Microwave Absorption Technique Research Articles

Enhanced microwave absorption and mechanical performance of polyvinylidene fluoride thin films embedded with novel activated carbon and manganese ferrite

AbstractThin composite films hold great promise for microwave devices for microwave absorption. This work mainly focuses on the investigation of the microwave absorption and mechanical studies of a composite thin film of activated carbon and manganese ferrite as conductive fillers within a polyvinylidene fluoride (PVDF) matrix. The production of activated carbon was successfully accomplished using a pyrolysis technique. Composite films consisting of polyvinylidene fluoride (PVDF) and activated carbon‐manganese ferrite were fabricated using a solution blending method. The composite films maintained a consistent concentration of activated carbon at 5 wt%. The characterization of the materials was conducted using scanning electron microscopy and X‐ray diffraction. The composite consists of 3 wt% of MnFe2O4 and 5 wt% Activated carbon/PVDF showed exceptional absorption properties and achieved a minimum reflection loss of around −38 dB at a frequency of 8–12 GHz at a thickness of 2 mm. Significantly, the composite film exhibited a greater tensile strength than the PVDF film. The results of our study highlighted the enhanced microwave absorption and economical manufacturing technique for producing composite films. These films exhibit promising microwave‐absorbing properties in stealth applications.Highlights A facile strategy to fabricate thin film PVDF composites was proposed. Novel activated carbon was synthesized to enhance the conductivity. Achieved reflection loss of around −38 dB at a frequency of 8–12 GHz. Synergistic effects of fillers enhanced dielectric and magnetic losses. Exhibited efficient mechanical performance and microwave absorption.

Detection of electromagnetic phase transitions using a helical cavity susceptometer.

Fast and sensitive phase transition detection is one of the most important requirements for new material synthesis and characterization. For solid-state samples, microwave absorption techniques can be employed for detecting phase transitions because it simultaneously monitors changes in electronic and magnetic properties. However, microwave absorption techniques require expensive high-frequency microwave equipment and bulky hollow cavities. Due to size limitations in conventional instruments, it is challenging to implement these cavities inside a laboratory cryostat. In this work, we designed and built a susceptometer that consists of a small helical cavity embedded into a custom insert of a commercial cryostat. This cavity resonator operated at sub-GHz frequencies is extremely sensitive to changes in material parameters, such as electrical conductivity, magnetization, and electric and magnetic susceptibilities. To demonstrate its operation, we detected superconducting phase transition in Nb and YBa2Cu3O7-δ, metal-insulator transitions in V2O3, ferromagnetic transition in Gd, and magnetic field induced transformation in meta magnetic NiCoMnIn single crystals. This high sensitivity apparatus allows the detection of trace amounts of materials (10-9-cc) undergoing an electromagnetic transition in a very broad temperature (2-400K) and magnetic field (up to 90 kOe) ranges.

Electromagnetic origin of the microwave absorption response of Fe3O4 thin films

Low-field microwave absorption techniques are ultrasensitive, nondestructive methods for probing electric and magnetic properties of solids. Nonresonant low-field microwave absorption techniques such as magnetic field modulated microwave spectroscopy (MFMMS) can easily detect electromagnetic phase transitions in minute and inhomogeneous samples. While this technique can easily and almost selectively identify superconducting transitions, magnetic phase transitions produce more varied responses. Here, we present a technique to investigate the electric and magnetic properties of a sample with complex electromagnetic responses. This technique involves taking a series of magnetic hysteresis loops and magnetoresistance measurements. These can be compared to MFMMS data to identify features having electric or magnetic origin. This approach is applied to magnetite $({\mathrm{Fe}}_{3}{\mathrm{O}}_{4})$, which possesses an electric, magnetic, and structural phase transition across its Verwey transition. By measuring high-quality ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ thin films in MFMMS and complementary techniques, the previously inscrutable MFMMS signal is analyzed. Furthermore, a model of the MFMMS signal can be calculated from the magnetic and electric data, which reproduces most of the features of the experimentally obtained MFMMS signal. This technique broadens the capabilities of MFMMS beyond the detection of superconductors.

First detection of low field microwave absorption in the disordered multiferroic double perovskite BiFe0.5Mn0.5O3

BiFe0.5Mn0.5O3 (BFMO) is an intriguing magnetic double perovskite, only obtainable through high pressure-high temperature synthesis. It shows bulk multiferroic properties, namely the coexistence between a spin canted antiferromagnetic structure superimposed to an externally induced electric polarization at least from 77 K. In particular, the system is characterized by a significant weak ferromagnetic hysteresis loop and by a very rare phenomenon: the spontaneous magnetization reversal (MRV) versus temperature in the low field regime. To clarify the BFMO exotic magnetic phase in the low field regime, the Electron Spin Resonance (ESR) and the low field microwave absorption (LFMA) techniques were used, providing the first observation of LFMA in the bulk BFMO as an additional functionality of this material. A striking feature is that the hysteresis in LFMA signals vanishes above 45 K, while the bulk M-H loop hysteresis, measured in the same field range of LFMA, persists till room temperature. The temperature at which LFMA hysteresis vanishes qualitatively matches the position of the magnetic susceptibility’s second derivative peak, corresponding to the temperature at which the local second order mechanism responsible for MRV is maximum. The line shape of LFMA completely changes above 45 K and the ESR linewidth starts decreasing above this temperature, indicating the role of defect/disorder induced inhomogeneity. The temperature evolution of LFMA hysteresis and line shapes as a measure of the competition between Fe- and Mn-rich clusters suggests a sort of local frustration at the microscopic scale, responsible for the peculiar magnetization reversal of this system.

Reply to “Comment on ‘Temperature range of superconducting fluctuations above Tc in YBa2Cu3O7−δ single crystals' ”

In a previous article [M. S. Grbi\ifmmode \acute{c}\else \'{c}\fi{} et al., Phys. Rev. B 83, 144508 (2011)] we reported data on the high-temperature superconductor ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ collected by a contactless microwave absorption technique, which provided evidence for superconducting correlations over a limited temperature range (10--20 K above ${T}_{c}$). The paraconductivity signal was determined by subtraction of zero-magnetic-field data from 16-T data. In the preceding Comment [D. S\'o\~nora et al., preceding Comment, Phys. Rev. B 102, 176501 (2020)] D. S\'o\~nora et al. argue that a 16-T magnetic field is not enough to quench all superconducting fluctuations in the microwave spectral range above ${T}_{c}$. As is obvious from the experimental data presented here, as well as from the data shown in the original paper, such conclusions disagree with our experimental results. Moreover, our initial experimental findings recently received independent robust confirmation from several different experimental techniques.

Observation of Pinning Strengthening in “MgB<sub>2</sub> – CNT – α-Fe” Nanocomposites Studied by Microwave Absorption Technique

Microwave absorption histeresis is measured in superconducting composites, prepared from MgB2 , carbon nanotubes, graphene particles and iron nanoparticles. Iron nanoparticles were synthesized with using of isopropanol in the supercritical fluid state. Measurements showed that the greatest increase in the pinning force is observed in the “MgB2 – (CNT-α-Fe)” composite.

Detection of an Anomalous Magnetic Transition in Hematite by Means of Derivative Microwave Absorption

We have investigated the derivative microwave power absorption (dP/dH) at X-band (8.8–9.8 GHz) in the hematite (a-Fe2O3) powders, for the 180-280 K temperature range. Two microwave absorption techniques are mainly used: the magnetically modulated microwave absorption spectroscopy (MAMMAS) and the extended field microwave absorption (EFMA). MAMMAS response showed distinctive features associated with the Morin transition at T Mo = 260 K, and below T Mo, a weak ferromagnetism is also observed. Additionally, EFMA technique is used to give further knowledge on this material, where this technique shows a change in microwave absorption dynamics in the vicinity of 200 K and it is associated with an anomalous magnetic transition, different to the Morin transition, and which is also confirmed by means of magnetic measurements and the low-field microwave absorption (LFMA).

Acoustic magnetoplasmons in a two-dimensional electron system with a smooth edge

Acoustic edge magnetoplasmon (AEMP) modes are detected in a wide range of magnetic fields by the resonant microwave absorption technique. It is established that in the case of a smooth edge of a two-dimensional electron system, the resonant signal is strongly enhanced with the AEMP excitation, whereas in the case of an abrupt edge, these modes are strongly damped. One fundamental mode and three AEMP modes are observed, and it is shown that the number of AEMP modes is directly related to the filling factor value. As a result, a sequence of abrupt terminations of AEMP modes is observed at the integer filling factors, in accordance with the number of edge compressible strips available in the electron system. Also the dependence of the relative AEMP amplitude versus the filling factor is addressed.

Paramagnetic resonance and non-resonant microwave absorption in iron niobate

An electron paramagnetic resonance (EPR) study of FeNbO 4 powder samples in monoclinic phase (wolframite-type) at X-band (8.8–9.8 GHz), in the 90–300 K temperature range, is presented. For all the temperatures, the EPR spectrum shows a single line associated with Fe 3+ ions. Changes in the lineshape of the EPR spectrum, which can be attributed to Fe 2+ ions, are detected at low temperatures. This behavior can be ascribed to a strong magnetic dipolar interaction between Fe 2+ and Fe 3+ ions. The non-resonant microwave absorption techniques: magnetically-modulated microwave absorption spectroscopy (MAMMAS) and low-field microwave absorption spectroscopy (LFMAS), were used for a further knowledge on this material. MAMMAS response suggests also the presence of Fe 2+ ions, that originates a change in microwave absorption regime for T < T p (=140 K), associated with the presence of short-range magnetic correlations. LFMAS spectra showed a linear behavior with positive slope and non-hysteretic traces. The profiles obtained by plotting the slope vs. temperature of the LFMAS line are similar to those detected by the MAMMAS technique, confirming that both types of measurement show the same processes of absorption.

Spectroscopy of metal related levels in Ge by transient infrared and microwave absorption techniques

Spectral analysis of deep levels in n-Ge implanted with different fluences of Co, Fe, Ti, and Cr ions is presented using transient infrared and microwave absorption techniques at room temperature. The activation energy E i of the deep levels observed with this novel approach, correlate rather well with those determined by capacitance-deep level transient spectroscopy (DLTS) and those extracted by simulation of the carrier recombination lifetime dependency on excitation level. It is shown that the synchronous variation of the carrier lifetime with deep level related parts of the microwave probed photo-conductivity spectrum reveals the recombination activity of specific centres.

Characterization of magnetic materials by low-field microwave absorption techniques

A low-field non-resonant microwave absorption has recently been observed in a variety of magnetically ordered materials at low DC fields (−1000 Oe ⩽ H DC⩽+1000 Oe), which is known as low-field microwave absorption (LFA). It has been shown that LFA is essentially similar to giant magnetoimpedance (GMI), and clearly different from ferromagnetic resonance (FMR). LFA strongly depends on the anisotropy field of the sample. In contrast with FMR (which can be described as the homogeneous precession of spins in the saturated state), LFA can be thought as a spin rotation process occurring during the magnetic saturation. In this work, we present a detailed study of the basic features of LFA in several types of materials: ferrites and amorphous microwires and ribbons; in particular the effects sample shape, temperature up to the Curie transition, the influence of easy axis and the effects of annealings. These examples show that once LFA is fully understood, it can become a powerful characterization tool.

Study of the Verwey transition in magnetite by low field and magnetically modulated non-resonant microwave absorption

We have investigated the Verwey phase transition (VPT) by two novel non-resonant microwave absorption techniques: low-field absorption (LFA) and magnetically modulated microwave absorption spectroscopy (MAMMAS). Measurements were carried out on sintered polycrystalline samples of Fe 3O 4, in the 77–300 K temperature range. LFA refers to the microwave absorption around the zero DC field range (−1000< H DC<+1000 G). LFA measurements showed an hysteretic behavior; a minimum in the hysteresis loop width was observed at 126 K, which can be associated with a minimum in magnetic anisotropy, as a consequence of charge localization below the VPT. MAMMAS experiments are based on the variations in microwave absorption (at constant H DC), as a function of temperature, and seem particularly well adapted to detect a wide range of phase transitions. In the magnetite case, a continuous increase in the microwave power absorption level was observed as temperature decreased, reaching a strong maximum at 130 K and a minimum at 100 K. An inflection point at 126 K was found, in very good agreement with LFA measurements. These results are discussed in detail.

On the Impact of Metal Impurities on the Carrier Lifetime in N-type Germanium

AbstractThe impact of metallic impurities on the carrier lifetime in n-Ge is studied using microwave reflection and absorption techniques. Co, Fe, Ti, Ni and Cr are introduced by ion implantation followed by a thermal anneal and quenching to room temperature. Excess carrier decay transients are examined by microwave reflection and absorption probing after pulsed light excitation. A detailed analysis allows to evaluate the ratio of the capture cross-sections for minority and majority carriers revealing an acceptor-like character of the metal induced traps. Cross-sectional lifetime measurements show an U-shaped depth distribution with the lowest lifetimes in the bulk of the wafer. The lifetime results are correlated with those of deep level transient spectroscopy in order to clarify the properties of the dominant metal related recombination centres. Fe and Co are the most effective lifetime killers in n-Ge while Cr has the least influence.

Dynamics of carrier decay in proton and γ-ray irradiated high-resistivity Si

Excess carrier lifetime variations in proton and ∞-ray irradiated MCz Si have been examined. The linear decrease of recombination lifetime with dose in ∞-ray irradiated material shows a dose-independent defect introduction rate, while protons of high uence induce changes of the type of the dominant radiation defects. The simultaneous recombination and trapping components in the carrier decay transients were revealed by microwave absorption technique. Two peaks were separated in recombination and trapping lifetime variations with temperature. Trap activation factors were extracted. The dominant defects acting as recombination and trapping centres are discussed.

Investigation of decay characteristics of photoelectrons in nanoparticales of cubic AgBr sensitized by NiS

NiS nanoparticales as a sensitizer is added to cubic silver bromide emulsion for the chemical sensitization. The decay curves of free photoelectrons and shallow-trapped electrons in the cubic silver bromide emulsion sensitized by NiS nanoparticales with the continuous increase of sensitization time are measured by the microwave absorption and dielectric spectrum techniques. The courses of decay of photoelectrons in the cubic silver bromide emulsion sensitized by NiS nanoparticales vis sensitization time is analyzed. The influence of electron traps in silver bromide microcrystals on the behavior of the photoelectrons' movement is discussed. The relationships between the effects of electron traps and the depth of traps with sensitization time are analyzed. By comparing the decay characteristics of the unsensitized and the sensitized samples, the optimum sensitization time under the present experimental conditions is 80 min.

Electrical and Optical Characterization of Thin Semiconductor Layers for Advanced ULSI Devices

An overview is given of analytical techniques for the characterization of the electrical and transport parameters in thin (&lt;1 µm) semiconductor layers. Some of these methods have been applied to the lifetime and diffusion length study in thin strain-relaxed buffer (SRB) layers of strained silicon (SSi) substrates, while a second group was dedicated to Silicon-on-Insulator (SOI) materials and devices. The employed techniques can be divided into two groups, whether a device structure (junction, MOS capacitor, MOSFET) is required or not. However, the MicroWave Absorption (MWA) technique can be used in both cases, making it a versatile tool to study both grown-in and processing-induced electrically active defects. The transport properties of SSi wafers are strongly determined by the density of threading and misfit dislocations, although the dependence of the recombination lifetime is weaker than expected from simple Shockley-Read-Hall (SRH) theory. This is related to the high injection regime typically employed, enabling the characterization of the 250-350 nm thick Si1-xGex layer only. At longer carrier decay times, multiple trapping events dominate that can be described by a stretched exponent approach, typical of disordered materials. For SOI substrates, transistor-based techniques will be demonstrated that enable to assess the generation or recombination lifetime in the thin silicon film (&lt;100 nm). The lifetime can be severely degraded by irradiation or hot-carrier degradation. Finally, it will be shown that Generation-Recombination (GR) noise spectroscopy as a function of temperature allows identifying residual ion-implantation-damage related deep levels, which are otherwise hard to detect even by Deep Level Transient Spectroscopy (DLTS).

Investigation of photoelectron action in cubic AgCl emulsion doped with formate ions

In recent years, the formate ion (HCO2−) as a kind of hole-to-electron converter has attracted much attention of photographic researchers. The formate ions can trap photo-generated holes, eliminate or reduce the electron loss caused by electron–hole recombination in latent image formation process. Through the hole-to-electron conversion, it can also release an extra electron or electron carrier, improving photosensitivity. In this paper the microwave absorption and dielectric spectrum detection technique is used to detect the time evolution behaviour of free photoelectrons generated by 35ps laser pulses in cubic AgCl emulsions doped with formate ions. The influence of different doping conditions of formate ions on the photoelectron decay kinetics of AgCl is analysed. It is found that when the HCO2− content is 10−3mol/mol Ag and the doping position is 90% the electron decay time and lifetime reach their maxima due to the efficient trap of holes by formate ions.

The influence of K 4 Fe(CN) 6 on the photosensitivity of cubic AgCl microcrystal

The influence of K4Fe(CN)6 at various doping concentrations on the photosensitivity of cubic AgCl microcrystals has been investigated by using the microwave absorption and phase-sensitive measurement technique. The time behaviour of free photoelectrons in AgCl microcrystals is analysed by using computer simulation as a function of three parameters of shallow electron trap (SET) including doping concentration, trap depth and capture cross-section (CCS). It is found that the three parameters of SET play different roles on the free photoelectron decay time (FDT). After considering the threshold effects of the three parameters and their collective effects, the trap depth value, the CCS value and the optimal doping concentration of the SET introduced by the dopant K4Fe(CN)6 in cubic AgCl microcrystals can also be determined and the best photosensitivity of cubic AgCl can be obtained.

Superconductivity of Carbon Materials - Unstable Phases

Difierent intercalated carbon systems: KxC60, RbxC60 as well as highly oriented pyrolytic graphite + potassium with unstable structural and superconducting behavior were investigated with the EPR and magnetically modulated microwave absorption techniques. Three characteristic, well-separated EPR signals were observed for AxC60 (A = K or Rb) systems at the early stages of doping process. We ascribed these signals to C + , C 1i 60 , and C 3i 60 radicals. Evolution of the EPR spectrum characterizes difierent phases of KxC60 system including superconducting one. Two superconducting phases with T (1)

Investigation of photoelectron temporal characteristics in silver halide microcrystals using the microwave absorption technique

The free photoelectron lifetime reflects to a large extent the latent image formation efficiency and sensitivity of silver halide material. The microwave absorption dielectric-spectrum technique enables measurement of the photoelectron decay process of silver halide emulsion exposed to 35ps laser pulse. For T-grain AgBr emulsion, the relationship between exciting energy and photoelectron action has been obtained and the influence of iodide dopants on photoelectron lifetime was measured and analysed. The photoelectron lifetime of dye-sensitized AgBr emulsion with tabular grains is shorter than that with cubic grains and the latent image formation efficiency of the former is higher than the latter.