Alexander S. Sokolov

Low loss factor Co2Z ferrite composites with equivalent permittivity and permeability for ultra-high frequency applications

Ferrite composites of nominal composition Ba3Co2+xIrxFe24−2xO41 were studied in order to achieve low magnetic and dielectric losses and equivalent permittivity and permeability over a frequency range of 0.3–1 GHz. Crystallographic structure was characterized by X-ray diffraction, which revealed a Z-type phase accompanied by increasing amounts of Y-type phase as the iridium amount was increased. The measured microwave dielectric and magnetic properties showed that the loss tan δe and loss tan δμ decreased by 80% and 90% at 0.8 GHz with the addition of iridium of x = 0.12 and 0.15, respectively. An effective medium approximation was adopted to analyze the composite ferrites having mixed phase structures. Moreover, adding Bi2O3 enabled equivalent values of real permittivity and real permeability over the studied frequency range. The resultant data give rise to low loss factors, i.e., tan δe/e′ = 0.008 and tan δμ/μ′ = 0.037 at 0.8 GHz, while characteristic impedance was the same as that of free space.

Crystallographically textured self-biased W-type hexaferrites for X-band microwave applications

We report the magnetic and structural properties of a series of W-type barium hexaferrites of composition BaZn2−xCoxFe16O27, where x = 0.15, 0.20, and 0.25. The anisotropy field of these barium ferrites (BaW) decreased with the substitution of divalent Co ions, while they maintained crystallographic c-axis texture. The measured anisotropy field was ∼10 kOe, and a hysteresis loop squareness Mr/Ms = 79% was obtained due to well-controlled grain size within the range of single domain scale. These two properties make the BaW suitable for applications in microwave devices at lower frequencies, such as self-biased circulators operated at X-band frequencies.

Giant magnetoresistance due to magnetoelectric currents in Sr3Co2Fe24O41 hexaferrites

The giant magnetoresistance and magnetoelectric (ME) effects of Z-type hexaferrite Sr3Co2Fe24O41 were investigated. The present experiments indicated that an induced magnetoelectric current in a transverse conical spin structure not only presented a nonlinear behavior with magnetic field and electric field but also depended upon a sweep rate of the applied magnetic field. More interestingly, the ME current induced magnetoresistance was measured, yielding a giant room temperature magnetoresistance of 32.2% measured at low magnetic fields (∼125 Oe). These results reveal great potential for emerging applications of multifunctional magnetoelectric ferrite materials.

Broadband ferromagnetic resonance linewidth measurement by a microstripline transmission resonator

The application of microstripline transmission resonators to ferromagnetic resonance linewidth measurements, and the linewidth extraction method based on the resonator resonant frequency dispersion are discussed. A detailed description of the spectrometer and the experimental procedure is presented. The limitations of the method and the origin of various errors that may occur in the measurements are considered. The geometry of the resonator was optimized by numerical methods and the radio-frequency magnetic field structure investigated. A series of magnetic films and bulk samples was measured by the spectrometer under varying circumstances, and results were compared to the ones obtained by conventional methods.

Single-Point FMR Linewidth Measurement by TE 10 Rectangular Transmission Cavity Perturbation

A method that allows the determination of magnetic permeability in magnetic bodies, as well as the ferromagnetic resonance (FMR) linewidth, by measuring only the quality factor and the resonant frequency of the perturbed microwave cavity at FMR is presented. The method is based on a set of equations derived for various types of magnetic media, including ferrites, single crystals, and polycrystalline materials. The evaluation of resonators containing magnetic medium is considered, and a detailed description of the spectrometer and the experimental procedure are presented. Independently measured yttrium iron garnet samples were used to validate the method.

Recent Advances in Numerical Simulation of Propagation of EM Waves in the Earth's Ionosphere

EM wave propagation in the magnetoactive terrestrial ionosphere was investigated by numerical methods, and associated software was developed. Two principal functions were realized: ray tracing and ionogram synthesis. For the first time, reference-standard models, including the International Reference Ionosphere (IRI) 2012, World Magnetic Model 2010, and World Geodetic System 1984, have been globally implemented for every component of the propagation medium, and fully manipulatable 3-D visualization was enabled in a study utilizing the bicharacteristic method. An assimilation model of the ionosphere was also used along with IRI. The focus was on the unveiling of multiskip long-range propagation effects. Effective numerical algorithms for solving the bicharacteristic system that allow rapid insight into an extremely large-scale EM problem and enable quasi-real-time operation were discussed. The simulation in the presence of abnormal locally increased or decreased electron density in the ionosphere of natural or artificial occurrence was considered. Experimental ionograms obtained by an ionosonde were compared to numerically synthesized ones.

The field domain FMR linewidth measurement by a microstrip line transmission resonator perturbation technique

In this paper, a method was proposed to make the FMR parameters studies easier and to accurately measure the resonances with widely varying intensities. A detailed description of this spectrometer is presented and the general theory of the FMR linewidth measurement is discussed and the associated working equations are also derived. A discussion is also given of the origin of various errors that may occur in the measurement.

3D crystallographic alignment of alumina ceramics by application of low magnetic fields

Abstract Non-cubic crystals exhibit anisotropic physical and functional properties. Microscopic crystallites as constituents of polycrystalline materials are randomly oriented, thus polycrystalline ceramics lack the anisotropic properties of their monocrystalline counterparts. We propose a technology that exploits the synergy between magnetic alignment and colloidal ceramics processing, and enables to independently tune the orientation of grains in different sample regions by infinitesimal magnetic fields (

Tailoring magnetic properties of self-biased hexaferrites using an alternative copolymer of isobutylene and maleic anhydride

Discussed is a novel self-biased hexaferrite gelling system based on a nontoxic and water-soluble copolymer of isobutylene and maleic anhydride. This copolymer simultaneously acts as a dispersant and gelling agent, and recently received much attention from the ceramics community. Herein its effects on the rheological conditions throughout magnetic-field pressing, and consequently, orientation, density and magnetic properties of textured hexaferrites were investigated. Ka-band FMR linewidths were measured, and the crystalline anisotropy and porosity induced linewidth broadening were estimated according to Schlomann’s theory. The copolymer allowed to reduce the friction between micron-sized magnetic particulates, resulting in higher density and degree of crystalline orientation, and lower FMR linewidth.