Resonant Microwave Absorption Research Articles

Band gap, ferromagnetic resonance and strong microwave absorption of BaFe12-2xZnxSnxO19 compounds and enhanced dielectric loss in BaFe12-2xZnxSnxO19/carbon nano tube composites

The forbidden bandgap energy, magnetic properties, natural ferromagnetic resonance and microwave absorption are studied on the Zn2+ and Sn4+ doped BaFe12-2xZnxSnxO19. The forbidden band gap of BaFe12-2xZnxSnxO19 increases with the increasing doping content x from 1.80 eV up to 1.97 eV. The coercive field, the anisotropy field and the magnetocrystalline constant of BaFe12-2xZnxSnxO19 decreases with the increasing doping. The natural ferromagnetic resonance is observed. The high permeability, accompanied with the ferromagnetic resonance, enables strong microwave absorption. BaFe10.4Zn0.8Sn0.8O19 has a wide effective microwave absorption bandwidth (RL≤-10dB) of 6 GHz with the material thickness below 2.5 mm in 11.4–17.4 GHz. The dielectric loss has little contribution to the excellent microwave absorption of BaFe12-2xZnxSnxO19. The BaFe12-2xZnxSnxO19/carbon nanotube (CNT) composites significantly increase the permittivity. The high dielectric loss and the high magnetic loss further improve the microwave absorption. With a very thin thickness of 1.8 mm, BaFe10.4Zn0.8Sn0.8O19/CNT has a bandwidth of 7.1 + GHz of 10.9–––18 + GHz. With the material thickness thinner than 3 mm, BaFe12-2xZnxSnxO19/CNT composites have very broad effective microwave absorption bandwidth of 11.4+, 10.7 and 10.7 GHz, with x = 0.6, 0.8 and 1.0, respectively. It illustrates that both BaFe10.4Zn0.8Sn0.8O19 and BaFe12-2xZnxSnxO19/CNT composites are very promising microwave absorbers.

Microwave-induced intersubband scattering and magneto-oscillation phenomena in an inhomogeneous 2D electron gas on liquid helium

The theoretical description of microwave-induced intersubband scattering in a 2D electron gas exposed to a magnetic field on liquid helium with the inhomogeneous distribution of subband excitation energies is presented. We demonstrate that the alignment of the staircases of Landau levels in the ground and the first excited subbands differently affects the microwave resonant absorption and in-plane magnetoconductivity. In contrast with previously used models, the microwave excitation rate is shown to have a strong dependence on the magnetic field with sharp minima at the Landau level alignment points even in a heavily inhomogeneous 2D electron system. The inhomogeneous distribution of transverse-quantization eigenfrequencies changes the lineshape of the intersubband resonance and substantially broadens conductivity magneto-oscillations. These results explain unexpected experimental observations reported previously.

Viscoelastic and microwave resonant absorption studies of NixFe3−xO4 (0 ≤ x ≤ 0.7) mixed ferrite nanoparticle based magnetic fluid

We investigated the effect of Ni substitution on oleic acid coated Fe3O4 magnetic nanoparticles of 10–13 nm size dispersed in kerosene medium with enhanced colloidal stability. Study of molecular dynamics and coating of oleic acid to ferrite NixFe3−xO4 (0 ≤ x ≤ 0.7) nanoparticles was confirmed by Fourier transform infrared spectroscopy (FTIR) measurements. The increase of Zeta potential (ζ) up on Ni concentration confirms the stability of Ni substituted Fe3O4 magnetic nano fluid. Magnetization measurements (M-vs-H) reveals the substitution of Ni2+ suppresses the magnetic moment thereby decrease of saturation magnetization while increase in Ni concentration from x = 0–0.7 of Fe3O4 nano magnetic fluid. Moreover, field dependent elastic (G′) and viscous (G′′) modulus recorded at varying strain rate confirms a cross-over from viscoelastic-to-viscous behavior of Ni substituted Fe3O4 nanofluid. Further, the induced spin-disorder upon Ni concentration in corroboration with microwave resonant absorption (EPR) measurements through the increase of resonance field and narrowing of peak-to-peak linewidth indicating the suppression of magnetic dipole interactions and enhancement of super-exchange-interactions (Fe3+-O-Ni2+/Fe2+-O-Ni2+) . Moreover, the dominance of spin-spin interactions over the spin-lattice interactions was determined by EPR relaxation-time-scales: spin-spin relaxation (τ2) increases and spin-lattice relaxation (τ1) decreases, which is in conjunction with lande-g-factor while increasing Ni concentration. The present results are useful for knowing the effect of Ni concentration on magnetic and viscoelastic properties of NixFe3−xO4 (0 ≤ x ≤ 0.7) magnetic nanoparticles. Also, it offers a chance to create such types of stable nano magnetic fluids that may be actively controlled by outside magnetic fields, which results in heat sink damping materials in sealed devices and biomedical applications, etc.

Effect of site disorder on the resonant microwave absorption in Co2Fe0.5Ti0.5Si Heusler alloy thin films

100 nm Co2Fe0.5Ti0.5Si (CFTS) thin films were grown on Si (100) substrate at different substrate temperatures TS to study the effect of site disorder on the magnetization relaxation processes and magnetic anisotropy in them. To this end, two different ferromagnetic resonance (FMR) techniques have been employed: broad-band (3 – 17 GHz) FMR at a fixed static magnetic field angle in the ‘in-plane’ (IP) sample geometry, and X-band (9.45 GHz) FMR at different polar field-angles, θH, in the ‘out-of-plane’ (OP) sample configuration. Increase in TS reduces the degree of disorder and improves crystalline order. Improvement in crystallinity, in turn, results in the decrease in the strength of magnetocrystalline anisotropy. Irrespective of the degree of disorder present, the OP anisotropy field, Hk⊥, is an order of magnitude higher than the IP counterpart, Hk‖. Out of the CFTS thin films studied, the film (TS500), deposited at TS = 500 °C, has the maximum L21 crystalline order, maximum saturation magnetization (MS = 779 G), and the least values for Hk‖ (5.3 Oe), Hk⊥ (17 Oe), Landé splitting factor, g = 2.02, and the Gilbert damping constant, α=5.5×10-3. These optimum values of MS, g and α, at room temperature, are conducive for spintronics applications. In the OP configuration, the FMR linewidth, ΔH⊥, mainly arises from the intrinsic Landau-Lifshitz-Gilbert damping and extrinsic angular spread of crystallite misorientation. However, the extrinsic two-magnon scattering contribution to ΔH⊥, is appreciable only in the angular range 70° ≤ θH ≤ 110° .

Resonant Absorption of Microwaves by Macromolecules

Resonant Absorption of Microwaves by Macromolecules

Microwave Detection, Disruption, and Inactivation of Microorganisms

This paper reviews three complex interactions between microwave energy and microorganisms (bacteria, fungi, and viruses). The first interaction comprises the detection of viruses within human blood using a 50-Ohm transmission-line vector net-analyzer (typically 0 to 10 dBm @ 2 to 8.5 GHz) where the blood is placed within a test chamber that acts as a non-50-Ohm discontinuity. The second interaction employs 1 to 6.5 W @ 8 to 26 GHz for microwave feed-horn illumination to inactivate microorganisms at an applied power density of 10 to 100 mW-2. The third interaction is within multi-mode microwave ovens, where microorganism cell membrane disruption occurs at a few 100 s of W @ 2.45 GHz and microorganism inactivation between 300 to 1800 W @ 2.45 GHz. Within the first microwave interaction, blood relaxation processes are examined. Whereas in the latter two microwave interactions, the following disruption, and inactivation mechanisms are examined: chemical cellular lysis and, microwave resonant absorption causing cell wall rupture, and thermodynamic analysis in terms of process energy budget and suspension energy density. In addition, oven-specific parameters are discussed.

Dual-band metamaterial absorber with stable absorption performance based on fractal structure

In this paper, two kinds of dual-band metamaterial absorbers (MMAs) with stable absorption performance based on fractal structures are proposed. As the key feature, with the increase in fractal order, the fractal MMAs can reduce the weight while keeping the absorption performance. The multi-band absorption property is analyzed by multiple L-C resonances generated by the fractal structure. By virtue of good impedance matching characteristics and the synergy of the circuit and electromagnetic resonance, effective and stable microwave absorption is readily achieved. Finally, two prototypes are fabricated for demonstration, and the measurement result is consistent well with the simulation one. As expected, the proposed fractal MMAs have the advantage of low-cost, light-weight, and dual-effective absorption bands, and have great potential in the application of multi-band radar stealth.

Electromagnetic Properties of Composite Materials Based on Nd-Compound Modified Fe-Si-Al Alloy

The investigation of microstructure, optical and electromagnetic properties of powder prepared from Nd-compound doped Al6Si9Fe85 (Alsifer) alloy. The result of energy-dispersive X-ray spectroscopy is an evidence that the complementary Nd-containing Nd-Fe phase is on the surface of Alsifer particles. The magnetic particles distribution histogram in powder after mechanical treatment was plotted and analyzed. The electromagnetic properties (components magnetic permeability and dielectric permittivity) for produced composites with Nd-compound doped Alsifer powder with 50-90% (wt.) was studied. The nonlinear concentration dependence of magnetic and dielectric permittivity versus concentration Nd-compound doped Alsifer particles was observed. The magnetic resonance in composites was detected that shifted in low frequencies with increased magnetic filler concentration. The resonance field, absorption linewidth, g-factor and microwave absorption ability of Nd-compound doped Alsifer powder were determined from FMR experiment.

Spectra of Two-Dimensional “Proximity” Plasmons Measured by the Standing-Wave Method

Plasma excitations in a two-dimensional electron system partially screened by a metal strip gate are investigated using the optical detection of resonance microwave absorption. The optimal geometric parameters of the structure for the observation of standing waves corresponding to “proximity” plasmons along the strip are found. Four standing modes of proximity plasma waves are detected. The spectrum of proximity plasma excitations is determined by identifying the wave vectors corresponding to each of the modes. The measured spectrum coincides with a high accuracy with the theoretical one.

Features of the Ferromagnetic Resonance of Amorphous FeSiBNbCu Ribbons with Different Compositions

The resonance microwave absorption in amorphous FeSiBNbCu ribbons with different compositions obtained by rapid quenching from the melt has been investigated. It is shown that the effective magnetization calculated from the resonance field linearly decreases with increasing boron and copper impurity concentrations and the total number of nonmagnetic impurity atoms in the iron-based FeSiBNbCu ribbons.

Temperature dependent resonant microwave absorption in perpendicular magnetic anisotropy epitaxial films of a spinel ferrite

We report on the temperature (T), magnetic field (μ0H), and angle (Θ, Φ) dependent resonant absorption of X-band microwaves in spinel ferrite epitaxial films subjected to two distinct states of growth strain. The polar angle (Θ) dependence of the resonance field (Hres) in films with ∼0.5% ab-plane expanded unit cell establishes a distinct perpendicular magnetic anisotropy (PMA). The anisotropy field (Han┴) for Θ = 0° increases monotonically on lowering the temperature from 300 K to 90 K following the behavior of the saturation magnetization (Ms) keeping Han┴/Ms ≈ 1. The narrow resonance linewidth (μ0ΔHres┴ = 3.7 mT at 300 K) and its negligible (±0.3 mT) variation with temperature establish the magnetic softness of these PMA films. The dependence of Hres on Θ, Φ, and T in films subjected to compressive stress shows in-plane cubic anisotropy whose strength is nonmonotonic in temperature. The ∼2.0% compression of the unit cell basal plane also appears to accentuate noncollinearity of sublattice magnetization of such films as indicated by the T-dependence of ΔHres. The thicker films with PMA display spin wave resonances whose position allows determination of the spin wave stiffness constant together with independent determination of Han┴. The resonance characteristics of the PMA films qualify them as potential candidates for frequency agile microwave devices and magnonic circuit elements.

Retardation Effects for “Dark” Plasma Modes in a Two-Dimensional Electron System

Plasma excitations with different symmetries in a single two-dimensional electron disk are studied by optical detection of resonant microwave absorption. The manifestation of retardation effects for “dark” axisymmetric plasma excitations is studied for the first time. It is shown that the hybridization of these excitations with light is much smaller than the hybridization of dipole active two-dimensional plasmons. An unusual crossing of the magnetic field dispersion of “bright” and dark plasmon-polariton modes is observed in a perpendicular magnetic field.

Co substituted BaFe12O19 ceramics with enhanced magnetic resonance behavior and microwave absorption properties in 2.6 – 18 GHz

Abstract BaFe12-xCoxO19 (BFCO, x ≤ 0.4) ceramics with superior magnetic and electromagnetic (EM) properties were prepared using Co3+ substituted BaFe12O19 (BFO) in 2.6–18 GHz. Compared with the ceramics without Co3+, the incorporation of 40 mol% Co3+ enhances magnetic properties; the saturation magnetization (MS) is improved by about 55.6 emu·g−1 due to the variety of magnetocrystalline anisotropy. The resonance behaviors of complex permeability are observed and resonance frequency shifts to lower frequency range from 6 to 3 GHz. The minimum reflection loss (RL) of −32.1 dB (

Microwave spectroscopy of superfluid He II

Experimentally observed with dielectric disc resonator technique the resonant microwave absorption- amplification in superfluid He II below λ-point has been interpreted theoretically as a phenomenon in electrically active dielectric medium with low-energy excitations which exist near the ground state of the He–He interatomic bond due to fine structure of spin subsystem in condensed helium phase. The experimentally registered microwave resonant absorption line with fres = 180.3 GHz at T = 1.4 K and fres = 150.0 GHz at T = 2.1 K is closely related to the standard value of roton gap Δ/kB = 8.64 K (179.36 GHz). The measured temperature dependence of the resonant absorption demonstrates an excellent agreement with the corresponding neutron diffraction data for Δ(T) known from different literature sources. From the common point of view the obtained resonant absorption provides a typical example of the microwave spectroscopy which occurs widely among molecular systems with rotational and vibrational degrees of f...

Drastic Reduction of Plasmon Damping in Two-Dimensional Electron Disks.

In this Letter, plasmon damping has been investigated using resonant microwave absorption of two-dimensional electrons in disks with different diameters. We have found an unexpected drastic reduction of the plasmon damping in the regime of strong retardation. This finding implies large delocalization of the retarded plasmon field outside the plane of the two-dimensional electron system. A universal relationship between the damping of plasmon polariton waves and the retardation parameter is reported.

Magnetic and microwave properties of Fe18Ni82 nanoparticles with close to zero magnetostriction

Magnetic nanoparticles (MNPs) with nominal Fe18Ni82 composition were prepared by the electrophysical technique of the electrical explosion of wire. Although we were not able to experimentally estimate the magnetostriction coefficient for MNPs, this iron–nickel ratio is usually associated to close to zero magnetostriction. Different values of the overheating rate K between 0.9 and 2.0, where K is the ratio between the electrical energy ejected into the wire and the sublimation energy of Fe18Ni82 alloy, were used for the fabrication of the MNPs. Structural characterization (X-ray diffraction, scanning electron microscopy, low temperature nitrogen adsorption), magnetic and microwave measurements were used for the comparative analysis of the properties of MNPs. The increase of the overheating rate led to the increase of the specific surface value (Ssp) of the MNPs and the decrease of their average size. The fabricated MNPs were spherical in shape with a very high degree of sphericity and very weakly aggregated with the average weighted diameter in the range of 94–301 nm depending on the specific surface value for each batch. For 0.9 ≤ K ≤ 1.2 the MNPs were in the γ-phase, cubic (S.G: Fm-3 m) face-centered cubic lattice and very a small amount of material had a cubic Pm-3 m structure with lost symmetry. For 1.2 < K ≤ 2.0 they were single γ-phase, cubic (S.G: Fm-3 m) face cubic centered structure. Magnetic measurements and microwave absorption showed that the MNPs of all batches have high saturation magnetization and exhibit both microwave phenomena: significant zero field microwave absorption and resonant absorption in the vicinity of the ferromagnetic resonance field. Contrary to previous studies of FeNi MNPs with non-zero magnetostriction, the field for ferromagnetic resonance was very close to the one expected for MNPs with small contributions arising from magnetocrystalline anisotropy, strains and the deviations from sphericity. The magnetic and microwave characteristics in combination with a very high value of the single batch are promising for technological applications of these MNPs including the development of composites consisting of a polymer matrix filled with MNPs.

Broadband ferromagnetic resonance measurements in thin-film structures for magnetoimpedance sensors

Thin magnetic films and multilayer structures are commonly used for the development of micromagnetic sensors, some of them operating at high frequencies, as it is the case of magnetoimpedance sensors. At the same time, high frequency characterization techniques as ferromagnetic resonance are extremely useful to investigate material properties. In this work, we analyze the microwave resonant absorption of magnetron sputtered Permalloy-based magnetic films suitable for magnetoimpedance sensor applications. The dispersion of values of the resonant frequencies obtained in broadband ferromagnetic resonance measurements is used to assess the quality and the repeatability of the samples. The thin-film structures under test are inserted in the measuring circuit, constituting itself a part of the microwave transmission line, i.e. they are characterized in a realistic magnetoimpedance configuration, in which the alternating current flows through the sample. The ferromagnetic resonance frequency as a function of the applied field is determined from the measurement of the transmission coefficient in a network analyzer. We also present the data reduction procedure, adapted to our particular measurement test-fixture, that allows determining the ferromagnetic resonance spectra accurately. The analysis of the resonance is performed among samples identically prepared, either in the same production batch or in different batches. The dispersion observed in the results is used to estimate the fluctuations that can be expected in this kind of measurements and the amount of repeatability of sample preparation. The investigation indicates that variations of the order of 10% can be expected in measurements that are usually considered to provide the same result.

ESR studies of transition from ferromagnetism to superparamagnetism in nano-ferromagnet La0.8Sr0.2MnO3

Electron Spin resonance spectroscopy (ESR) was used to determine the magnetic properties of nanocrystalline samples of La0.8Sr0.2MnO3 having an average crystallite size ranging from 9 to 57 nm. The control of the size is performed by adopting the autocombustion method with two-step synthesis process. The main objective of this study is to determine the effect of crystallite size on the magnetic behavior of materials. As well as the domains of the sizes correspond to the superparamagnetic state and to single-domain and multi-domain ferromagnetic states. However, we have noticed that Electron Magnetic Resonance (EMR) and Low Field Microwave Absorption (LFMA) spectra are highly crystallites size dependent. Significant changes in the line shape, LFMA signal, resonance field, and linewidth (ΔHpp) can be used as indices of magnetic state transitions. Moreover, the discontinuity observed in ΔHpp and g-factor is attributed to the changes of magnetic states. Magnetic measurements showed a good agreement with ESR results. Therefore, samples having crystallite size less than 24.5 nm are in a superparamagnetic state and those between 24.5 and 32 nm are single-domain ferromagnetic. The multi-domains ferromagnetic are manifested for sizes greater than 32 nm. Above 24.5 nm in accordance with the core-shell structure, the increase in g-factor with crystallites size is attributed to the increase in core size magnetic. In the superparamagnetic zone, the practically constant value of the g-factor assumes that the volume of the core magnetic is not affected by increasing crystallites size. This contradictory observation with the core-shell model was explained by the phenomenon of phase separation often reported for manganites perovskite. Thus, the low size crystallites consist of several small ferromagnetic volumes distributed in a paramagnetic matrix, that we called crystallite multi-core superparamagnetic.

Temperature-dependent energy levels of electrons on liquid helium

We present measurements of the resonant microwave absorption by the Rydberg energy levels of surface state electrons on the surface of superfluid liquid helium, in the frequency range 165 - 220 GHz. The resonant frequency was strongly temperature dependent from 0.1 to 2 K. The experiments are in agreement with recent theoretical calculations of the renormalisation of the electron energy levels due to zero-point and thermal ripplons, analogous to a condensed matter Lamb shift. The temperature-dependent contribution to the linewidth for excitation to the first excited state at 189.6 GHz is compared with other measurements and theoretical predictions.

Azbel'-Kaner-like cyclotron resonance in a two-dimensional electron system

Resonant microwave absorption of a two-dimensional electron system in an AlGaAs/GaAs heterostructure excited by a near-field technique was investigated. Along with collective magnetoplasmon modes, we observed resonance that precisely follows the cyclotron resonance (CR) position and revealed no signs of a collective plasma depolarization shift. We show that the discovered CR mode is absent in the Faraday geometry, and it is localized at the edge of the exciting metal electrode. Such behavior points in favor of the single-particle Azbel'-Kaner nature of the discovered resonance.