The phase-structural state, magnetic structure parameters, static and microwave magnetic properties of Fe81 – 74Zr2 – 5N17 – 21 films prepared by reactive magnetron deposition were studied. As the Zr and N contents increase, the film structure changes from, so-called, mixed structure (grain size 3 – 6 nm of nanocrystalline α-Fe(Zr,N) + fcc nitride) to amorphous one. The film magnetic structure parameters (effective anisotropy field of stochastic domains D1/2〈Ha〉 = 11.8 ± 8 Oe and effective local anisotropy field D1/2Ha = 880 ± 700 Oe), saturation magnetization Ms (1.8 ± 0.3 T), coercive field Hc (11 ± 2 Oe), and the real permeability µ′ (80 ± 30), retaining its value at frequencies up to 3 GHz, were determined. The values of the initial permeability µ0 (70 ± 21) obtained from the hysteresis and the real permeability µ′ (80 ± 30) obtained from microwave measurements are in good agreement. The correlation between the static magnetic properties, the stochastic magnetic structure parameters and the real permeability µ′ was considered. There is no any sign on ferromagnetic resonance in the studied frequency range. However the Acher’s coefficient value (less than 0.38) for this frequency range indicates the possibility of ferromagnetic resonance at higher frequencies.

Changes in the microwave permeability of permalloy films with an increase in the film thickness are studied. Measurement data on the evolution of microwave permeability with film thickness are analyzed in the framework of a model for the film with a regular stripe domain structure and out-of-plane magnetic anisotropy. A correlation between the microwave magnetic properties and magnetic structure of permalloy films is established. It is demonstrated that the observed decrease in the ferromagnetic resonance frequency and the static permeability with a growth in the film thickness can ascribed to the appearance of perpendicular anisotropy and the formation of a stripe domain structure. The calculated dependences of the ferromagnetic resonance frequency and static permeability on the film thickness are in reasonable agreement with the measurement results. Based on the analysis of these dependences, the domain width in the permalloy films is estimated. It is found that for thick permalloy films, the domain width is of the order of the film thickness. The results obtained may be useful for high-frequency applications of soft magnetic films.

Nanocrystalline Fe100–56.8Ti0–13.5B0–34.2 films 1.4 µm thick are prepared by dc magnetron deposition on glass substrates. The structure and magnetic properties of the films are comprehensively characterized and analyzed. The lattice parameter and the grain size of the bcc Fe-based phase (2-25 nm), parameters of the stochastic magnetic structure (the relative size and the effective anisotropy field of stochastic domains D1/2 and the local magnetic anisotropy field at the grain scale), saturation magnetization Ms, coercive field Hc, static permeability mst, and ferromagnetic resonance frequency fr of the films are quantitatively estimated and their interrelations are studied. As Ti and B contents increase, the phase composition of the films changes in accordance with the sequence αFe - αFe(Ti) - αFe(Ti) + TiB2 - amorphous, wherein Ms decreases from 2.1 to 0.3 T. The Hc values vary in the interval 7–70 Oe determined by the D1/2 field or by other magnetic anisotropy sources. The μst values vary in the interval 20–140. The μst values obtained by the Lorentzian dispersion law adequately fall within the range of the calculated permeabilities, which is limited by the coercive field obtained from the hysteresis loops and the anisotropy field determined by Kittel equation. The μst values of the films are kept up to the frequencies of at least 1.5 GHz. The frequency dependences of the permeability of the films are analyzed taking into account the influence of skin effect and are considered in terms of Acher’s law. The obtained values of the Acher’s constant (less than 0.3) indicate the possibility of reaching the higher values of μst and fr at the expense of elimination of the perpendicular anisotropy, in particular, via the formation of thinner films as compared to the films under study (1.4 µm thick). According to the available literature data, the presented investigations, using the FeTiB films as an example, were performed for the first time.

The relationship between the chemical purity of one-size particles and microwave properties in ferromagnetic materials is not clearly studied. Ferromagnetic nanostructured iron powders were synthesized from iron nitrate solution using ultrasonic spray-pyrolysis and then reduced in H2 flow at 350, 400, 450, and 500 °C. A rise in the concentration of solutions of a precursor from 10 to 20 wt. % led to an increase in mean particle size. The interrelationship was studied between chemical composition and the microwave dispersion of the powders obtained. An increase in the temperature of reduction changes the chemical composition and increases the amplitude of complex microwave permeability, which was studied using solid-state physics methods (XRD, STA, SEM, and VNA). It was found that annealing at 400 °C is the optimal treatment that allows the production of iron powders, consisting of about 90% of α-Fe phase, possessing a particle surface with low roughness and porosity, and demonstrating intense microwave absorption. Annealing at a higher temperature (500 °C) causes an even higher increase in permeability but leads to the destruction of nanostructured spheres into smaller particles due to grain growth. This destruction causes an abrupt increase in permittivity and therefore significantly reduces potential applications of the product. The insight into chemical–magnetic relationships of these materials enhances the data for design applications in magnetic field sensing.

Mixing rules may be extremely useful for predicting the properties of composite materials and coatings. The paper is devoted to the study of the applicability of the mixing rules to permittivity and permeability and the possibility of retrieving the intrinsic properties of inclusions. Magnetically soft Ni-Zn ferrites are chosen as the object of the study due to their low permittivity and the negligible influence of the skin effect. Due to this, the microwave properties of bulk ferrites may be measured by standard techniques. It is suggested to perform the analysis of the microwave properties of composites filled with Ni-Zn ferrite powder in terms of the normalized inverse susceptibility defined as the volume fraction of inclusions divided by the effective dielectric or magnetic susceptibility of the composite. The measured properties of the bulk ferrite are compared with those obtained by mixing rules from composite materials. The experimental evidence for difference between the mixing rules for permittivity and permeability of a composite, which was previously predicted only theoretically, is obtained. The reason for the difference is considered to be the effect of non-ideal electrical contacts between neighboring inclusions. It is also experimentally shown that the measured permeability of the bulk material may differ from the retrieved one. The measured static permeability is 1400 and the retrieved one is 12. The reason for the discrepancy is the difference between the domain structures and demagnetizing fields of particles and bulk ferrite.

A theoretical and numerical investigation of the quality criteria for radar absorbing coatings (RACs) has been conducted. Simple analytical expressions for the minimum of the reflection coefficient of a homogeneous absorbing layer have been obtained. It is shown that this value is not a correct characteristic of radar absorption. To describe the quality of RACs, it is necessary to use either the relative bandwidth of its operating waveband or the ratio of the operating waveband to the layer’s thickness. The former of these quantities may be useful when it is important to achieve a high broadband absorbency, while the latter is useful for achieving a small thickness of an absorber.

A study is performed of using a transverse electromagnetic wave approximation to measure magnetic permeability in symmetric and asymmetric strip transmission lines via electrodynamic modeling. A longitudinal component of a wave that results in a substantial measuring error is shown to arise upon the nonuniform filling of a line’s cross section with the studied samples.

The Fe99.2 divided by 56.6Ti0 divided by 13.2B0 divided by 34 films were prepared by de magnetron deposition on glass substrates. X-ray diffraction analysis was used to determine the phase composition, volume fractions of formed crystalline phases, their grain sizes and microstrains on the grain scale. The magnetic hysteretic properties, saturation induction B-s (2.1-0.3 T), coercive field H-c (0.6-6 kA/m), relative remanence B-r/B-s (0.07-0.37), were determined using measured hysteresis loops. Their shapes indicate the existence of intergranular exchange interaction in all studied films. The magnetic structure parameters of the films were determined by correlation magnetometry. All films are characterized by a stochastic magnetic structure. The magnetic properties and the magnetic structure parameters of the films are substantiated by their phase and structural states. The values of the local parameters of the magnetic structure (rms local anisotropy field (DHa)-H-1/2 and exchange field H-R) follow the parameters of the grain (lattice parameter and grain size), while the macroscopic parameters (H-c and rms stochastic domain anisotropy field D-1/2 ) have a more complex behavior.

The work is devoted to the results of calculating the effective microwave dielectric properties of a composite material based on periodically arranged conductive inclusions in the form of flat square plates. The calculation was carried out by the finite element method. It is shown that among the currently known mixing formulas, the best agreement with the obtained calculated data is observed when using the Odelevsky formula, in which the effective form factor of an ellipsoid inscribed in the inclusion volume is used. A significant contribution of surface effects at the boundary of an inhomogeneous material to its effective permittivity is demonstrated.

A composite containing about 30% volume of micrometer-size powder of gadolinium in paraffin wax is synthesized mechanochemically. The composite permittivity and permeability are measured within the frequency range from 0.01 to 15 GHz and the temperature range from ~0 °C to 35 °C. The permittivity is constant within the measured ranges. Curie temperature of the composite is close to 15.5 °C, the phase transition is shown to take place within a temperature range about ±10 °C. The effect of temperature deviation from Curie point on reflection and transmission of a composite layer filled with Gd powder is studied experimentally and via simulation. Constitutive parameters of the composite are measured in cooled coaxial lines applying reflection-transmission and open-circuit-short-circuit techniques, and the measured low-frequency permeability is in agreement with the values retrieved from the published magnetization curves. The effect of temperature on permeability spectrum of the composite is described in terms of cluster magnetization model based on the Wiener mixing formula. The model is applied to design a microwave screen with variable attenuation; the reflectivity attenuation of 4.5 mm-thick screen increases from about −2 dB to −20 dB at 3.5 GHz if the temperature decreases from 25 °C to 5 °C.

Hollow ferromagnetic powders of iron were obtained by means of ultrasonic spray pyrolysis. A variation in the conditions of the synthesis allows for the adjustment of the mean size of the hollow iron particles. Iron powders were obtained by this technique, starting from the aqueous solution of iron nitrate of two different concentrations: 10 and 20 wt.%. This was followed by a reduction in hydrogen. An increase in the concentration of the solution increased the mean particle size from 0.6 to 1.0 microns and widened particle size distribution, but still produced hollow particles. Larger particles appeared problematic for the reduction, although admixture of iron oxides did not decrease the microwave permeability of the material. The paraffin wax-based composites filled with obtained powders demonstrated broadband magnetic loss with a complex structure for lesser particles, and single-peak absorption for particles of 1 micron. Potential applications are 5G technology, electromagnetic compatibility designs, and magnetic field sensing.

Znx-1NixFe2O4 samples have been synthesized by solid-state reactions. Data on the chemical composition and the surface morphology of the samples have been obtained using a scanning electron microscope. X-ray powder diffractometer has been used to establish the phase purity and to determine the unit cell parameters. It has been found that the obtained samples had the spinel structure with Fd-3m (No. 227) space group. The unit cell parameters decrease with increasing nickel concentration. The magnetic characteristics of the obtained samples are determined and discussed. The Curie point of obtained samples varies in the range of 803.5-572.7 K. The maximum spontaneous magnetization of similar to 74.6 emu/g at room temperature was fixed for the solid solution with x = 0.6. The maximum value of the mu(/) real part of similar to 12 and mu(//)imaginary part of 6 of the permeability in the frequency range of 50 MHz-10 GHz is observed for the composition with x = 0.3. The composite samples for microwave study were prepared by mixing the ferrite powders with molten paraffin wax. The largest value of the mu(/) real part of similar to 3 and mu(//) imaginary part of 0.63 of permeability is found for the x = 0.4 composite. The formation of the composite significantly reduces permeability. (C) 2021 Elsevier Ltd. All rights reserved.

Composite materials filled with ferromagnetic inclusions are useful in the development of various microwave devices. The performance of such devices is determined both by material properties (such as the saturation magnetization and the permeability) and by the demagnetization effects. The paper is devoted to the study of the demagnetization effect on the permeability measurements of composites under external magnetic bias. The microwave permeability of composites filled with flake sendust (Fe-Si-Al alloy) particles is measured as a function of frequency and the external magnetic field. The measurements are carried out by the Nicolson–Ross–Weir technique in a 7/3 coaxial line in the frequency range of 0.1 to 20 GHz by a vector network analyzer. It is found that the magnetic loss peak is split under external fields of more than 1.5 kOe. The main aim of this paper is to study the causes of this splitting and to interpret the observed magnetic loss peaks. To study this effect, the samples of various thicknesses and the samples with isotropic and anisotropic orientations of particles are measured. The particles in the anisotropic samples are oriented by a strong uniform magnetic field. At a small fraction of inclusions, the permanent magnetic field is demagnetized on the individual particles rather than the whole sample. The splitting of the magnetic loss peak of the isotropic sample is caused by different orientations of particles in the sample. At a high fraction of inclusions, the permanent magnetic field is demagnetized on the whole sample and the magnetic loss peak of the isotropic sample is not split. The saturation magnetization of the material is found by measurements under the external magnetic field of the anisotropic sample.

The frequency dependencies of microwave permeability of composites filled with flake-shaped sendust (Fe-Al-Si alloy) powder particles are measured. A technique for determining the physical mechanisms resulting in the appearance of magnetic loss peaks is proposed. The technique consists in analysis of the data on microwave permeability obtained under external magnetic field. It is shown that the mixing rule, which correctly retrieves the intrinsic permeability in the absence of the external field, is not valid under magnetic bias. Based on the measured data, the physical mechanisms leading to the appearance of loss peaks are determined, and the invalidity of mixing rules under magnetic bias is explained. It is shown that the main peak of magnetic loss is attributed to the Polder-Smith modes. An increase in the external field leads to vanishing of the domain structure and to an increase in the interaction between powder particles. The permeability under bias should be described by mixing rules that take into account the interaction between inclusions. The found frequencies of the Polder-Smith modes are in good agreement with the permeability data.

Reactive sputtering of permalloy typically reduces crystallite size in films, if the concentration of reactive gas is low. This advances magnetic properties: decreases coercivity and increases good in-plane magnetic anisotropy. But oxygen is rarely applied for this purpose. Here, peculiar deposition conditions, including the geometry of the vacuum deposition system and large polymer substrate, allowed for the deposition of supermalloy with unusually high admixture of oxygen. Evolution of both static and dynamic magnetic properties with an increase in oxygen concentration proved to be dealt with the balance between grain size, surface structure, disordering of atomic structure and internal stresses. The critical concentration when supermalloy shows indications of oxidation is 5% of O2. Coercivity, resistivity, roughness and ferromagnetic resonance frequency are the parameters that are most sensitive to oxidation.

The possibility of determining the magnetic susceptibility of metal inclusions from measured material parameters of binary composites using known mixing models is considered. A model is proposed that considers the inversion of the matrix structure of the Maxwell–Garnett model for a filler concentration close to the percolation threshold. It is determined that restructuring the composite occurs gradually in the region of transition concentrations the width of which is an additional parameter determined from the experiment. The proposed model is compared with the effective medium model in terms of the dependences of complex susceptibility on filling and frequency. The reliability of the model is illustrated by processing the material parameters of composites with carbonyl nickel as a filler measured in the frequency range 0.1–20 GHz.

Increase in the spontaneous magnetization up to x < 0.5 is a sequence of appearance of the additional magnetic moment of 2 μB of the Ni2+ cations in tetrahedral positions. For x > 0.5, the moment decreases in octahedral positions and the spontaneous magnetization also decreases.

This paper presents a comparative analysis of the mixture models used in radio physics to describe the effective properties of composite materials and formulates the limits of their applicability and criteria for checking their correctness. Two of the most general models have been chosen, which take into account the transformation of the composite structure upon changes in its composition. The correspondence of the chosen model to the experimental results is considered. The effect of the composite structure on the frequency of the maximum and the shape of its dielectric absorption line is shown. The analysis shows that the complication of the models in comparison with those already known and an increase in the number of parameters determined from the experiment is inappropriate at the existing level of the composition and material parameters (permittivity and permeability) measuring error and due to a significant contribution of the size and surface effects, which is not taken into account in quasi-static mixture models.