A. Radkovskaya

Mechanism of subwavelength imaging with bilayered magnetic metamaterials: Theory and experiment

We present a theoretical and experimental study of a bilayered metamaterial structure for subwavelength imaging of magnetic field. The simplest version of such a structure consists of one or two linear arrays of capacitively loaded split pipe resonators. Its subwavelength physics is governed by strongly anisotropic magnetic coupling between individual resonators and by propagation of magnetoinductive waves with wavelength much shorter than the wavelength of the electromagnetic radiation in free space. It is shown that magnetoinductive waves propagating in the lateral direction are undesirable because they spread the image. Good subwavelength imaging is achieved when, due to the strong interlayer coupling, a stop band in the vicinity of the resonant frequency appears in the dispersion characteristics. The imaging properties of the single and double lens are compared and it is shown that the double lens has a superior performance. Excellent agreement is obtained between experimental and theoretical results ...

Mapping inter-element coupling in metamaterials: Scaling down to infrared

The coupling between arbitrarily positioned and oriented split ring resonators is investigated up to THz frequencies. Two different analytical approaches are used, one based on circuits and the other on field quantities that includes retardation. These are supplemented by numerical simulations and experiments in the GHz range, and by simulations in the THz range. The field approach makes it possible to determine separately the electric and magnetic coupling coefficients which, depending on orientation, may reinforce or may cancel each other. Maps of coupling are produced for arbitrary orientations of two co-planar split rings resonant at around 2 GHz and then with the geometry scaled down to be resonant at around 100 THz. We prove that the inertia of electrons at high frequencies results in a dramatic change in the maps of coupling, due to reduction of the magnetic contribution. Our approach could facilitate the design of metamaterials in a wide frequency range up to the saturation of the resonant frequency.

Magnetic properties of short amorphous microwires

Abstract Magnetic properties of amorphous Fe- and Co-rich wires with different magnetostriction are studied in dependence on their length (8 mm L L * is estimated. It is shown that for L L * the influence of demagnetizing fields on the wire magnetization is substantial and the wire remanent magnetization is proportional to the square of the wire length. The oscillations of the magnetic moment component parallel to the wire axis during the cross remagnetization processes are found.

The thermal and stress effect on GMI in amorphous wires

We report results on the magneto-impedance (MI) effect in CoFeSiB amorphous wires with and without glass cover and a composite wire under heating up to about 180°C which is much less than the crystallisation one and internal longitudinal stresses. The changing of the MI effect with low-temperature heating was irreversible and strongly depended on the processing environment.

An electro-chemical magnetic field sensor

Abstract We have presented some results of the investigation of extreme large value of the electrode potential (EP) for the ferromagnetic electrodes in the external magnetic field. The measured EP value is about 20 mV for the Fe-electrode in the magnetic field of 3 kOe, which is three orders of magnitude larger in comparison with the theoretical estimation. The issues presented are as follows: the EP dependencies on the magnetic field for electrodes from various ferromagnetic (Fe and Ni) and paramagnetic (Cu) metals and EP dependencies on the magnetic field for the various types and concentration of electrolytes (FeCl3 and H2SO4). A very significant EP hysteresis and strong EP relaxation for ferromagnetic electrodes have been found and the correlation between them has been discussed. The EP relaxation is measured at different rates of the magnetic field variation and the long-time character of relaxation is observed. The width of EP hysteresis loop strongly depends on the magnetic field orientation relatively to the electrode plane. This fact can be considered as the evidence of the correlation between the EP and the magnetic state of the electrode. This effect can be used either for the definition of the magnetic state of the electrode as well as for measuring the external magnetic field. A model sensor for measuring the magnetic field is proposed.

Magnetic and magnetooptical properties of (Fe30Co70)xAg1 − x systems

Abstract The magnetic properties and magnetooptical spectra (MOS) of granular alloys (Co70Fe30)xAg1 − x were studied in the 1.5–4 eV range of incident light at room temperature. The MOS of the granular alloys are quite different from those of homogeneous Fe or Co films. The MOS behavior of (Co70Fe30)xAg1 − x granular alloys is explained in the framework of the effective medium approximation.

The magnetic properties of magnetic nanoparticles produced by microwave flash synthesis of ferrous alcoholic solutions

Summary form only given. Microwave heating is an emerging technology, which uses the ability of some liquids and solids to transform electromagnetic energy into heat. Recently it has been shown that it is possible to produce hematite, magnetite and iron-magnetite nanoparticles by microwave heating of alcoholic solutions of a ferrous salt. The developed original microwave applicator associated with autoclave for one flash synthesis of nanoparticles by disproportionation of ferrous alcoholic solutions as well as operation conditions is described elsewhere Caillot et al. (Journ. of Mat. Sci. (2002)). We present the results of experimental study of magnetic and structural properties of magnetic nanoparticles fabricated by this technique. Comparing with similar nanoparticles fabricated by using a conventional heating, we obtained much lower grain size (up to 10 nm) and very stable properties. The hysteresis loop for samples of the non-oriented assembly of hematite particles has a coercive force about 100 Oe with squareness about 0.4. Some quantity of superparamagnetic fraction exists in our sample. The particles distribution on the anisotropy fields has a maximum about 200 Oe. The possibility of improvement of the magnetic properties is under investigation.

Influence of boundary conditions on the parameters of the low-temperature ΔT effect

The influence of boundary conditions on the parameters of the low-temperature ΔT effect is investigated. Our analysis provides a new evidence supporting the physical model of the low-temperature ΔT effect. The developed model has been experimentally verified. This model explains the published experimental data and can serve as the basis for developing macroscopic mechanisms responsible for irreversible changes of the structure and the physical properties of amorphous metallic alloys after the low-temperature treatment.

Superdirectivity from arrays of strongly coupled meta-atoms

We explore the possibility of achieving superdirectivity in metamaterial-inspired endfire antenna arrays relying on the good services of magnetoinductive waves. These are short-wavelength slow waves propagating by virtue of coupling between resonant meta-atoms. Magnetoinductive waves are capable of providing a rapidly varying current distribution on the scale of the free space wavelength. Using dimers and trimers of magnetically coupled split ring resonators with only one element driven by an external source, we introduce an analytical condition for realising superdirective current distributions. Although those current distributions have been known theoretically for a good 60 years, this is the first time that a recipe is given to realise them in practice. Our key parameters are the size of the array, the resonant frequency and quality factor of the elements, and their coupling constant. We compare our analytical results for coupled magnetic dipoles with numerical results from CST simulations for meta-atoms of various shapes. The calculated bandwidth of 5 MHz for a dimer operating at 150 MHz indicates that, contrary to popular belief, superdirective antennas exist not only in theory but may have practical applications.We explore the possibility of achieving superdirectivity in metamaterial-inspired endfire antenna arrays relying on the good services of magnetoinductive waves. These are short-wavelength slow waves propagating by virtue of coupling between resonant meta-atoms. Magnetoinductive waves are capable of providing a rapidly varying current distribution on the scale of the free space wavelength. Using dimers and trimers of magnetically coupled split ring resonators with only one element driven by an external source, we introduce an analytical condition for realising superdirective current distributions. Although those current distributions have been known theoretically for a good 60 years, this is the first time that a recipe is given to realise them in practice. Our key parameters are the size of the array, the resonant frequency and quality factor of the elements, and their coupling constant. We compare our analytical results for coupled magnetic dipoles with numerical results from CST simulations for meta-ato...