Table of contents

Fermi surface of a half-metal compound with cubic Fm–3m symmetry: Heusler alloy or double perovskite. The inset shows the spin resolved density of state for a CCFA Heusler compound. ( This illustration appears on the cover of the print edition. )

This cluster issue of Journal of Physics D: Applied Physics is devoted to magnetic Heusler alloys. This class of materials is currently considered to contain the most attractive half-metallic ferromagnets due to their high Curie temperatures and their structural relation to conventional semiconductors. The first compound to be identified as a half-metallic ferromagnet, by de Groot et al back in 1983, was the half-Heusler alloy NiMnSb. After a period of rather slow research, magnetic Heusler alloys have received a strong boost of interest in the last few years owing to their potential application in magneto-electronic devices. Due to their half-metallic character, they may have a spin polarization of 100% at the Fermi level. The highest Curie temperature for Heusler alloys is 1100 K, found in the full-Heusler alloy Co₂FeSi.

Although there is no proof of their half-metallicity, Heusler alloys are, up to now, the only half-metallic materials that show a high magnetoresistance effect in tunnel junctions at room temperature. A breakthrough was achieved by using Co₂MnSi (by Reiss's group in Bielefeld and Miyazaki's group in Sendai) and Co₂Cr_0.6Fe_0.4Al (by Inomata in Sendai and Yamamoto in Sapporo) Heusler alloys as electrodes. This has resulted in a pronounced increase in magnetoresistance at room temperature. In this cluster issue, Miyazaki's group report their current world record, a magnetoresistance effect of 159% at 2 K with an Al-oxide tunnel barrier and Co₂MnSi bottom electrode.

The purpose of this cluster issue is to cover the various aspects (theory, sample preparation, characterization, devices) related to Heusler alloys with high-spin polarization. Galanakis et al present in their review article an overview of the basic electronic and magnetic properties of both Heusler families, the half- and full-Heusler compounds. In many Heusler compounds the total magnetic moment follows a simple electron-counting rule based on the Slater–Pauling behaviour.

Kandpal et al demonstrate the relation between half-Heusler compounds with 8 or 18 valence electrons and classical semiconductors like silicon and ZnS. A half-metallic ferromagnet with a half-Heusler structure can be considered as being built up of a zinc blende framework filled with an electropositive magnetic ion.

Besides the high Curie temperature, Co₂FeSi exhibits the highest magnetic moment of 6 μ_B per formula unit. Kallmeyer et al have therefore investigated the magnetic properties of Co₂Mn_1-xFe_xAl Heusler alloys. Their experimental findings indicate that the magnetization determined near the Co₂Mn_1-xFe_xAl/Au interface is smaller than expected from the band structure for a defect-free Heusler structure, while the bulk properties agree well with prediction. It has been known for a long time that surface or interface states or surface segregation might be responsible for the reduced spin polarization measured by surface-sensitive methods like photoemission.

The interface and the surface are the subjects of theoretical study in two papers. The continuing drama of the half-metal–semiconductor interface is addressed in the paper of Attema et al. They propose semiconductors based on transition metals rather than the main group metals as appropriate interfaces for Heusler compounds. The semiconducting nonmagnetic half-Heusler compound NiScSb has only a very small lattice mismatch with NiMnSb and shows a genuine half-metallic interface with NiMnSb for all interfaces of low index. Leciac et alhave predicted the scanning tunnelling microscopy surfaces of NiMnSb(001) using ab initio methods.

The quaternary Heusler compounds Co₂Mn_1-xFe_xAl were investigated theoretically and experimentally by Wurmehl et al. To avoid the problem of surface states, the authors investigated the electronic structure using higher photon energy for greater bulk sensitivity of excitation. The discrepancy between the calculated density of states and the measured density of states by resonant and high-energy photoemission suggests the presence of correlations.

It is well established that well-ordered films are a precondition for large magnetoresistance effects in Heusler compounds. Inomata et al present here their recent results for tunnel junctions of Co₂Mn_1-xFe_xAl and Co₂FeSi with an Al-oxide tunnelling barrier. A maximum TMR (tunnelling magnetoresistance) effect of 52% was achieved at room temperature for less-ordered Co₂Mn_1-xFe_xAl and of 41% for well-ordered Co₂FeSi. The lattice mismatch of Co₂FeSi with the MgO substrate probably accounts for the low effect in the well-ordered samples. Very recently, effects of about 300% at room temperature have been achieved with simple transition metals like Fe by using the spin filter effect of a MgO tunnelling barrier, and Yamamoto et al present here their first very promising results. However, the best electrode material is still well-ordered Co₂MnSi in combination with Al-oxide with 159% at 2 K and 70% at room temperature as reported by Oogane et al in this issue.

X-ray resonant magnetic scattering studies of multilayers of Co₂MnGe have shown experimentally a nonferromagnetic interface layer of about 0.6 nm at the bottom of the Co₂MnGe layer and 0.45 nm at the top. Bergmann et al interpret this as the natural explanation for the small GMR (giant magnetoresistance) amplitude in [Co₂MnGe/V]_n.

In addition to the high-spin polarization, Heusler compounds are also promising candidates for magneto-optics. The article by Picozzi et al focuses on magneto-optical properties using first-principles calculations. Their results may pave the way to magneto-optical material design.

Semiconducting full-Heusler compounds are candidates for heavy fermion behaviour. The review by Ślebarski shows that the disorder in Heusler compounds strongly influences the magnetic and transport properties and can also be responsible for unusual low-temperature behaviour.

Another related topic is magneto-elastic effects in Heusler alloys, on which the review by Entel et al nicely completes the wide field of potential applications of Heusler alloys.

We hope this cluster of papers will help to push forward a better understanding of this very interesting class of materials, which have the potential to revolutionize magnetism-based electronics.

Intermetallic Heusler alloys are amongst the most attractive half-metallic systems due to their high Curie temperatures and their structural similarity to binary semiconductors. In this review we present an overview of the basic electronic and magnetic properties of both Heusler families: the so-called half-Heusler alloys like NiMnSb and the full-Heusler alloys like Co₂MnGe. Ab initio results suggest that both the electronic and magnetic properties in these compounds are intrinsically related to the appearance of the minority-spin gap. The total spin magnetic moment M_t scales linearly with the number of the valence electrons Z_t, such that M_t = Z_t − 24 for the full-Heusler and M_t = Z_t − 18 for the half-Heusler alloys, thus opening the way to engineer new half-metallic alloys with the desired magnetic properties.

Half-Heusler compounds XYZ, also called semi-Heusler compounds, crystallize in the C1_b MgAgAs structure, in the space group . We report a systematic examination of band gaps and the nature (covalent or ionic) of bonding in semiconducting 8- and 18-electron half-Heusler compounds through first-principles density functional calculations. We find that the most appropriate description of these compounds from the viewpoint of electronic structures is one of a YZ zinc blende lattice stuffed by the X ion. Simple valence rules are obeyed for bonding in the 8-electron compound. For example, LiMgN can be written Li⁺ + (MgN)⁻ and (MgN)⁻, which is isoelectronic with (SiSi), forms a zinc blende lattice. The 18-electron compounds can similarly be considered as obeying valence rules. A semiconductor such as TiCoSb can be written Ti⁴⁺ + (CoSb)⁴⁻; the latter unit is isoelectronic and isostructural with zinc-blende GaSb. For both the 8- and the 18-electron compounds, when X is fixed as some electropositive cation, the computed band gap varies approximately as the difference in Pauling electronegativities of Y and Z. What is particularly exciting is that this simple idea of a covalently bonded YZ lattice can also be extended to the very important magnetic half-Heusler phases; we describe these as valence compounds, i.e. possessing a band gap at the Fermi energy albeit only in one spin direction. The local moment in these magnetic compounds resides on the X site.

Co₂Mn_1−xFe_xSi Heusler alloys with Fe concentration x = 0–0.4 as prepared by arc melting show a L2₁ long range order for all Fe concentrations. Magnetic properties of Co₂Mn_1−xFe_xSi Heusler alloys were investigated by magnetometry and circular magnetic dichroism. The magnetization of the Fe doped Heusler alloys is in agreement with the Slater–Pauling values expected for half-metallic ferromagnets. Element specific magnetic moments as determined by x-ray absorption using the total electron yield method are in disagreement with theoretical predictions for x = 0 but approach the predicted values as the Fe concentration increases. Surprisingly small Fe concentration increases the magnetic moments of all constituents considerably.

In this article, based on electronic structure calculations, the conditions are discussed under which a genuine half-metallic interface between a heusler C1_b half-metal and a semiconductor can exist. An explanation is given why for the III–V semiconductors the double anion terminated (111) interface is the only possible interface. For semiconductors, based on transition metals, a much wider variety of interfaces are found to be possible.

We present a first-principles study of the unreconstructed (001) surfaces of the half-metallic ferromagnet NiMnSb. Both terminations (MnSb and Ni) are considered. We find that half-metallicity is lost at the surfaces. After a discussion of the geometric relaxations and the spin-polarized surface band structure, we focus on topography images which are expected to be found with spin-polarized scanning tunnelling microscopy. For the MnSb-terminated surface we find that only the Sb atoms are visible, reflecting a geometric buckling caused by relaxations. For the Ni-terminated surface we find a strong contrast between the images of forward and reverse tip-sample-bias of 0.5 eV, as well as a stripe-like image for reverse bias. We interpret these findings in terms of highly directional surface states which are formed in the spin-down gap region.

Quaternary Heusler alloys Co₂Cr_1−xFe_xAl with varying Cr to Fe ratio x were investigated experimentally and theoretically. The electronic structure and spectroscopic properties were calculated using the full relativistic Korringa–Kohn–Rostocker method with coherent potential approximation to account for the random distribution of Cr and Fe atoms as well as random disorder. Magnetic effects are included using spin dependent potentials in the local spin density approximation.

Magnetic circular dichroism in x-ray absorption was measured at the L_2,3 edges of Co, Fe and Cr of the pure compounds and the x = 0.4 alloy in order to determine element specific magnetic moments. Calculations and measurements show an increase of the magnetic moments with increasing iron content. Resonant (560–800 eV) soft x-ray as well as high resolution–high energy (⩾3.5 keV) hard x-ray photo emission was used to probe the density of the occupied states in Co₂Cr_0.6Fe_0.4Al.

We have investigated the structure and magnetization of Co₂(Cr_1−xFe_x)Al (0 ⩽ x ⩽ 1) and Co₂FeSi full-Heusler alloy films deposited on thermally oxidized Si (SiO₂) and MgO (001) single crystal substrates by ultra-high vacuum sputtering at various temperatures. The films were also post-annealed after deposition at room temperature (RT). Magnetic tunnel junctions with a full-Huesler alloy electrode were fabricated with a stacking structure of Co₂YZ (20 nm)/Al (1.2 nm)-oxide/Co₇₅Fe₂₅ (3 nm)/IrMn (15 nm)/Ta (60 nm) and microfabricated using electron beam lithography and Ar ion etching with a 10² µm² junction area, where Co₂YZ stands for Co₂(Cr_1−xFe_x)Al or Co₂FeSi. The tunnel barriers were formed by the deposition of 1.2 nm Al, followed by plasma oxidization in the chamber. The x-ray diffraction revealed the A2 or B2 structure depending on heat treatment conditions and the substrate, but not L2₁ structure for the Co₂(Cr_1−xFe_x)Al (0 ⩽ x ⩽ 1) films. The L2₁ structure, however, was obtained for the Co₂FeSi films when deposited on a MgO (001) substrate at elevated temperatures above 473 K. The maximum tunnelling magnetoresistance (TMR) was obtained with 52% at RT and 83% at 5 K for a junction using a Co₂(Cr_0.4Fe_0.6)Al electrode. While the junction using a Co₂FeSi electrode with the L2₁ structure exhibited the TMR of 41% at RT and 60% at 5 K, which may be improved by using a buffer layer for reducing the lattice misfit between the Co₂FeSi and MgO (001) substrate.

Cobalt-based full-Heusler alloy thin films have recently attracted much interest as highly desirable ferromagnetic electrodes for spintronic devices because of the half-metallic ferromagnetic nature theoretically predicted for some of these alloys and because of their high Curie temperatures, which are well above room temperature (RT). In this study, Co-based full-Heusler alloy thin films of Co₂Cr_0.6Fe_0.4Al (CCFA) and Co₂MnGe (CMG) were epitaxially grown on MgO-buffered MgO (001) substrates using magnetron sputtering. The films were deposited at RT and subsequently annealed in situ at temperatures ranging from 400 to 600 °C. X-ray pole figure measurements of the CCFA films indicated that these films were epitaxial and crystallized in the B2 structure. X-ray pole figure measurements of the annealed CMG films showed (111) peaks with four-fold symmetry, which provides direct evidence that these films were epitaxial and crystallized in the L2₁ structure. Furthermore, cross-sectional transmission electron microscope images of a fabricated CMG film indicated that it was single-crystalline. The annealed films of CCFA and CMG had sufficiently flat surface morphologies with roughness of about 0.23 nm rms for 100 nm thick CCFA films and 0.26 nm rms for 45 nm thick CMG films. Using these epitaxially grown thin films, we fabricated fully epitaxial magnetic tunnel junctions (MTJs) consisting of a Co-based full-Heusler thin film of either CCFA or CMG as a lower electrode, a MgO tunnel barrier and a Co₅₀Fe₅₀ (CoFe) upper electrode. All layers were successively deposited in an ultrahigh vacuum chamber through the combined use of magnetron sputtering and electron beam evaporation. Reflection high-energy electron diffraction patterns observed in situ for each layer during preparation clearly indicated that all layers grew epitaxially in both the CCFA/MgO/CoFe and CMG/MgO/CoFe MTJ layer structures. The microfabricated epitaxial CCFA/MgO/CoFe MTJs demonstrated relatively high tunnel magnetoresistance (TMR) ratios, for MTJs using a full-Heusler alloy, of 42% at RT and 74% at 55 K. On the other hand, the microfabricated epitaxial CMG/MgO/CoFe MTJs showed strongly temperature-dependent TMR characteristics with typical TMR ratios of 14% at RT and 70% at 7 K. These results confirm the promise of epitaxial MTJs as a key device structure for clarifying and utilizing the potentially high spin-polarization of Co-based full-Heusler alloy thin films.

We fabricated B2-ordered Co₂MnAl and L2₁-ordered Co₂MnSi Heusler alloy films by optimizing various fabrication conditions (substrate, composition of sputtering target, substrate and post-annealing temperature, etc) and applied these films to bottom electrodes of magnetic tunnel junctions (MTJs). We used Al-oxide insulating tunnel barriers for our MTJs and varied oxidation times of Al films to control qualities of the Al-oxide insulating layer and Heusler-alloy/Al-oxide interface. Observed tunnel magnetoresistance (TMR) ratios were extremely sensitive to the structure and surface morphology of the prepared Heusler alloy films. Epitaxially grown Heusler alloy films showed good structural quality, very flat surfaces and enhanced TMR ratios. The behaviour of the TMR ratios towards oxidation time for the preparation of the Al-oxide barriers and the measurement temperature dependence of the TMR ratios were quite different between the MTJs with Co₂MnAl and Co₂MnSi electrodes. The obtained TMR ratio of 83% at 2 K in the MTJ with epitaxially grown B2-ordered Co₂MnAl was large among the MTJs with an amorphous Al-oxide tunnel barrier. This result suggests that B2-ordered Co₂MnAl is a highly spin-polarized material, as predicted by our theoretical calculation. Moreover, we observed a very large TMR ratio of 159% at 2 K in the MTJ with a high-quality epitaxially grown L2₁-ordered Co₂MnSi electrode. This TMR ratio is the highest value to date in MTJs using an amorphous Al-oxide tunnel barrier. Spin-polarization of the Co₂MnSi bottom electrode obtained from Julliere's formula was about 0.89. This value is also the largest achieved to date for a Heusler material and is much larger than those of conventional ferromagnetic materials such as Co–Fe. This large spin-polarization is attributed to a half-metallic band structure, as predicted by theoretical calculations.

We study the structure and magnetism of the ferromagnetic half metallic Heusler compound Co₂MnGe in high-quality [Co₂MnGe/Au]_n- and [Co₂MnGe/V]_n-multilayers by hard x-ray scattering and soft x-ray resonant magnetic scattering. The diffuse hard x-ray scattering reveals that in [Co₂MnGe/Au]_n at the interfaces correlated roughness dominates and interdiffusion is negligible, whereas in [Co₂MnGe/V]_n the roughness is uncorrelated and dominated by interdiffusion. An analysis of energy-dependent scans in the soft x-ray regime at the superlattice Bragg peaks allows a determination of the element-specific magnetic moment density profile within the Co₂MnGe layers. We find that the magnetic moment density profile determined for Co and Mn is definitely different; moreover, it is narrower than the chemical density profile and asymmetric with respect to the growth direction. For [Co₂MnGe/Au]_n at room temperature a non-ferromagnetic interface layer exists with a thickness of about 0.6 nm at the bottom and 0.45 nm at the top of the Co₂MnGe layers; for [Co₂MnGe/V]_n at the bottom and the top the corresponding thicknesses are 0.5 nm and 0.35 nm, respectively.

Accurate first principles calculations were performed to predict the optical and magneto-optical properties of a large set of Heusler compounds. The optical properties are explained in terms of relevant transitions from the underlying band-structure, suggesting that the joint density of states plays a role in shaping the optical spectra. As for the magneto-optical properties, our results show that most of the Heusler compounds have peaks in the Kerr rotations of the order of 0.5°, typical for 3d-based materials. The Kerr spectra are rather similar for all considered Heuslers, consistent with their overall comparable band-structure; the peaks in the Kerr angles were traced back to their optical versus magneto-optical contributions. This careful ab initio analysis is extremely helpful in view of engineering the Kerr rotation for technological purposes.

Recent electrical transport, specific heat and magnetic measurements indicated that the Heusler-type Fe₂TiSn alloy could be a candidate for a 3d heavy-fermion system with a quasi-particle effective mass of ∼40 times the free electron mass. Ab initio electronic structure calculations yield a nonmagnetic ground state with a pseudogap and a very small number of the density of states at the Fermi level. However, the atomic disorder strongly influences the magnetic and transport properties of Fe₂TiSn samples. In contrast to the thermodynamical properties of Fe₂TiSn, the infrared studies do not support the notion that the Kondo interaction plays a dominant role in this alloy and rather support the interband transition across a pseudogap responsible for the mass enhancement. The many-body calculations, however, have shown that the narrow d band resulting from Fe/Ti site exchange (i.e. Fe impurity atoms) can be responsible for the unusual temperature dependences of the physical properties of the Fe₂TiSn alloy. In this review paper we discuss the implications of our findings for the ground-state properties of Fe₂TiSn, in particular with respect to the role of atomic disorder.

We have modelled the phase diagram of magnetic shape memory alloys of the Heusler type by using the phenomenological Ginzburg–Landau theory. When fixing the parameters by realistic values taken from experiment we are able to reproduce most details of, for example, the phase diagram of Ni_2+xMn_1−xGa in the (T, x) plane. We present the results of ab initio calculations of the electronic and phonon properties of several ferromagnetic Heusler alloys, which allow one to characterize the structural changes associated with the martensitic instability leading to the modulated and tetragonal phases. From the ab initio investigations emerges a complex pattern of the interplay of magic valence electron per atom numbers (Hume–Rothery rules for magnetic ternary alloys), Fermi surface nesting and phonon instability. As the main result, we find that the driving force for structural transformations is considerably enhanced by the extremely low lying optical modes of Ni in the Ni-based Heusler alloys, which interfere with the acoustical modes enhancing phonon softening of the TA₂ mode. In contrast, the ferromagnetic Co-based Heusler alloys show no tendency for phonon softening.

The experimental study of InN and In-rich InGaN by a number of structural, optical and electrical methods is reviewed. Recent advances in thin film growth have produced single crystal epitaxial layers of InN which are similar in structural quality to GaN films made under similar conditions and which can have electron concentrations below 1 × 10¹⁸ cm⁻³ and mobilities exceeding 2000 cm² (Vs)⁻¹. Optical absorption, photoluminescence, photo-modulated reflectance and soft x-ray spectroscopy measurements were used to establish that the room temperature band gap of InN is 0.67 ± 0.05 eV. Experimental measurements of the electron effective mass in InN are presented and interpreted in terms of a non-parabolic conduction band caused by the k · p interaction across the narrow gap. Energetic particle irradiation is shown to be an effective method to control the electron concentration, n, in undoped InN. Optical studies of irradiated InN reveal a large Burstein–Moss shift of the absorption edge with increasing n. Fundamental studies of the energy levels of defects in InN and of electron transport are also reviewed. Finally, the current experimental evidence for p-type activity in Mg-doped InN is evaluated.

The equivalent ellipsoid for magnetized bodies of arbitrary shape can be determined by imposing the equality between the demagnetization factors of the two shapes of equal volume. It is shown that the 'commonsense' criterion for mapping two different shapes by imposing the equality of the demagnetization factors for equal aspect ratios often results in large errors. We propose a general method for the rigorous determination of the equivalent ellipsoid. The cases of the exact equivalent ellipsoids for discs, cylinders with elliptical cross section and prisms are worked out and discussed.

Fine particles of lithium ferrite were synthesized by the sol-gel method. By subsequent heat treatment at different temperatures, lithium ferrites of different grain sizes were prepared. A structural characterization of all the samples was conducted by the x-ray diffraction technique. A grain size of around 12 nm was observed for Li_0.5Fe_2.5O₄ obtained through the sol-gel method. Magnetic properties of lithium ferrite nanoparticles with grain size ranging from 12 to 32 nm were studied. Magnetization measurements showed that Li_0.5Fe_2.5O₄ fine particles exhibit a deviation from the predicted magnetic behaviour. The as-prepared sample of lithium ferrite showed a maximum saturation magnetization of 75 emu g⁻¹. Variation of coercivity is attributed to the transition from multi-domain to single domain nature. Dielectric permittivity and ac conductivity of all the samples were evaluated as a function of frequency, temperature and grain size. Variation of permittivity and ac conductivity with frequency reveals that the dispersion is due to the Maxwell–Wagner type interfacial polarization.

As-deposited Fe–C granular films doped with N fabricated using facing-target sputtering at room temperature are amorphous. Annealing at temperatures ranging from 200 °C to 600 °C causes the crystallization of metal granules, the enlargement of the particle size from ∼2 nm to ∼50 nm and the decrease of the N atomic fraction. With the increasing nitrogen partial pressure (P_N), the dominant phases of the particles in 600 °C annealed films develop from N-poor phases to N-rich phases, and the CN matrix still retains the amorphous state after the annealing. Meanwhile, high-temperature annealing turns superparamagnetism of the as-deposited films synthesized at P_N < 0.1 Pa and paramagnetism at P_N ≥ 0.1 Pa into ferromagnetism. The saturation magnetization of 600 °C annealed films decreases with the increasing P_N, and both their saturation magnetization and coercivity drop with the rise of measuring temperature. In addition, the coercivity of the films increases with increasing annealing temperature initially and then declines due to the contribution of phase segregation and interparticle interactions.

Bright blue organic light-emitting diodes (OLEDs) based on 1, 4, 5, 8, N-pentamethylcarbazole (PMC) and on dimer of N-ethylcarbazole (N, N'-diethyl-3, 3'-bicarbazyl) (DEC) as emitting layers or as dopants in a 4, 4'-bis(2, 2'-diphenylvinyl)-1, 1'-biphenyl (DPVBi) matrix are described. Pure blue light with the CIE coordinates (x = 0.153, y = 0.100), electroluminescence efficiency η_EL of 0.4 cd A⁻¹, external quantum efficiency η_ext of 0.6% and luminance L of 236 cd m⁻² (at 60 mA cm⁻²) were obtained with PMC as an emitter and the 2, 9-dimethyl-4, 7-diphenyl-1, 10-phenantroline (BCP) as a hole-blocking material in five-layer emitting devices. The highest efficiencies η_EL of 4.7 cd A⁻¹ and η_ext = 3.3% were obtained with a four-layer structure and a DPVBi DEC-doped active layer (CIE coordinates x = 0.158, y = 0.169, λ_peak = 456 nm). The η_ext value is one the highest reported at this wavelength for blue OLEDs and is related to an internal quantum efficiency up to 20%.

Steady-state self-fields are derived for a relativistic electron beam in a helical wiggler with ion-channel guiding by a self-consistent method. The effect of self-fields on electron dynamics is studied and modified steady-state orbits and their stabilities have been analysed. Variation of electron energy is included in the stability analysis and it is found that γ variation has significant effects on the orbit stability. Results of the orbit stability analysis were found in agreement with the single particle simulation.

Use of fused silica microcapillary tubing for supersonic molecular beam nozzles is reported. Speed ratio measurements were made with nozzles of various diameters and sidewall profiles, demonstrating that these nozzles perform as well as the best conventional nozzles. Commercial microcapillary nozzles therefore offer an effective and inexpensive alternative to conventional metal nozzles.

We have studied the time-dependent development of electric double-layers (ionic sheaths) in saline solutions by simultaneously solving the sodium and chlorine ion continuity equations coupled with Poisson's equation in one dimension. The study of the effects of time-varying electric fields in solution is relevant to the possible health effect of radio-frequency electric fields on cells in the human body and to assessing the potential of using external electric fields to orient proteins for attachment to surfaces for biosensing applications. Our calculations, for applied voltages of 10–175 mV between the electrode and the solution, predict time scales of ∼0.1–110 µs for the formation of double-layers in solutions of concentration between 0.001 and 1.0 M. We develop an empirical equation that can predict the double-layer formation time to within 10% over this wide parameter range. The method has been validated by comparing the solutions obtained, once the program has run to a steady state, with the standard non-linear Poisson–Boltzmann equations. Excellent agreement is found with the Gouy–Chapman solution of the non-linear Poisson–Boltzmann equation. Thus the method is not restricted in accuracy and applicability as is the case for the linear Poisson–Boltzmann equation. The method can also provide solutions for cases where there are orders of magnitude changes in the ion densities; this has not been the case for previous studies where small perturbation analysis has been employed. The method developed here can readily be extended to two and three dimensions using time-splitting methods.

Surface thickness allocations of a polymer film grown in a glow discharge on a flat solid substrate were studied as the total effect produced by several simultaneous discharge processes: surface and volumetric mechanisms of polymerization, heat and mass transfer flows induced by the discharge and gravitational drift of the polymeric microparticles. Some kinds of motion of aerosols entrained by gas flows were visualized using laser probing. Scanning the film thickness in various directions at the substrate surface was carried out by multiple-beam microinterferometry. The results are presented for pure hexamethyldisiloxane and hexamethyldisilazane glow discharges, since these monomers were most capable of generating polymeric particles, though all the other monomers used gave qualitatively identical results. Peculiarities observed in a film relief are interpreted as results of deposition of the polymeric microparticles from corresponding flows of a dispersive gas–polymer mixture.

In this paper the authors discuss the effects of particles (fillers) mixed in a composite polymer on the space charge measurement techniques. The origin of particle-induced spurious signals is determined and silica filled epoxy resin is analysed using the laser-induced-pressure-pulse (LIPP) method, the pulsed-electro-acoustic (PEA) method and the laser-induced-thermal-pulse (LITP) method. A spurious signal identified as the consequence of a piezoelectric effect of some silica particles is visible for all the method. Moreover, space charges are clearly detected at the epoxy/silica interface after a 10 kV mm⁻¹ poling at room temperature for 2 h.

In this article it is shown that high quality single crystalline Ga-doped ZnO (GZO) films could be achieved on ac-plane sapphire using conventional rf magnetron sputtering. High-resolution x-ray diffractometry, transmission electron microscopy (TEM), and scanning electron microscopy investigations clearly confirmed that the GZO films with low Ga doping levels up to 1wt% were of high quality single crystal, which is featured by the (0002) rocking curve as narrow as 0.14°, symmetric six poles in pole figure, sharply defined spot pattern in the TEM diffraction diagram of the interfacial region, and the flat surface. It was also estimated from the Hall measurements and photoluminescence spectroscopy that these single crystalline GZO films possessed good optical and electrical characteristics including the narrow band-width and higher intensity of exciton-related emission peak, Hall mobility as large as 66 cm² V⁻¹ s⁻¹, and the resistivity as low as 1.69 × 10^{− 3} Ω cm.

The structure and chemistry of two electrical trees (designated Tree A and Tree B) grown in low density polyethylene have been studied by a combination of confocal Raman microprobe spectroscopy, optical microscopy and scanning electron microscopy. Despite being grown under similar conditions (A, 30 °C and 13.5 kV; B, 20 °C and 13.5 kV), these two trees exhibit very different structures. Tree A exhibits a branched structure while Tree B is more bush-like. In Tree A, the very tips of the structure are made up of hollow tubules, which exhibit just the Raman signature of polyethylene. On moving towards the high voltage needle electrode, fluorescent decomposition products are first detected which, subsequently, are replaced by disordered graphitic carbon. From the relative intensity of the graphitic sp² G and D Raman bands, the constituent graphitic domains are estimated to be ∼4 nm in size, which leads to a local tree channel resistance per unit length of 1–10 Ω µm⁻¹. These structures are therefore sufficiently conducting to prevent local electrical discharge activity. In Tree B, the observed fluorescence increases continuously from the growth tips to the needle. Here, the tree channels are not sufficiently conducting to prevent electrical discharge activity within the body of the tree. These results are discussed in terms of mechanisms of tree growth and, in particular, the chemical processes involved.

In order to clarify the basic reason why Ce doping can dramatically decrease the leakage current in Ba_0.5Sr_0.5TiO₃ (BST) as reported in our previous work (Wang et al2005 J. Phys. D: Appl. Phys.38 2253), we have employed x-ray photoelectron spectroscopy (XPS) and the optical transmittance technique to study the electronic structure of undoped and 1.0 at% Ce-doped BST (CeBST) films fabricated by pulsed laser deposition. XPS results show that Ce doping has a strong influence on the valence band and core levels of BST films, and that the Fermi level is lowered by about 0.35 eV by Ce doping. Optical transmittance measurements demonstrate that the energy gap is expanded with Ce doping. These Ce-doping effects can induce an increase in the barrier height for the thermionic emission and eventually reduce leakage current in CeBST thin films.

This paper employs linear and nonlinear stability theories to conduct a numerical characterization of thin micropolar film flows travelling down a vertical moving plate. Having applied the long-wave perturbation method to derive the generalized kinematic equations for a free film surface condition, the multiple scales method is used to solve the micropolar film flow in the cases of a stationary vertical plate, a vertical plate moving in the downward direction and a vertical plate moving in the upward direction. The numerical results indicate that both subcritical instability and supercritical stability conditions may occur in the micropolar film flow system. No obvious change in the supercritical stability condition is observed when the plate moves vertically in the upward or downward direction. The numerical results indicate that a downward motion of the vertical plate tends to enhance the stability of the film flow.

A general model of an irreversible fluid flow system is established, in which the linear and nonlinear fluid flow resistances and the friction forces of different types are taken into account. The influence of the piston speed and friction force on the instantaneous power output and efficiency of the system is analysed. The instantaneous power output is maximized. The characteristics of instantaneous power outputs versus efficiency are revealed. The optimum criteria of some important parameters are given. Some typical cases are discussed in detail. The results obtained here are general and can include the important conclusions of the various special cases investigated in the literature.