Increasing Microwave Frequency Research Articles

Microwave-assisted switching in CoCrPt granular medium under continuous microwave fields

We have fabricated a device for microwave-assisted switching (MAS) experiments with perpendicularly magnetized CoCrPt-SiO2 granular film with anisotropy field Hk = 18.8 kOe. The device was carefully designed to be able to apply a microwave field of 0.95 kOe in amplitude continuously, allowing direct evaluation of microwave-assisted switching properties of the medium without any assumptions about time-dependent switching behavior. The coercivity of CoCrPt-SiO2 medium linearly decreases with increasing microwave frequency, and the maximum coercivity reduction ratio reaches 80% for hrf = 0.95 kOe at frf = 25 GHz after subtracting the effect of temperature rise due to microwave field application. The linear slope of coercivity against the frequency becomes steeper with increasing microwave field amplitude, which is a unique frequency and amplitude dependencies in granular media. Correlation length obtained by X-ray magnetic circular dichroism microscopy measurement decreases by applying a microwave field with higher frequency, suggesting that the magnetic clustering dimensions of the medium possibly depend on microwave frequency during microwave-assisted switching.

Activation of endoplasmic reticulum stress in rat brain following low-intensity microwave exposure.

The present study was designed to explore the effects of low-intensity microwave radiation on endoplasmic reticulum stress and unfolded protein response. Experiments were performed on male Wistar rats exposed to microwave radiation for 30days at 900 MHz, 1800 MHz, and 2450 MHz frequencies on four groups of animal: sham-exposed group, 900 MHz exposed (SAR 5.84 × 10-4 W/kg), 1800 MHz exposed (SAR 5.94 × 10-4 W/kg), and 2450 MHz exposed (SAR 6.7 × 10-4 W/kg) groups. Expressions of mRNA were estimated at the end of exposure in rat brain by real-time quantitative PCR. Microwave exposure at 900, 1800, and 2450MHz with respective SAR values as mentioned above significantly (< 0.05) altered mRNA expression of transcription factors ATF4, CHOP, and XBP1 in accordance with increasing microwave frequency. The result of the present study reveals that low-intensity microwave exposure at frequencies 900, 1800, and 2450MHz induces endoplasmic reticulum stress and unfolded protein response.

Frequency dependence of microwave-assisted switching in CoCrPt granular perpendicular media

The microwave-assisted switching properties of a CoCrPt–TiO2 granular perpendicular medium are investigated by detecting the anomalous Hall effect. We improved the device structure to be able to generate a linearly polarized microwave field of 950 Oe without a significant increase in temperature. Coercivity decreases linearly with increasing microwave frequency and the microwave assistance effect is most enhanced by the application of microwaves of 18 GHz frequency with a coercivity reduction ratio of 55%.

Microwave pyrolysis of sewage biosolids: Dielectric properties, microwave susceptor role and its impact on biochar properties

Microwave assisted pyrolysis (MWAP) is an alternative heating approach to convert biosolids into value-added products, such as biochar, biogas and bio-oil. Studying the dielectric properties of biosolids is fundamental to understand the behaviour of this material under microwave irradiation and to design microwave assisted pyrolysis systems. This study examined the dielectric properties of biosolids with changes in moisture content and applied microwave frequency. Results demonstrated that the dielectric constant decreases with decreasing moisture content and with increasing microwave frequency, but the dielectric loss factor of dry biosolids is almost zero. Simulations demonstrated that moisture content of biosolids impacts on the distribution and intensity of electromagnetic field. Because of the poor dielectric properties of dry biosolids, a microwave susceptor must be added to the biosolids to attract microwave energy so that the materials can reach temperatures required for pyrolysis. Therefore, this study also investigated the impact of four microwave susceptors (activated carbon, charcoal, biochar and glycerol) on biosolids pyrolysis and on biochar properties produced from biosolids via microwave assisted pyrolysis at 600°C. The choice of microwave susceptor influences the heating rate of biosolids and the specific surface area of the resultant biochar. Results show that activated carbon favours the heating process, increases surface area, and the biochar produced with activated carbon has the highest carbon stability and energy value.

Long-range correlations in rectangular cavities containing pointlike perturbations.

We investigated experimentally the short- and long-range correlations in the fluctuations of the resonance frequencies of flat, rectangular microwave cavities that contained antennas acting as pointlike perturbations. We demonstrate that their spectral properties exhibit the features typical for singular statistics. Hitherto, only the nearest-neighbor spacing distribution had been studied. In addition we considered statistical measures for the long-range correlations and analyzed power spectra. Thereby we could corroborate that the spectral properties change to semi-Poisson statistic with increasing microwave frequency. Furthermore, the experimental results are shown to be well described by a model applicable to billiards containing a zero-range perturbation [T. Tudorovskiy et al., New J. Phys. 12, 123021 (2010)NJOPFM1367-263010.1088/1367-2630/12/12/123021].

Cantilever detected ferromagnetic resonance in thin Fe50Ni50, Co2FeAl0.5Si0.5 and Sr2FeMoO6 films using a double modulation technique

In this work we introduce a new method, which employs commercial piezo-cantilevers, for a ferromagnetic resonance (FMR) detection from thin, nm-size, films. Our setup has an option to rotate the sample in the magnetic field and it operates up to the high microwave frequencies of 160GHz. Using our cantilever based FMR spectrometer we have investigated a set of samples, namely quasi-bulk and 84nm film Co2FeAl0.5Si0.5 samples, 16nm Fe50Ni50 film and 150nm Sr2FeMoO6 film. Low frequency and room temperature test of our setup using 84nm Co2FeAl0.5Si0.5 film yielded a result identical to a standard X-Band spectrometer, namely a single line with quite small linewidth. Our measurements at low temperatures and high frequencies revealed a quite strong FMR response detected in all samples. The FMR spectra share common features, such as the emergence of the second line with an opposite angular dependence, and a drastic increase of the linewidths with increasing microwave frequency. We believe that these findings are results of the complicated dynamics of the magnetization at low temperatures and high frequencies, which we were able to probe using our cantilever based FMR setup.

Electromagnetic interference shielding properties of nanocomposites for commercial electronic devices

A novel functional nanocomposites microwave shielding material using natural rubber (NR) loaded with binary active fillers barium hexaferrite (BF) (BaFe12O19) and carbon nanoparticle (CB) were fabricated by conventional roll milling technique. Nanocrystalline barium hexaferrite with particle size of 16 nm were successfully synthesized by hydrothermal approach at low temperature. The surface morphology of the nanocomposites was investigated using X-ray, field emission scanning electron microscopy and transmission electron microscopy. The electrical resistivity of the nanocomposites is decreased with BF loading level. The dielectric constant of the composites is decreased with increasing microwave frequency. NR/BF nanocomposites were screened for electromagnetic interference (EMI) shielding effectiveness (SE) over a frequency band of 1---12 GHz. A maximum SE value is approximately 75 dB in the frequency range of 1---12 GHz for BF content of 20 wt % and thickness of 1 mm. An excellent agreement between the theoretically predicted SE and the measured data was found. These features collectively demonstrated the potential of NR/BF nanocomposites for versatile applications in microwave shielding application and commercial electronic devices issues.

Development research of thermocouple-based microwave power sensors using AC/DC substitution method

In this work, a thermocouple-based microwave power sensor using AC/DC substitution method is developed. The reason that the output voltage is different under different microwave frequency is explained. An improved package solution of the thermocouple-based microwave power sensor is designed at X-band (8---12 GHz). The fabrication of this thermocouple-based microwave power sensor is divided into a front side and a back side processing using GaAs MMIC process and MEMS technology. The measurement results show that the output voltage of the microwave power sensor decreases with increasing microwave frequency, and the COB (Chip on Board) technology brings an economic package solution for the thermocouple-based microwave power sensor. The microwave power sensor after package can be applied into the radar system and other microwave system.

A miniature electrothermal thruster using microwave-excited plasmas: a numerical design consideration

A miniature electrothermal thruster has been proposed using azimuthally symmetric microwave-excited plasmas, and numerical investigations have been conducted for design consideration. The microthruster consists of a microplasma source and a micronozzle. The former, made of a dielectric chamber 1 mm in radius and 10 mm long covered with a grounded metal, produces high temperature plasmas in Ar at around atmospheric pressures. The latter converts such high thermal energy into directional kinetic energy through supersonic nozzle expansion to obtain the thrust required. The numerical model consists of three modules: a global model and an electromagnetic model for microplasma sources and a fluid model for micronozzle flows. Simulation was conducted separately for the plasma source and nozzle flow. The numerical results indicated that the microwave power absorbed in plasmas increases with increasing microwave frequency and relative permittivity of dielectrics, to achieve plasma density in the range 1019–1022 m−3, electron temperature in the order of 104 K and heavy particle temperature in the range 103–104 K at a microwave input power of ≤ 10 W; in practice, surface waves tend to be established in the microplasma source at high frequencies and permittivities. A certain combination of frequency and permittivity was found to significantly enhance the power absorption, enabling the microplasma source to absorb almost all microwave input powers. Moreover, the micronozzle flow was found to be very lossy because of high viscosity in thick boundary layers, implying that shortening the nozzle length with increasing half-cone angles suppresses the effects of viscous loss and thus enhances the thrust performance. A thrust of 2.5–3.5 mN and a specific impulse of 130–180 s were obtained for a given microwave power range of interest, which is applicable to a station-keeping manoeuvre for microspacecraft less than 10 kg.

Gilbert damping in single and multilayer ultrathin films: role of interfaces in nonlocal spin dynamics.

Unique features of the Gilbert damping in magnetic multilayers were investigated by ferromagnetic resonance (FMR) using magnetic single and double layer structures prepared by molecular beam epitaxy. The FMR linewidth for the Fe films in the double layer structures was larger than the FMR linewidth in the single Fe films having the same thickness. The additional FMR linewidth scaled inversely with the film thickness, and increased linearly with increasing microwave frequency. These results demonstrate that a transfer of electron angular momentum between the magnetic layers leads to additional relaxation torques.

Unusual microwave vortex dissipation of Nd 1.85Ce 0.15CuO 4

We have measured the microwave surface impedance Z S= R S+i X S of the electron-doped cuprate Nd 1.85Ce 0.15CuO 4 in magnetic field perpendicular to the superconducting layers. A broad peak was observed in the field dependence of flux flow resistivity ρ f obtained from Z S. This peak shifts to higher field with increasing microwave frequency, indicating that the peak originates not from the vortex pinning effect but from the electronic excitations. We discussed several possible origins for this peak.

Multifrequency electron paramagnetic resonance of irradiated l-alanine

The radical generated by γ-irradiation of crystalline l-alanine was examined by continuous wave (CW) and pulsed electron paramagnetic resonance (EPR) at 1.8, 3.2, 4.9, 9.1 and 19.4 GHz. The spin-flip satellite lines that make a prominent contribution to the saturated spectra at 9.1 GHz are less conspicuous at lower frequencies because of overlap with the allowed transitions. The spin-lattice relaxation times measured by long-pulse saturation recovery and phase memory times measured by electron spin echo increase with increasing microwave frequency.

Magnetic properties of monocrystalline epilayer sandwich of magnesium ferrite

The ferrimagnetic monocrystalllne epilayer Mg 0.5 Fe 2.5 O 4 and superparamagnetic interface between this layer at the MgO monocrystalline substrate formed the sandwich. The sandwich was obtained by CVD on the /100/ crystallographical surface of the substrate. The magnetic properties of this sandwich were studied by ferromagnetic resonance. Measurements at room temperature at 9.26, 25.94, 33.04 and 69.61 GHz permitted the determination of the resonance field H r vs. the angle θ between the normal to the layer and the external field direction in the crystallographical plane /010/. In this data was defined the magnetic parameters characterizing the two layers of the sandwich as functions of the microwave frequency. Epilayer Mg 0.5 Fe 2.5 O 4 exhibited at ferromagnetic resonance non-collinearity between its magnetisation and the external field dependent on θ and frequency. The interface, of weaker magnetisation, exhibited superparamagnetic properties - the magnetisation increased with increasing microwave frequency.