Andrey N. Lagarkov

Theory of giant magneto-impedance effect in amorphous wires with different types of magnetic anisotropy

Abstract The effect of various types of magnetic anisotropy on the giant magneto-impedance spectra of amorphous wires with low magnetostriction is studied theoretically. For the first of the model considered the easy anisotropy axis is supposed to be parallel to the wire axis, whereas for the second model it has circumferential direction. In case of a wire with axial type of magnetic anisotropy the transverse magnetic permeability is obtained as a function of external magnetic field amplitude H 0 and other magnetic parameters of the wire. A strong dependence of this quantity on H 0 is shown to explain the giant magneto-impedance (GMI) effect in this type of amorphous wire. Besides, the amplitude of the GMI effect depends substantially on the value of a phenomenological damping parameter. In case of a wire with circumferential anisotropy the classical expression [Landau and Lifshitz, Electrodynamics of Continuous Media, 2nd ed, Pergamon, New York, 1984] for the wire impedance is not valid in the range of external magnetic field 0⩽ H 0 ⩽ H a , where H a is the anisotropy field. In this range of external magnetic field the wire impedance becomes a tensor. It has both longitudinal and transverse components that can be measured experimentally [Antonov et al., IEEE Trans. Magn. 33 (1997) 3367]. In case of a wire with circumferential anisotropy the peak of the longitudinal component of wire impedance as the function of H 0 corresponds to the anisotropy field, whereas in case of axial anisotropy it is found at zero magnetic field. The results obtained show that the good magnetic softness of amorphous wire is one of the most important conditions to observe the GMI effect; neither any domain structure nor circumferential anisotropy are strictly necessary.

Dielectric properties of fiber-filled composites

Results are presented of the comprehensive experimental investigation of the dielectric properties of composites filled with conducting fibers results. The results apply to the percolation threshold values and microwave dielectric dispersion of these materials. The fiber-filled composites exhibit a great variety of dielectric dispersion behavior, and offer wide possibilities of tailoring their dielectric dispersion. The experimental data are in good agreement with the recent theoretical approach [see A. N. Lagarkov and A. K. Sarychev, Phys. Rev. B 53, 6318 (1996)] as regards the values of percolation threshold and the shape of dielectric dispersion dependences. This agreement is a validation of the original assumption that the permittivity of effective medium is a scale dependent function in fiber-filled composites.

Resonance Properties of Bi-Helix Media at Microwaves

ABSTRACT In this paper, a novel inclusion shape (bi-helix) for artificial magnetic composites is studied. The developed analytical model is used to analyze the eigenmodes and frequency dispersion of the polarizabilities. Experimental investigations (cavity measurements) reveal the main features of the physical phenomena of electromagnetic excitation of individual bi-helix particles. Sheet samples of bi-helix composites are studied by free space measurements. The results lead to the conclusion that the use of new particles can help to improve electromagnetic parameters of magnetic composites. In particular, the magnetic moments appear to be larger than in conventional inclusions, and the resonance frequency of the magnetic permeability is different from that of the effective permittivity.

Shift of resonance frequency of long conducting fibers embedded in a composite

An experimental study on the dipole resonance of long high-conducting fibers embedded in an inhomogeneous composite sheet was conducted. The location of the resonance characterizes the effect of the inhomogeneous environment on the electromagnetic response of the fibers. It is shown that the resonance frequency is determined completely by the thickness and permittivity of the composite sheet, in particular, with the anisotropy of the permittivity. No effect due to inhomogeneity of the environment is observed. This is in disagreement with the scale-dependent effective medium theory (SDEMT) that is conventionally exploited to model the permittivity of fiber-filled composites, because this theory shows that the response of the fibers depends on the inhomogeneity scale of the environment. Therefore, although the SDEMT provides qualitative agreement with the observed behavior of fiber-filled composites, it must be further improved to obtain better quantitative agreement with experimental data. The experimental...

Development and simulation of microwave artificial magnetic composites utilizing nonmagnetic inclusions

Abstract The paper deals with the selected results of investigation of composite materials exhibiting artificial magnetic properties at microwave frequencies owing to the resonant phenomena in nonmagnetic inclusions. In particular, inclusions are investigated in the form of small (compared with the wavelength) dielectric particles prepared of a material of high permittivity and wire helixes of different shapes. It is demonstrated that the composites exhibit effective magnetic properties in the vicinity of the eigenfrequencies of the inclusions. The frequency dispersion of the effective magnetic permeability of such materials is resonant. It is demonstrated that conducting helical inclusions may be used to prepare composite materials characterized, in some frequency band, by the negative values of the real parts of the permeability and permittivity simultaneously. The properties of unusual “left-handed” (V.G. Veselago) materials are discussed. A numerical simulation of the electromagnetic properties of such composites is performed. The calculated effective parameters turn out to be close to the measured values.

Wide-angle Absorption by the Use of a Metamaterial Plate

The problem regarding top possible (hopefully, total) fleld suppression of a fll- amentary source placed above nonuniform impedance plane is discussed. New designs of the electromagnetic fleld absorbers and resonators are suggested which may be engineered with the use of metamaterials. 1. INTRODUCTION | ABSORPTION OF THE ENERGY OF A POINT SOURCE BY A HALF-SPACE Interest to the fleld propagation along the imperfect surface has about century-old history, the beginnings of which trace back to the Sommerfelds solution of the classical problem for the dipole radiating above the plane with flnite conductivity. Later, as the radio broadcasting evolved, a lot of publications appeared which dealt with electromagnetic fleld propagation in the presence of an absorbing half-space with the aim to minimize the signal losses. Now certain practical needs (1) set questions about what value of impedance of a plane should be chosen to absorb the maximum portion of energy radiated by a point source (say, fllamentary current), how much the amount of the absorbed energy is and how to create such an impedance. Note, in view of the symmetry of the radiation pattern of the fllament, at the absence of the plate (in the free space) equal power ∞uxes are radiated into upper and lower half-spaces, and exactly one half of the radiated energy penetrates through the plane y = 0, see Fig. 1.

Nondiagonal impedance of amorphous wires with circular magnetic anisotropy

Abstract The effect of the nondiagonal magnetoimpedance of amorphous wires with low magnetostriction is studied experimentally and theoretically. The nondiagonal effect was observed and investigated. For a wire with circumferential magnetic anisotropy the tensor of magnetic permeability is obtained as a function of external magnetic field amplitude H and other parameters. A strong dependence of this quantity on H is shown in the domain of the external magnetic field, H ⩽ H A where H A is the anisotropy field. The nondiagonal component of the surface impedance is responsible for the measured effect.

Possible origin for the bamboo domain structure in Co-rich amorphous wire

It is shown that the bamboo domain structure in the outer shell of the Co-rich amorphous wire may arise due to small disturbances in the axial symmetry of the wire. As an example, an elliptical deformation of the wire cross-section is considered. If the anisotropy constant has sufficient value near the wire surface, even small elliptical deformation may lead to a finite period of the domain structure of the order of wire radius.

Nonlinear magnetization reversal of Co-based amorphous microwires induced by an ac current

The magnetization reversal of ferromagnetic Co-based amorphous microwires carrying ac current is studied. The frequency spectra of the voltage V induced in the pick-up coil wound around the microwires were analysed and were found to depend on the ac current amplitude I0 and external longitudinal dc magnetic field He. The even harmonics are shown to dominate in the frequency spectra of V if the ac current amplitude exceeds some threshold value, which strongly depends on He. A high sensitivity of the even harmonic amplitudes to the longitudinal dc field is demonstrated. The experimental data are described in terms of the Faraday law and the quasi-static Stoner-Wohlfarth model. The results obtained may be of importance for the design of weak magnetic field sensors.

Experimental studies on antenna miniaturisation using magneto-dielectric and dielectric materials

Dielectric or magneto-dielectric materials can be used to miniaturise antennas, but there are many important parameters that must be considered when selecting which material to use. The authors discuss these figures of merit and a rigorous method to compare antennas with different material fillings. A meandered planar inverted-F antenna (PIFA) loaded with magneto-dielectric and dielectric materials is presented as an example antenna for testing. The magneto-dielectric material used is composed of mylar substrate and Fe–SiO2 sheets. Measurement results for the permeability of the material are presented. The radiation mechanism of the meandered PIFA is studied, and the proper position for dielectric and magneto-dielectric filling is discussed and identified. Miniaturisation by commercial dielectric and the presented magneto-dielectric fillings is measured and compared at the same resonance frequency using the radiation quality factor as the figure of merit. It is seen, that the antenna can benefit from the magneto-dielectric filling material in terms of the radiation quality factor only if the placement of the antenna filling is carefully selected.