ChemInform Abstract: Ferromagnetic Nanowire Metamaterials: Theory and Applications
An overview of ferromagnetic nanowire (FMNW) metamaterials is presented. First, FMNW metamaterials are placed in the historical context of antique composites and 20th Century artificial dielectrics, and presented as an example of second-generation metamaterials following the microstructured metamaterials developed in the first part of the decade. Next, the fabrication processes of FMNW metamaterials and subsequent planar devices are detailed. It is then shown how the geometrical properties of the FMNW structure, such as the wire diameter and the wire nanodisk thicknesses, determine the dc and RF responses of the material. Upon this basis, the modeling of the metamaterial is presented, using a two-level approach where the microscopic (with respect to the wires) susceptibility is derived by solving the Landau-Lifshitz equation and the macroscopic (metamaterial) permittivity and permeability tensors are obtained by effective medium theory. Next, a review of FMNW microwave devices, such as circulators, isolators, and phase shifters, is provided, and the example of an FMNW dual-band edge-mode isolator is studied. Finally, spintronic effects and applications of FMNW metamaterials, such as dc to RF generators and detectors based on the spin-torque transfer phenomenon, are reviewed.
Available from: Alejandro Álvarez Melcón
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ABSTRACT: The behavior of leaky and surface modes in uniaxially anisotropic grounded slabs is investigated. First, a transverse magnetic and transverse electric modal parametric analysis of the structure is performed, based on dispersion relations, comparing the nondispersive and Drude/Lorentz dispersive anisotropic slabs with an isotropic nondispersive slab. This analysis reveals that in the case of the isotropic slab, the leaky wave pointing angle is restricted to the end-fire region. In contrast, it is shown, for the first time, that the proposed anisotropic dispersive grounded slab structure provides efficient (in particular highly directive) leaky wave radiation with a high design flexibility. Toward its lower frequencies, the dominant leaky mode provides full-space conical-beam scanning. At higher frequencies, it provides fixed-beam radiation (at a designable angle) with very low beam squint. A vertical dipole source is placed inside the slab to excite the relevant leaky wave mode. The radiation characteristics obtained for this structure confirms the novel low-beam squint and high-directivity operation of the dominant leaky mode. Further validation is included using the commercial software tool CST. The structure could be used to conceive antennas either for conical beam scanning (lower-frequency range) or for point-to-point communication and radar systems (higher-frequency range).
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ABSTRACT: This paper presents what, in the vision of the author, will be next-generation metamaterials for microwave systems. For this purpose, it first introduces some fundamentals on metamaterials and describes the different types of currently existing metamaterial structures, called micro-scale metamate-rials, in reference with their typical smallest dimension. Upon this basis, it presents a few micro-scale metamaterial properties, structures and principles, from the popular negative index of refraction, via various dispersion-engineered devices, to a very recent non-reciprocal magnet-less metamaterial. After pointing out the limitations of micro-scale metamaterial, the paper introduces next generation metamaterials, namely multi-scale metamaterials, which incorporate nano-scale and atomic-scale structures and substances. For the nano-scales, it presents the example of ferromagnetic nanowire metamaterials and their applications, while for the atomic scale it describes recent innovations in ferrite based structures, including a uniform waveguide CRLH leaky-wave antenna and a perfect electromagnetic conductor wall, as well as most recent graphene-based structures and devices. Finally, a tri-scale structure, incorporating a graphene atomic-scale, ferromagnetic wire nano-scale and a metasurface micro-scale is discussed. In conclusion, the paper suggests that, although technology is eminently unpredictable, the wealth of novel properties of metamaterials augurs a long and lasting impact of this area.
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ABSTRACT: Electromagnetics is a mature field which is ubiquitous in everyday life. All communication systems rely, to a certain extent, on electromagnetic devices such as antennas and waveguides. Furthermore, the ever increasing demand for high capacity and small size systems calls for novel avenues for miniaturization and integration. Conventional electromagnetic devices suffer from a fundamental tradeoff between size and functionality, originating from the fact that the interaction between all classical materials and electromagnetic waves reduces as their size decreases. Therefore, new materials immune of these fundamental drawbacks must be explored. Nanotechnology, the study of materials and structures with at least one of dimension in the order of the nanometer , offers a potential solution. Conceptualized for the first time in 1956 by Richard Feynman in his famous talk “There's plenty of room at the bottom” , nanotechnology can now routinely produce nanomaterials with controlled structural and functional parameters in the nano or even atomic scale. Some of these nanomaterials are not only able to strongly interact with electromagnetic waves, despite their dimensions being much smaller than the wavelength, but they also exhibit totally new phenomena, not found in conventional materials. Here, we provide three illustrative examples from our recent work on nanoelectromagnetics, which show how the combination of electromagnetics and nanotechnology can lead to devices with unprecedented characteristics.
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