Microwave properties of composites with glass coated amorphous magnetic microwires

The complex permittivity and permeability of composites filled with Fe-based microwires are measured in a coaxial line at frequencies from 0.01 to 10 GHz.The samples under study consist of closely packed wire sections with various orientations relative to wave vector. The composites, where the wires are collinear to the coaxial axis, are found to be low permeable. Their permittivity has frequency dispersion governed by the length of the wire and its linear impedance. The middle section of the wire has higher impedance than that of the end sections where the regular domain structure is distorted. Magnetic bias parallel to the wire axis affects the linear impedance and parameters of dielectric absorption of a composite, the effect is proportional to bias strength. The samples of a coil-type structure, where the wires are wound around the coaxial axis, display the intensive magnetic absorption attributed to the domain wall motion. The absorption takes place in the megahertz band, at microwaves the permeability is close to unity. The microwave properties of diluted composites filled with randomly oriented permeable wires are discussed.     

Microwave waveguide method for the measurement of electron mobility and conductivity in GaAs/AlGaAs heterostructures

The microwave waveguide method for contactless determination of the electron mobility and conductivity of thin active layers is reported. The method is based on relative measurements of the magnetic field dependences of the derivative of the reflection coefficient with respect to the magnetic field from a semiconductor wafer bridging the waveguide.Experiments are performed on GaAs/AlGaAs heterostructures at microwave frequency = 36.4 GHz and liquid nitrogen temperature. For the analysis of the experimental data the theoretical basis for arbitrary frequencies is developed. The main advantage of the proposed method is that this method enables one to determine material parameters - mobility and conductivity - without careful calibration of the microwave system and does not require the accurate measurements of the absolute values of the reflection coefficient and phase of the reflected wave.     

Ferromagnetic Antiresonance and Giant Microwave Magneto-Impedance in Polycrystalline La0.49Sr0.51MnO3

The ferromagnetic antiresonance （FMAR） phenomenon, i.e., the minimum of the microwave absorption, in polycrystalline La0.49Sr0.51MnO3 is observed near Curie temperature TC = 282 K. Temperature-dependences of magnetization μ0M are obtained from the FMAR. The results show that as μ0H = 0, by fitting the scaling law M ∝ （TC -T）^β to temperature-dependences of μ0M at the different microwave frequencies, it yields TC=281.2 K and β = 0.47. However, temperature-dependences of μ0M under different μ0H are not in agreement with the scaling law. Due to FMAR, about 40% giant microwave magneto-impedance at 11.9 GHz can occur under a low field μ0H = 0.03 T.     

Sum rules and figures-of-merit on the microwave permeability of nanocrystalline microwires

The microwave permeability spectra of three nanocrystalline microwires were investigated, and dynamic magnetization M_μ(F) was determined from these spectra. An experimental verification of the theoretical prediction that this quantity was lower than the saturation magnetization was carried out, and it was further shown that the theoretical result provided straightforward guidelines for the conception of high-permeability materials, including successful tradeoffs between a higher resistivity and an increased saturation magnetization. M_μ(F) was determined to be an appropriate figure-of-merit for several applications, including high-frequency noise suppressors.     

Generalized surface current method in the macroscopic theory of diffraction radiation

The surface current method known in the theory of electromagnetic waves diffraction has been generalized to be applied to the problems of diffraction radiation generated by a charged particle moving nearby an ideally-conducting screen in vacuum. An expression for induced surface current density leading to the exact results in the theory of transition radiation has been derived, and by using this expression several exact solutions of diffraction radiation problems are found. Limits of applicability for the earlier known models based on the surface current conception are indicated. Properties of radiation from a semi-plane and from a slit in cylinder are investigated at the various distances to observer.     

On reflection from interfaces with some spatially dispersive metamaterials

Plane-wave reflection from interfaces with single and double wire media is considered. Such media exhibit strong spatial dispersion even at very low frequencies which causes appearance of additional waves. The problem of additional boundary conditions (ABC) in application to wire media is discussed and an ABC-free approach, known in solid state physics, is used. Expressions for the fields and Poynting vectors of refracted waves are derived. The directions and values of the power density flow of refracted waves are found and conservation of the power flow through the interface is checked.     

Mössbauer spectroscopy studies of spin reorientations in amorphous and crystalline (Co0.2Fe0.8)72.5Si12.5B15 glass coated micro-wires

Thermo-gravimetric, differential scanning calorimetry and comprehensive ⁵⁷Fe Mössbauer spectroscopy studies of amorphous and crystalline ferromagnetic glass coated (Co_0.2Fe_0.8)_72.5Si_12.5B₁₅ micro-wires have been recorded. The Curie temperature of the amorphous phase is T_C(amorp) ∼730 K. The analysis of the Mössbauer spectra reveals that below 623 K the easy axis of the magnetization is axial-along the wires, and that a tangential or/and radial orientation occurs at higher temperatures. At 770 K, in the first 4 hours the Mössbauer spectrum exhibits a pure paramagnetic doublet. Crystallization and decomposition to predominantly α-Fe(Si) and Fe₂B occurs either by raising the temperature above 835 K or isothermally in time at lower temperatures. Annealing for a day at 770 K, leads to crystallization. In the crystalline material the magnetic moments have a complete random orientation. After cooling back to ambient temperature, both α-Fe(Si) and Fe₂B in the glass coated wire show pure axial magnetic orientation like in the original amorphous state. The observed spin reorientations are associated with changes in the stress induced by the glass coating.     

Contactless measurement of the conductivity of II–VI epitaxial layers by means of the partially filled waveguide method

We report the contactless determination of the conductivity, the mobility and the carrier concentration of II–VI semiconductors by means of the technique of the partially filled waveguide at a microwave frequency of 9 GHz. The samples are CdHgTe epitaxial layers, grown on CdZnTe substrates by molecular beam epitaxy. The conductivity is determined from the transmission coefficient of the sample in the partially filled waveguide. For the analysis of the experimental data, the complex transmission coefficient is calculated by a rigorous multi-mode matching procedure. By varying the conductivity of the sample, we obtain an optimum fit of the calculated data to the experimental results. Comparison with conductivity data determined by the van der Pauw method shows that our method allows to measure the conductivity with good accuracy. The behaviour of the transmission coefficient of the sample is discussed in dependence on the layer conductivity, the layer thickness and the dielectric constant of the substrate. The calculations require to consider in detail the distribution of the electromagnetic fields in the sample region. The usual assumption of a hardly disturbed TE₁₀ mode cannot be used in our case. By applying a magnetic field in extraordinary Voigt configuration, galvanomagnetic measurements have been carried out which yield the mobility and thus the carrier concentration. These results are also in good agreement with van der Pauw transport measurements.     

Barium ferrite powders prepared by microwave-induced hydrothermal reaction and magnetic property

We have prepared barium ferrite particles by the microwave-induced hydrothermal method. The crystallization of the barium ferrite particles is promoted within a short time by microwave irradiation because the seeds of barium ferrite having large permeability are rapidly heated through the interaction of barium ferrite with the magnetic component of the microwaves. Crystals having unusually low thickness were obtained compared with the conventional hydrothermal method. The magnetic properties of barium ferrite particles are discussed.     

Spin-deposited nanocrystalline lithium ferrite thin films: Fabrication and characterization

Thin films of lithium ferrite (with general composition Li_0.5Fe_2.5O₄) were fabricated at low temperatures (up to 650 °C) by citrate-route using spin-deposition technique. Deposited films consisted of nanometer-sized grains as evidenced by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and atomic force microscopy (AFM) techniques. XRD patterns for annealed films showed broad peaks exhibiting a spinel phase. Size of nanocrystallites is estimated to be 3-7 nm using Scherrer's equation. Average grain size ∼8.5 nm is observed from TEM images of films annealed at 650 °C. Scanning electron micrographs show the formation of spherical aggregates of around 130 nm in diameter. The AFM analysis clearly evidenced the development of nanograins even at low (∼500 °C) annealing temperatures. Significant decrease in complex dielectric permittivity (∈′ − j∈″) with frequency is observed in the low frequency (100 Hz-1 MHz) as well as in X-band microwave frequency (8-12 GHz) region. ∈′ is found to be in the range of 15.7-33.9 in low frequency region, whereas in X-band microwave frequency region, it is found to lie between 3.9 and 4.9. Similarly, ∈″ is found to be 0.16-5.9 in the low frequency region, and 0.002-0.024 in the X-band microwave frequency region. Room temperature dc resistivity of these films is estimated to lie in the range of 10⁶-10⁸ Ω cm. These results strongly suggest that citrate-route processed nanocrystalline lithium ferrite thin films are promising candidates for monolithic microwave integrated circuits (MMICs).     

Mechanical properties of polymeric composites with carbon dioxide particles

Nanocomposites consisting of a polymethylmethacrylate or polystyrene matrix with embedded silicon dioxide nanoparticles surface-modified by silazanes have been prepared by melting technology. The influence of particles on viscoelastic properties of the nanocomposites has been studied using dynamic mechanical analysis. It has been revealed that the addition of 20 wt % of SiO₂ raises the flexural modulus of the nanocomposites by 30%.     

Transmission characteristics of wave modes in a rectangular waveguide filled with anisotropic metamaterial

In this paper, transmission characteristics of wave modes in a rectangular waveguide filled with lossless anisotropic metamaterial are theoretically investigated. The wave equation and dispersion relations for TE and TM modes in the waveguide are obtained and analyzed. It is shown that the negative constitutive parameters of the filling anisotropic metamaterial can be used to control the wave magnitude and the phase velocity direction in the waveguide over the entire frequency domain, both below and above the cutoff frequency. Particularly, not only backward waves, but also forward waves can propagate below the cutoff frequency in the waveguide. Furthermore, a typical example is calculated to demonstrate transmission characteristics of waves inside the waveguide. Numerical results are obtained in the paper and compared with theoretical predictions: a good agreement is found.     

Polarization dependence of resonant inelastic X-ray scattering spectroscopy in correlated electron systems

Resonant inelastic X-ray scattering (RIXS) at the transition-metal K-edge is studied as a tool to detect the electronic structure in correlated electron systems. We, in particular, focus on the polarization dependence of RIXS intensity and symmetry of the electronic excitations. It is shown that by analyzing the polarization of the initial and scattered X-rays, the symmetry of the 4p orbitals are selected. Combined effects of the polarization of X-rays and the momentum transfer in the scattering are also studied.     

Enhancement of the thermal properties of silver-diamond composites with chromium carbide coating

The present work reports the enhancement of the thermal properties in Ag/diamond matrix composites reinforced with chromium carbide coated diamond particles. The coated diamond particles were characterized by x-ray diffraction, x-ray photoelectron spectroscopy and Raman spectra. The composites were synthesized by spark plasma sintering. The chromium carbide coating on the diamond particles resulted in composites exhibiting improved wettability and strong interfacial bonding between the diamond particles and Ag matrix. The composites with coated diamonds showed a low coefficient of thermal expansion of 8.24 × 10^?6/K and a high thermal conductivity of 695 W/mK at 60 % particle volume fraction, which greatly outperformed the composites with uncoated diamonds at the same particle volume fraction. The obtained results are useful for synthesizing Ag/diamond composites with greatly improved thermal performance.     

Magnetization of ballistic quantum dots induced by a linear-polarized microwave field

On a basis of extensive analytical and numerical studies we show that a linear-polarized microwave field creates a stationary magnetization in mesoscopic ballistic quantum dots with two-dimensional electron gas being at a thermal equilibrium. The magnetization is proportional to a number of electrons in a dot and to a microwave power. Microwave fields of moderate strength create in a one dot of few micron size a magnetization which is by few orders of magnitude larger than a magnetization produced by persistent currents. The effect is weakly dependent on temperature and can be observed with existing experimental techniques. The parallels between this effect and ratchets in asymmetric nanostructures are also discussed.     

Unusual electronic properties of InN

Electronic structure of zinc-blende and wurtzite InN using linear combination of atomic orbitals and the latest approach of generalised gradient approximation within full potential linearised augmented plane wave schemes is reported. An unusual small band gap and real space analysis of our first ever experimental Compton profile are discussed.     

Form-factor measurements on chromium with high-energy synchrotron radiation

Results of high-energy magnetic X-ray diffraction on pure antiferromagnetic chromium are presented. The temperature dependence of the propagation vector of the spin-density wave (SDW) and the strain-wave (SW) could be reproduced. The temperature dependence of the magnetic integrated intensity could be measured in the transversally as well as in the longitudinally polarised SDW phase. The magnetic form-factor has been determined in the transversally polarised SDW phase with five magnetic satellites. For the first time a spin-orbit separation has been performed by comparing X-ray to neutron data. The small orbital contribution to the magnetisation density turns out to be negligible, in agreement to our relativistic band-structure calculations. In addition, measurements of strain-wave reflections have been undertaken, and the results complement previous studies. Received 17 August 1998 and Received in final form 10 August 1999     

Defect structure and dielectric properties of Bi-based pyrochlores probed by positron annihilation

Positron annihilation lifetime (PAL) and Doppler broadening (DB) techniques have been performed to identify structural defects of the bismuth based pyrochlore systems with generic formula (Bi_1.5Zn_0.5)(Zn_0.5−x/3Ti_xNb_1.5−2x/3)O₇ (x = 0, 0.25, 0.5,1.0, 1.5). We found that all studied compounds contain substantial amount of the lattice vacancy defects, the variation of the annihilation lifetime suggests that the defects structure undergoes significant changes. The complex defects could be produced with increasing content of Ti, resulting in a drop in the intensity I₂ in the Ti-rich sample. At 1 MHz their dielectric constant (?′) varies from 150 for Ti-poor system to 210 for Ti-rich system and loss tangent (tan δ) remains rather low level. The high dielectric constant response of the BZTN ceramics is attributed to loosening state of cations located in the center of octahedral, so favor off-center displacement. The occurrence of complex defects help to enhance the dielectric constant.     

Structural and magnetic properties of chromium dioxide at high pressures

Ab initio calculations were performed on CrO₂ to study its behavior and possible similarity to silica under high pressures. At the rutile→CaCl₂-type phase transition, the lattice constants, cell volume and total energy change continuously, indicating the second-order nature of the phase transition, consistent with the experimental observations. The current calculations have demonstrated that the rutile→CaCl₂-type phase transition is driven by the softening of the Raman active B_1g mode, weakly coupling with the elastic shear modulus C_s. Further phase transitions of CrO₂ to denser packed phases of α-PbO₂-type and pyrite have been well predicted by total energy calculations. Our electronic calculations revealed that CrO₂ is still a half-metallic ferromagnet up to pressure of 95 GPa. The present results confirm the analogy of the phase sequence between silica and CrO₂ at high pressures.     

Electrical properties of composites based on polystyrene with hybrid silicon dioxide particles

Composites based on polysterene and hybrid core-shell nanoparticles are studied; the nanoparticle core consists of silicon dioxide, and ethylphenylic groups (organic shell) are grafted to the core surface. It is shown that the permittivity, the volume resistivity, the thermostimulated depolarization current spectra, and the glass transition temperature of these materials depend on the nanofiller content and, what is more important, the nanofiller distribution over the polymer volume in the form of particles or their aggregates.