D. A. Petrov

Structure, magnetostatic properties, and microwave characteristics of mechanoactivated nanocrystalline Fe and Fe-Si powders

The effect of the chemical composition and structural and morphological features of Fe and Fe87Si13 powders milled in an inert atmosphere of Ar and a 3% solution of oleic acid in heptane (H + OA) on the magnetostatic and microwave properties has been studied in this work. Irrespective of the milling medium, all the samples were nanocrystalline. The powder particles obtained in Ar have a stonelike shape; those obtained in H + OA, have a platelike (flaky) shape. The effect of the particle shape manifested itself in the processes of magnetization and in the qualitative and quantitative type of the frequency dependences of the dielectric permittivity and magnetic permeability in a frequency range from 0.1 to 3 GHz.

Microwave absorbing properties of Fe powders milled in various media

Principal factors determining the microwave-absorption material parameters (shape, size, and chemical and phase compositions of the particles) and their dispersion relations in a range from 0.1 to 3 GHz were determined for composites containing milled Fe particles as the filling agent. The basic physical mechanisms of the effect of the aforementioned factors were assumed to be the domain-wall resonance and ferromagnetic resonance.

Laminates of Thin Ferromagnetic Films for Microwave Applications

Thin ferromagnetic films are known to exhibit the highest possible microwave permeability of known magnetic materials. Magnetic materials with high microwave magnetic performance are useful for many technical applications. However, bulk rather than planar materials are frequently needed for the applications. Bulk materials with high microwave permeability may be produced as laminated structures of thin ferromagnetic films, possibly patterned. The paper presents experimental data on the microwave permeability of such laminated structures based on Fe films. Possible technical applications of the materials under study include thin wideband radar absorbers and miniaturized patch antennas.

Microwave properties of powders produced by high-energy milling of iron with paraffin

The structural and phase composition and magnetostatic and microwave properties of powders produced by simultaneous milling of carbonyl iron and paraffin taken in volume ratios ranging from 10 : 90 to 80 : 20 have been studied. It has been shown that the shape of the powder particles depends substantially on the amount of Fe in the mixture. At an Fe content of 30 vol %, the particles acquire a platelet shape; at an Fe content of more than 40 vol %, the agglomeration of the particles is observed. At an Fe content above 30 vol %, the Fe3C phase is formed in the particles, which leads to an increase in the coercive force and a decrease in the specific saturation magnetization of the powder. The frequency dependences of the microwave magnetic and dielectric permeabilities of the composite materials containing the prepared powders have been measured, and the frequency dependences of the intrinsic magnetic permeability of these powders have been determined. It has been shown that the static magnitude of the intrinsic magnetic permeability reaches a maximum in powders containing 30 vol % Fe in the initial mixture.

Structural-phase composition, structure of the surface, magnetostatic and microwave properties of powders produced by milling of Fe in polystyrene with additions of surfactants

The effect of additions of surfactants (stearic and perfluorononanoic acids, stearylamine, and of their mixtures) on the morphology, structural-phase composition, structure of the surface, and magnetostatic and microwave properties of ferromagnetic powders obtained by the joint high-energy milling of Fe and polystyrene has been investigated. It has been shown that the use of a mixture of stearic and perfluorononanoic acids during milling made it possible to obtain particles of plate-like shape with minimum changes in the phase composition and to produce shells on their surface that consist of the surfactant molecules. All of these factors have positively affected the microwave properties of the composites prepared from thus obtained powders.

Influence of shape and chemical and phase compositions of iron-containing particles on microwave performance of composites with an insulating matrix

The structure and microwave magnetic properties of Fe powders grounded in argon or acetone and also of Fe-Si-C and amorphous Fe-Co-Si-C powders mechanically alloyed in argon are studied using X-ray diffraction, Mössbauer spectroscopy, granulometric and microscopic analyses, magnetostatic measurements, and microwave spectroscopy. The goal of investigation is to determine the influence of factors (shape, size, and chemical and phase compositions of grains) governing the microwave material parameters of composites based on these alloys in the frequency range 0.1–3.0 GHz. It is shown that the difference in the grain shape is the basic reason for the difference in the microwave permeability at low frequencies (3 GHz or lower). At higher frequencies, the magnetic properties are related to the skin effect and depend largely on the grain size. The differences in the microwave properties of the composites are not significant and are concealed by the above effects.

Metamaterials Fabricated of Amorphous Ferromagnetic Microwires: Negative Microwave Permeability

Microwave permeability of glass-coated ferromagnetic amorphous microwire exhibiting a weak negative magnetostriction has been studied. The diameter of the microwire was about 20 m and the diameter of the metal core was about 12 m. The microwire was wound to comprise a 7/3 washer-shaped composite sample with the volume fraction of magnetic constituent of about 10%. The permeability of the composite sample was measured in a coaxial line in the frequency range from 0.1 to 10 GHz. The composite was found to exhibit a negative permeability within the frequency range from approximately 0.7 to 1.5 GHz, with the permeability being as low as −0.4. Therefore, microwire-based composites, particularly, crossed arrays of microwires may be employed to develop metamaterials for microwave applications. In the composite, the negative microwave permeability is due to the natural ferromagnetic resonance and the negative microwave permittivity is due to the inherent inductance of the wire. Such metamaterials are advantageous in simple design, isotropic in-plane performance, and possible tunability of performance by external magnetic bias. However, for a feasible metamaterial fabricated from microwire arrays, the wires have to exhibit higher magnitude of the ferromagnetic resonance, higher quality factor, and higher resonance frequency.

Effect of Perpendicular Anisotropy and Eddy Currents on the Microwave Performance of Single-Layer and Multi-Layer Permalloy Films

In this letter, microwave and magnetostatic measurements of single- and multilayer Permalloy films are made to find physical reasons causing the decrease of the microwave permeability of the films with increase in film thickness. The permeability is measured in the frequency range of 0.1-10 GHz by a coaxial technique. From the measured permeability, a contribution to the effective field from the magnetoelastic effect is determined. The decrease of microwave permeability in thick films is shown to arise mainly due to the perpendicular magnetic anisotropy rather than from the effect of eddy currents, as is frequently suggested. It is also shown that both these effects are reduced in multilayer films comprising alternating magnetic and dielectric layers. Therefore, such films are promising materials for developing magnets that possess high permeability at high frequencies.

Microwave Properties of Mechanosynthesized Fe-Paraffin Nanocomposites

The structure and microwave properties of composites consisting of paraffin as a dielectric host matrix and Fe particles as inclusions have been studied. The composites were prepared by the mechanochemical synthesis and contain 7.7% vol. concentration of the inclusions. The microwave properties were studied in 0.1 to 6 GHz frequency range. The microwave properties of the composites are shown to depend mostly on the size and shape of the inclusions, with the phase composition of the inclusions exerting only a slight effect. With decreasing the inclusions size, the low-frequency peak (300−400 MHz) attributed to the domain wall motion diminishes.

Microwave properties of FeCo–SiO 2 systems obtained by high-energy milling

Structural and phase composition and magnetostatic and microwave properties of Fe70Co30–SiO2 systems obtained by high-energy milling in acetone for different times (from 6 to 48 h) are studied. It is found that, after the longest milling time, the obtained particles have sizes from 0.5 to 5 μm, are covered with a thin layer of SiO2, and contain 10 wt % iron carbides. Milling-induced change in the phase composition of the particles influences their specific magnetization of saturation and coercive force, but has a small effect on the microwave permittivity of composite materials containing these particles in the frequency range of 0.1–6 GHz. As the milling time grows, the imaginary part of the microwave permeability decreases at frequencies below 1 GHz.