma–wall interactions are discussed to explain the sharp decrease of the energy driving the shock wave outside the cavity.

In semi-solid die-casting, a metallic billet is first heated in an induction furnace until it reaches a semi-solid state (partially liquid and partially solid). Then, it is injected into a die and kept there until it is solidified. Subsequently, the die opens, the part is ejected and the cycle starts again. The liquid–solid fraction and its spatial distribution within the billet at the end of the heating phase are of prime importance for the success of the process and the quality of the final product. These parameters are strongly correlated with temperature gradients within the billet and their evolution in time through the heating cycle. There is presently no inspection method that could reasonably be used to control the billet temperature in a production environment. In this work, we investigate the suitability of using infrared thermography to meet the heating requirements. With this technique, it is possible to non-intrusively monitor the temperature distribution on the entire surface of the billet and to obtain information on how thermal energy is dissipated. Moreover, with the combination of surface infrared measurements and inverse heat conduction formalism, it is also possible to recover some information about the temperature distribution inside the billet. Effects of some process operating conditions such as heating power magnitude, power input cycles, location of the billet inside the induction coil, and the coil overhang are considered and discussed.