Microwave Absorption Spectra Research Articles

Microwave magnetic excitations in U-type hexaferrite Sr4CoZnFe36O60 ceramics

Microwave (MW) transmission, absorption, and reflection loss spectra of the ferrimagnetic U-type hexaferrite Sr4CoZnFe36O60 ceramics were studied from 100 MHz to 35 GHz at temperatures between 10 and 390 K. Nine MW magnetic excitations with anomalous behavior near ferrimagnetic phase transitions were revealed. They also change under the application of the weak bias magnetic field (0–700 Oe) at room temperature. Six pure magnetic modes are assigned to dynamics of the magnetic domain walls and inhomogeneous magnetic structure of the ceramics, to the natural ferromagnetic resonance (FMR), and to the higher-frequency magnons. Three modes are considered the magnetodielectric ones with the dominating influence of the magnetic properties on their temperature and field dependences. The presence of the natural FMR in all ferrimagnetic phases proves the existence of the non-zero internal magnetization and magnetocrystalline anisotropy. Splitting of the FMR into two components without magnetic bias was observed in the collinear phase and is attributed to a change in the magnetocrystalline anisotropy during phase transition. The high-frequency FMR component critically slows down to phase transition. At room temperature, FMR splitting and essential suppression of the higher-frequency modes were revealed under the weak bias field (300–700 Oe). The highly nonlinear MW response and FMR splitting are caused by the gradual evolution of the polydomain magnetic structure to a monodomain one. The high number of magnetic excitations observed in the MW region confirms the suitability of using hexaferrite Sr4CoZnFe36O60 ceramics as MW absorbers, shielding materials and highly tunable filters.

Variance properties of the microwave absorption spectrum of an ensemble of nitrogen vacancy centers in diamond

This work presents an original method based on the variance properties of the microwave absorption spectrum of an ensemble of nitrogen vacancy centers in diamond. The spectrum is measured optically. A compact and simple device is designed to optimize the photon collection. We conduct a quantitative comparison of the ensemble's optical signal in both the visible and near infrared range. Using the enhanced signal-to-noise ratio achieved through the device geometry we perform real-time DC magnetometry at moderate light and microwave powers. Under these conditions, the amplitude of a DC magnetic field can be extracted from the variance of the microwave absorption spectrum in a fast and reproducible manner, without the burden of complex fitting techniques.

Enhanced microwave absorption properties of magnetite nanoparticles decorated multi walled carbon nanotubes loaded polyaniline/polyvinyl alcohol nanocomposites

AbstractElectromagnetic radiation interference (EMI) or pollution is increased significantly in proportion to the expansion of electronic and communication industries, which lead to the disturbance of the functioning of electronic gadgets and medical equipments, in addition to the human health damage. The Microwave absorbing (MA) composite comprised of polyvinyl alcohol (PVA), polyaniline (PANI), carbon nanotube (MWCNT) and iron oxide (Fe3O4) has been developed with various proportions, denoted as PVA/PANI/MWCNT/Fe3O4 (1%, 3%, 5%). Fe3O4 was prepared using a hydrothermal method, and PVA/PANI/MWCNT/Fe3O4 (1%, 3%, 5%) nanocomposites was made by solvent casting techniques. The PVA composite films were also subjected to a range of analyses including IR, TGA, DTA, DTG, SEM, TEM, dielectric measurements and EMI shielding effectiveness (EMI SE), to describe their characteristics. The results of various studies indicate that the PVA/PANI/MWCNT/Fe3O4 (1%, 3%, 5%) composites possess enhanced thermal and dielectric characteristics, for instance the PVA/PANI/MWCNT/5% Fe3O4 blend nanocomposites possess 10% initial decomposition at 220°C and dielectric constant of 3.2 × 107 at 1 MHz. The total EMI SE value for the synthetic PVA/PANI/MWCNT/Fe3O4 was found at 9.2 and 10 GHz is 48 and 54 dB, respectively, exhibiting synergistic properties of the composites toward EMI shielding. The microwave absorption spectra indicate two absorption peaks in the region around 8 to 12 GHz, with the absorption reflection loss (RL) value of −20 dB at 10 GHz for the PVA/PANI/MWCNT/5% Fe3O4 nanocomposites.

Plasmons in a square of two-dimensional electrons

Microwave absorption spectra of a single square of two-dimensional electrons (2DESs) have been investigated using an optical detection technique. Fundamental dipole and harmonic quadrupole plasmon modes have been identified and compared to those in the disk geometry. In the square-shaped 2DES, a strong interaction is discovered between the neighboring plasmon modes, whereas no such hybridization is observed in the disk-shaped geometry. We establish a rigid theoretical platform to analytically describe the magneto-optical response of confined two-dimensional systems. The developed theory provides a proper description of the obtained experimental results.

Температурные зависимости межслойной обменной константы трехслойных пленок FeNi/Dy/FeNi, исследованные динамическим методом

Three-layer FeNi/Dy/FeNi films were studied by the ferromagnetic resonance method in the temperature range from 4 to 300 К. The detected acoustic and optical peaks in the microwave absorption spectra demonstrate that there is exchange coupling between the ferromagnetic FeNi layers of the planar system, which allows us to define the sign and value of the interlayer exchange interaction constant. The temperature dependences of the interlayer exchange constant of the three-layer films with an interlayer thickness of Dy equal to 5 and 10 nm show a number of features (change of the sign and extreme point) which reflect the transformation of the Dy magnetic structure

The analysis of the microwave absorption spectrum of trans-ethyl methyl ether in the skeletal torsion v30 = 2 state

The microwave absorption spectrum of the trans-ethyl methyl ether in the skeletal torsional v30 = 2 state was analyzed for the first time. The trans-ethyl methyl ether molecule has two inequivalent methyl group internal rotors. Splitting due to the methyl group connected to the oxygen was dominant and observed in the v30 = 2 state. This result was supported by considering the torsional force field in the same way as in the ground and the v30 = 1 states. Over 800 lines up to J = 55 and Ka = 6 were assigned. The spectrum was analyzed using an IAM-like matrix formulation. Three rotational constants, eight centrifugal distortion constants, and four O-CH3 internal rotation tunneling parameters were determined for the v30 = 2 state. It was also made clear in the present analysis that the tunneling parameter dominant concerning the splitting due to the O-CH3 internal rotation has the same sign as that of the v30 = 1 state, and the sign opposite to that of the ground state.

A tunable broadband microwave absorber based on coherent population trapping

A scheme is proposed for a tunable broadband microwave absorber using a four-level Rydberg atom system with competition between electromagnetically induced transparency (EIT) and coherent population trapping (CPT). Two laser fields drive the Rydberg atoms from the ground state to a Rydberg state, forming a cascade-type EIT/CPT system. When a microwave (MW) field couples the Rydberg transition, a strong MW absorption peak appears. A narrow band MW absorber could be obtained via EIT, however, using CPT, the MW absorption spectrum can be significantly broadened and enhanced to have a bandwidth about ten times larger than that obtained with EIT and whose peak value can also be enhanced by a factor of ten, based on simulation.

Microwave Giant Magnetoresistance and Ferromagnetic and Spin-Wave Resonances in (CoFe)/Cu Nanostructures

The microwave phenomena that occur in magnetic multilayer (CoFe)/Cu nanostructures, which have a giant magnetoresistance, are studied. The transmission of waves through a nanostructure is used to investigate the microwave giant magnetoresistance effect. The changes in the transmission coefficient at frequencies of 29–38 GHz are found to exceed the relative magnetoresistance, which distinguishes the system under study from the nanostructures studied earlier. Ferromagnetic and spin-wave resonances are used to study the angular dependences of the microwave absorption spectra of a multilayer (CoFe/Cu)n nanostructure. The following parameters are determined: the critical angle that determines the boundaries of the ranges of excitation of uniform and nonuniform spin modes, the type of boundary conditions describing the pinning of spins on the outer nanostructure surfaces, and the surface anisotropy and exchange interaction constants.

Delocalization in two and three-dimensional Rydberg gases

As was recently shown in Abumwis et al (2020 Phys. Rev. Lett. 124 193401), many eigenstates of a random Rydberg gas with resonant dipole–dipole interactions are highly delocalized. Although the high degree of delocalization is generic to various types of power-law interactions and to both two and three-dimensional systems, in their detailed aspects the coherence distributions are sensitive to these parameters and vary dramatically between different systems. We calculate the eigenstates of both two and three-dimensional gases and quantify their delocalization throughout the atoms in the gas using a coherence measure. By contrasting the angular dependence of the dipole–dipole interaction with an isotropic interaction we obtain additional information about the generic physical principles underlying random interacting systems. We also investigate the density of states and microwave absorption spectra to obtain information about the types of measurements where these delocalized states play a role, and to check that these delocalized eigenstates are robust against various types of perturbation.

Linear chains of nanomagnets: engineering the effective magnetic anisotropy.

This paper investigates the control of effective magnetic anisotropy in Permalloy linear chain arrays, achieved by tuning the symmetry arrangement of the ellipsoidal nanomagnets and the film thickness. When the ellipsoidal nanomagnets are coupled along their easy axis, stronger effective magnetic anisotropy is achieved compared to when the nanomagnets are coupled along their hard axis. A clear transition from a single domain state to a combination of complex flux closure states such as a vortex or double vortices is observed at different applied field angles when the film thickness is varied in the range from 20 nm to 100 nm. Tunable microwave absorption spectra, obtained by ferromagnetic resonance spectroscopy, established the complex interplay between the shape anisotropy and magnetostatic interactions, which becomes more intriguing at different film thicknesses and applied field angles. The micromagnetic simulations are in good agreement with the experimental results. Our results demonstrate possible ways of manipulating the effective magnetic anisotropy in arrays of nanomagnets for magnonic and microwave applications.

Ferromagnetic and Spin-Wave Resonance in the [CoFe/Cu]N Superlattice Thin (30-nm) Film

Angular dependences of the microwave absorption spectra of a (CoFe/Cu)N multilayer film have been studied by the ferromagnetic and spin-wave resonance techniques. The critical angle θc indicating the ranges of excitation of uniform and nonuniform spin modes, type of the boundary conditions, and surface anisotropy and exchange coupling constants have been established. It is shown that the accuracy of identification of individual modes in the spectra plays a key role in the analysis of the detected curves.

Observation of “Dark” Axisymmetric Plasma Modes in a Single Disk of Two-Dimensional Electrons

The microwave absorption spectra of a two-dimensional electron system based on GaAs/AlGaAs heterostructures in a perpendicular magnetic field are experimentally investigated. A unique noninvasive technique for the excitation of “dark” axisymmetric plasma oscillations in single disks of two-dimensional electrons is developed. The excitation conditions and physical properties of the new dark plasma modes are studied in detail. The experimental results are in excellent agreement with theoretical calculations.

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.

Resonant Dipolar Coupling of Microwaves with Confined Acoustic Vibrations in a Rod-shaped Virus

In this letter, we treat a rod-shaped virus as a free homogenous nanorod and identify its confined acoustic vibration modes that can cause strong resonant microwave absorption through electric dipolar excitation with a core-shell charge distribution. They are found to be the n = 4N-2 modes of the longitudinal modes of the nanorods, where N is an integer starting from 1 and n is the mode order quantum number. This study was confirmed by measuring the microwave absorption spectra of white spot syndrome virus (WSSV), which is a rod-shaped virus. This is also the first study to identify the “dipolar-like” mode in a rod-shaped nano-object. Our study is not only an important step to achieve rapid and sensitive detection of rod-shaped viruses based on their microwave spectroscopic features and a non-contact method to measure the Young’s modulus of rod-shaped viruses, but also is critical to formulate an efficient epidemic prevention strategy to deactivate viruses with the structure-resonant microwaves.

Temperature-dependent magnetic damping of yttrium iron garnet spheres

We investigate the temperature dependent microwave absorption spectrum of an yttrium iron garnet sphere as a function of temperature (5 K to 300 K) and frequency (3 GHz to 43.5 GHz). At temperatures above 100 K, the magnetic resonance linewidth increases linearly with temperature and shows a Gilbert-like linear frequency dependence. At lower temperatures, the temperature dependence of the resonance linewidth at constant external magnetic fields exhibits a characteristic peak which coincides with a non-Gilbert-like frequency dependence. The complete temperature and frequency evolution of the linewidth can be modeled by the phenomenology of slowly relaxing rare-earth impurities and either the Kasuya-LeCraw mechanism or the scattering with optical magnons. Furthermore, we extract the temperature dependence of the saturation magnetization, the magnetic anisotropy and the g-factor.

Magnetomechanical coupling and ferromagnetic resonance in magnetic nanoparticles

We address the theory of the coupled lattice and magnetization dynamics of freely suspended single-domain nanoparticles. Magnetic anisotropy generates low-frequency satellite peaks in the microwave absorption spectrum and a blueshift of the ferromagnetic resonance (FMR) frequency. The low-frequency resonances are very sharp with maxima exceeding that of the FMR, because their magnetic and mechanical precessions are locked, thereby suppressing Gilbert damping. Magnetic nanoparticles can operate as nearly ideal motors that convert electromagnetic into mechanical energy. The Barnett/Einstein-de Haas effect is significant even in the absence of a net rotation.

Spin pumping and the inverse spin hall effect in single crystalline Fe/Pt heterostructure

Spin pumping effect in single crystalline Fe/Pt bilayer has been systematically studied by the measurements of the microwave absorption spectrum and the inverse spin hall voltage detection. The gilbert damping constant of Fe first increases with Pt thickness and then saturates at tPt>1.5 nm. The spin diffusion length can be determined as 1.5±0.4 nm, and the spin mixing conductance is (3.4±0.4)×1019 m-2. The inverse spin hall voltage is quantitatively separated from the spin rectification effect through the measurement of the magnetization angular dependence, and the estimated spin hall angle of Pt is 0.048±0.015, in consistent with the values determined in polycrystalline Pt films.

Spin-Crossover Materials towards Microwave Radiation Switches.

Microwave electromagnetic radiation that ranges from one meter to one millimetre wavelengths is finding numerous applications for wireless communication, navigation and detection, which makes materials able to tune microwave radiation getting widespread interest. Here we offer a new way to tune GHz frequency radiation by using spin-crossover complexes that are known to change their various physical properties under the influence of diverse external stimuli. As a result of electronic re-configuration process, microwave absorption properties differ for high spin and low spin forms of the complex. The evolution of a microwave absorption spectrum for the switchable compound within the region of thermal transition indicates that the high-spin and the low-spin forms are characterized by a different attenuation of electromagnetic waves. Absorption and reflection coefficients were found to be higher in the high-spin state comparing to the low-spin state. These results reveal a considerable potential for the implementation of spin-crossover materials into different elements of microwave signal switching and wireless communication.

Non-resonant Microwave Absorption in Nano Nickel Added YBCO Powders: Observation of Multiple Phase Reversals

We report on the non-resonant microwave absorption in the system of a nominal 2 wt % nano nickel particles added into YBCO powders (Ni–YBCO). With this dilute mixture of nano nickel particles, one is expected to have groups of normal Josephson junctions (JJs) and π JJs due to YBCO–nickel–YBCO interparticle weak links and as nickel is ferromagnetic. We experimentally show, for the first time multiple phase reversals in the non-resonant microwave absorption (NRMA) spectra from Ni–YBCO possibly due to the formation of π JJs. We also showed that these multiple phase reversals then depend on microwave power and temperature. We argue that microwave power induced coherence among some groups of JJs and breaking of some of the weaker JJs can then lead to the disappearance of multiple phase reversals at higher microwave power levels. Further, we also reported a role of pair breaking effects that shall give a linear field dependence of the derivative microwave absorption signal, essentially which is the NRMA signal. This pair breaking effect dominates at closer temperatures to T c which is expected thermodynamically.

Thickness and Composition Tailoring of K- and Ka-Band Microwave Absorption of BaCo x Ti x Fe(12−2x)O19 Ferrites

The goal of this research is to investigate the electromagnetic and microwave absorption properties of M-type barium hexaferrites with chemical formula BaCo x Ti x Fe(12−2x)O19 (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) in K and Ka band. Characterization techniques such as x-ray diffraction analysis and scanning electron microscopy were applied to confirm ferrite formation. The frequency dependence of the complex permittivity and complex permeability was studied for prepared ferrite samples in the frequency range from 18 GHz to 40 GHz. Factors such as the quarter-wavelength condition, impedance matching, high dielectric–magnetic losses, as well as ferromagnetic resonance were investigated to determine their contribution to the absorption characteristics. It was found that the quarter-wavelength (λ/4) model could be successfully applied to predict and understand the position as well as number of reflection peaks in the microwave absorption spectrum. The origin of the reflection loss peaks is explained and verified based on calculations of input impedance, loss tangent, and ferromagnetic resonance. Reflection loss analysis revealed that all six compositions exhibited reflection loss peaks (absorption >90%) at their matching thicknesses and frequencies. Therefore, these ferrites are potential candidates for use in electromagnetic shielding applications requiring low reflectivity in K and Ka band.