Table of contents

The magnetic properties of Ba₆Mn₂₄O₄₈ nanoribbons and bulk are studied by zero-field-cooled and field-cooled magnetization, and ac susceptibility. Upon cooling, we find that both nanoribbons and bulk experience multiple magnetic phase transitions, i.e. paramagnetic, antiferromagnetic and re-entrant spin glass (RSG) phases at a low temperature. The main focus of this work is to understand the origin of RSG behaviour and to investigate the surface effect on magnetism in Ba₆Mn₂₄O₄₈ nanoribbons. The dc susceptibility measurement shows that the paramagnetism in these nanoribbons involves the Curie–Weiss paramagnetism and temperature-independent contributions to magnetic susceptibility due to Pauli and Van Vleck paramagnetism. The Pauli paramagnetic contribution to susceptibility might give rise to uncompensated spins due to the breaking of sublattice pairing on the surface of nanoribbons. An exchange bias phenomenon is found in both Ba₆Mn₂₄O₄₈ nanoribbons and bulk. The exchange field (H_E) in the nanoribbons is larger than that of the corresponding bulk, which may also be induced by the uncompensated surface spins. The resistivity of Ba₆Mn₂₄O₄₈ bulk presents the relation ln(ρ) ∝ T^−1/2, and the possible conduction mechanism is suggested.

FePt dot arrays with dot sizes ranging from 100 down to 15 nm were fabricated using sputtering, annealing and patterning techniques. The dot diameter distribution and dot position deviation are greater for smaller dot arrays than for larger dot arrays. The dot arrays produced through a sequence of annealing followed by patterning have a large perpendicular magnetic anisotropy resulting from the highly L1₀-ordered structure and the perpendicular orientation of the [0 0 1] crystalline axis, whereas samples of annealing after patterning display a magnetic vortex structure. As the dot size reduces from 100 to 29.9 nm, the arrays produced by annealing and then patterning exhibit an increase in the remanent coercivity H_cr from 15.7 to 21 kOe because fewer defects are contained by the smaller dots. This result is explained by nucleation and domain wall propagation mechanisms. For 15.4 nm dot arrays, a model of dot with damaged edge may interpret the decreasing coercivity behaviour.

Composite BaTiO₃–CoFe₂O₄ nanostructures deposited on a SrTiO₃ substrate are known to produce the magnetoelectric coupling effect. Although the coupling efficiency is significantly influenced by the composite layout, there appears no systematic simulation approach for configuring optimal layouts. In this study, a formulation to find optimal heterostructures using the topology optimization method is developed. For the formulation, the macroscopic extrinsic magnetoelectric coupling factor is maximized while numerical calculation is performed by finite element analysis. The proposed method yields an optimal piezoelectric and piezomagnetic material distribution. Numerical simulations are used to explain why the optimized distribution indeed maximizes the magnetoelectric coupling effect. As an application of the developed method, a magnetic read head sensor using the magnetoelectric effect is also designed.

We investigate the influence of nanoparticle height on light trapping in thin-film solar cells covered with metal nanoparticles. We show that in taller nanoparticles the scattering cross-section is enhanced by resonant excitation of plasmonic standing waves. Tall nanoparticles have higher coupling efficiency when placed on the illuminated surface of the cell than on the rear of the cell due to their forward scattering nature. One of the major factors affecting the coupling efficiency of these particles is the phase shift of surface plasmon polaritons propagating along the nanoparticle due to reflection from the Ag/Si or Ag/air interface. The high scattering cross-sections of tall nanoparticles on the illuminated surface of the cell could be exploited for efficient light trapping by modifying the coupling efficiency of nanoparticles by engineering this phase shift. We demonstrate that the path length enhancement (with a nanoparticle of height 500 nm) at an incident wavelength of 700 nm can be increased from ∼6 to ∼16 by modifying the phase shift at the Ag/air interface by coating the surface of the nanoparticle with a layer of Si.

We present optimized design of cylindrical invisibility cloak with minimum layers of non-magnetic isotropic materials. Through an optimization procedure based on genetic algorithm, simpler cloak structure and more realizable material parameters can be achieved with better cloak performance than that of an ideal non-magnetic cloak with a reduced set of parameters. We demonstrate that a cloak shell with only five layers of two normal materials can result in an average 20 dB reduction in the scattering width for all directions when covering the inner conducting cylinder with the cloak. The optimized design can substantially simplify the realization of the invisibility cloak, especially in the optical range.

Temperature distributions in Pt/SrZrO₃/SrRuO₃ and Pt/TiO₂/Pt thin film heterostructures were imaged by infrared thermography while under electrical bias. Local hot spots with lateral sizes between 5 and 30 µm appear during electroforming, they reappear during switching, and they show temperature increases from 50 to above 250 °C. Over 90% of conductivity increases produced by electroforming were confined to the hotspot locations. In some structures, thermography demonstrated that two separate conductive paths could be formed using opposite biases, and their conductivities could be repeatedly switched on and off with opposite voltage dependences. Direct evidence of large temperature increases supports the existence of Joule heating within the conductive channel during resistance switching of oxide heterostructures.

We describe shock-tunnel-based experiments carried out to evaluate a magnetohydrodynamic electrical power generator equipped with a convexly divergent supersonic channel. Two-dimensional structure of the streaming MHD plasma and the temporal behaviour of electron temperature are examined. The spatial profile of MHD power-generating plasma and the energy-conversion efficiency in the convexly divergent channel are compared with those from a linearly divergent channel. For an understanding of the basic scaling of the channel geometry modification effect, a convexity parameter is proposed. With this simple and fundamental scaling parameter, the dependence of plasma–fluid properties and energy-conversion efficiency on the channel convexity is quantitatively examined. The quality of MHD plasma and the generator performance are improved at the convexity parameter of 0.35 (a slight enhancement of the channel convexity) rather than at the convexity parameter of zero (no convexity or concavity). This paper is the first part of a duology.

We describe quasi-three-dimensional numerical calculations based on large eddy simulation model for magnetohydrodynamic (MHD) electrical power generators equipped with modified wall configurations. The wall profile of the MHD channel is finely tuned in four types of geometry, that is, a concavely divergent channel, a linearly divergent channel, a convexly divergent channel and a highly convexed channel. The plasma–fluid properties and energy conversion efficiency are examined in detail. Although the deterioration in the plasma–fluid behaviour is not completely overcome, the advantages of the convexly divergent channel are notable. The convexly divergent channel exhibits the highest energy conversion performance, which is followed by the highly convexed, linearly and concavely divergent channels in order. The effect of the channel geometry modification on the generator performance is clearly quantified using a convexity parameter. This paper is the second part of a duology.

In this paper, we report the effects of hydrogen plasma treatment on the structural and electrical properties of SnO₂ thin films prepared by the sputtering method. Whereas the hydrogen plasma treatment led to etching of SnO₂ films and subsequent degradation of crystalline quality and optical transmittance, the plasma-treated films exhibited an improvement in the electrical conductivity. Hall measurements indicated an increase in the carrier concentration of SnO₂ films which, following x-ray diffraction and secondary ion mass spectrometry measurements, was attributed to the generation of oxygen vacancies rather than the incorporation of hydrogen shallow donors in undoped SnO₂ films.

The electron temperature in a low-field helicon mode has been characterized in the presence of a diverging magnetic field (B₀ < 5 mT), using a number of electrostatic probes. At the low pressures investigated (<0.3 Pa) the electron energy probability function (measured with an rf compensated Langmuir probe) shows a depleted tail in both the upstream and downstream regions, with the tail temperature being about half that of the bulk electron temperature. Independent measurements of the tail temperature have been made with a retarding field energy analyser (operated in electron collection mode), which closely match those from the rf compensated probe. A global model is developed to predict the electron temperature, plasma potential and plasma density during the low-field mode, and is found to be in very good agreement with the experimental measurements.

The paper presents a transient three-dimensional model of an anti-phase-synchronized pulsed tandem gas–metal arc welding process, which is used to analyse arc interactions and their influence on the gas shield flow. The shielding gases considered are pure argon and a mixture of argon with 18% CO₂. Comparison of the temperature fields predicted by the model with high-speed images indicates that the essential features of the interactions between the arcs are captured. The paper demonstrates strong arc deflection and kinking, especially during the low-current phase of the pulse, in agreement with experimental observations. These effects are more distinct for the argon mixture with 18% CO₂. The second part of the paper demonstrates the effects of arc deflection and instabilities on the shielding gas flow and the occurrence of air contamination in the process region. The results allow an improved understanding of the causes of periodic instabilities and weld seam imperfections such as porosity, spatter, heat-tint oxidation and fume deposits.

The main aim of this work was to study and understand the influence of SiC particles on the corrosion and tribocorrosion of Al-matrix composite materials. For that, Al–SiC_p functionally graded composites were produced by centrifugal casting and different SiC_p contents were achieved. Their mechanical properties were improved by age-hardening heat treatments. The tribocorrosion behaviour was studied in 0.05M NaCl solutions using a reciprocating motion tribometer involving an alumina ball sliding against the Al-based samples. Above critical SiC particles' content the matrix alloy surface was found to be protected against wear by SiC particles protruding from the surface. Below this threshold content, the SiC reinforcement was inefficient and the wear rate of the composite was the same as the non-reinforced alloy.

The electrical and dielectric properties of CuFe_1−xCr_xO₂ (0 ⩽ x ⩽ 1) powders, doped with 3% of Mg and prepared by solid-state reaction, were studied by broadband dielectric spectroscopy in the temperature range from −100 to 150 °C. The frequency-dependent electrical and dielectric data have been discussed in the framework of a power law conductivity and complex impedance and dielectric modulus. At room temperature, the ac conductivity behaviour is characteristic of the charge transport in CuFe_1−xCr_xO₂ powders. The substitution of Fe³⁺ by Cr³⁺ results in an increase in dc conductivity and a decrease in the Cu⁺–Cu⁺ distance. Dc conductivity, characteristic onset frequency and Havriliak–Negami characteristics relaxation times are thermally activated above −40 °C for x = 0.835. The associated activation energies obtained from dc and ac conductivity and from impedance and modulus losses are similar and show that CuFe_1−xCr_xO₂ delafossite powders satisfy the BNN relation. Dc and ac conductivities have the same transport mechanism, namely thermally activated nearest neighbour hopping and tunnelling hopping above and below −40 °C, respectively.

High-constant dielectrics have gained considerable attention due to their wide applications in advanced devices, such as gate oxides in metal–oxide–semiconductor devices and insulators in high-density metal–insulator–metal capacitors. However, the theoretical investigations of these materials cannot fulfil the requirement of experimental development, especially the requirement for the accurate description of band structures. We performed first-principles calculations based on the hybrid density functionals theory to investigate several typical high-k dielectrics such as Al₂O₃, HfO₂, ZrSiO₄, HfSiO₄, La₂O₃ and ZrO₂. The band structures of these materials are well described within the framework of hybrid density functionals theory. The band gaps of Al₂O₃, HfO₂, ZrSiO₄, HfSiO₄, La₂O₃ and ZrO₂are calculated to be 8.0 eV, 5.6 eV, 6.2 eV, 7.1 eV, 5.3 eV and 5.0 eV, respectively, which are very close to the experimental values and far more accurate than those obtained by the traditional generalized gradient approximation method.

The shock-induced shear strength of a commercial silastomer, trade name Sylgard 184™, has been determined using laterally mounted manganin stress gauges. Shear strength has been observed to increase with increasing shock amplitude, in common with many other materials. Shear strength has also been observed to increase slightly behind the shock front as well. It is believed that a combination of polymer chain entanglement and cross linking between chains is responsible. Finally, a ramp on the leading edge of the lower amplitude stress traces has been observed. It has been suggested that this is due to shock-induced collapse of free space between the polymer chains. Similar explanations have been used to explain the apparent non-linearity of the shock velocity with particle velocity at low shock amplitudes.

Raman spectra of monolayer graphene at various temperatures (303–473 K) are measured. In Raman scattering with wave numbers ranging from 1200 to 3400 cm⁻¹, the four main Raman peaks (G, 2D, T + D and 2D') show temperature-dependent behaviour, but have different frequency shifts with increase in temperature. We propose that the peak frequency shift is related mainly to the elongation of C–C bond due to thermal expansion or anharmonic coupling of phonon modes, and oxygen-induced strong hole doping on the graphene surface. The doping effect can be confirmed from the frequency shifts, full-width at half-maximum as well as the area and intensity ratios of G and 2D peaks in temperature-dependent Raman scattering of graphene, room-temperature Raman spectra of pristine graphene and graphene cooled down after Raman measurement at 473 K in air. Therefore, the oxygen doping effect and temperature effect coexist in temperature-dependent Raman scattering of monolayer graphene.

This work reports on the study of surface properties of CaF₂ films (30 and 10 nm thick) grown on (1 1 1) Si by molecular beam epitaxy at substrate temperatures from 400 to 700 °C. Reflection high-energy electron diffraction (RHEED) analysis indicated that CaF₂ films with smooth surfaces were obtained in temperature ranges 500–550 °C and 620–700 °C, while at temperatures from 400 to 500 °C and in the vicinity of 600 °C the films showed grains randomly oriented on top of the surface. Atomic force microscopy (AFM) investigation corroborated with the RHEED results and confirmed the presence of grains on the film surface, with an evident transition near 600 °C. The dependence of grain density on the growth temperature followed the expectation from the RHEED analysis. The arithmetical average roughness of the CaF₂ surface obtained from the AFM images remained below 1 nm for the best quality films. The x-ray reflectivity curves of all samples exhibited well-defined interference fringes, whose oscillation damping behaviour agreed with the RHEED and AFM results. The CaF₂ layer thickness and roughness were accurately determined by a best-fit procedure applied to the x-ray reflectivity data. By combining all results, the temperature range between 525 and 550 °C was found to be the most suitable to grow CaF₂ layers on (1 1 1) Si. For growth temperatures above 650 °C, pinholes and cracks started to reduce the CaF₂ surface quality.

Solid-state synthesis and investigation of crystal structure and magnetic properties of Bi_0.86La_0.14−xSm_xFeO₃ (0 ⩽ x ⩽ 0.14) ceramics were performed. It was found that a rhombohedral to orthorhombic phase transition took place in the series with decreasing average ionic radius of the substituting elements occupying the A-site of the ABO₃ perovskite. Magnetic properties of the compounds were shown to correlate with evolution of their structural state. Pure rhombohedral samples 0 ⩽ x ⩽ 0.06 were obtained in a mixed antiferromagnetic/weak ferromagnetic state. A small residual magnetization characteristic of the compounds was found to weakly depend on change of the chemical composition. Progressive increase of the residual magnetization was observed upon the rhombohedral-to-orthorhombic transition. Reasons for the appearance of the weak ferromagnetism in orthorhombic and rhombohedral phases of the Bi_0.86(La, Sm)_0.14FeO₃ compounds were analysed.

We present finite element analysis of the deflections of clamped, ultrathin single crystals under electrostatic pressure. The crystals form concave mirrors that are suitable for low-aberration reflective focusing of inert atom beams, such as those required for the construction of a scanning helium microscope. The electrostatic and elastic aspects of the problem are coupled in the simulation. Additionally, realistic inhomogeneities in the crystal thickness are considered and it is shown that adaptive optic protocols using a minimal number of optimized electrodes can correct the resultant mirror aberrations.