TUNEABLE MICROWAVE COMPOSITES BASED ON FERROMAGNETIC WIRES CONCEPTUAL REVIEW, EXPERIMENTAL EVIDENCE, AND FUTURE WORK

DMITRIY MAKHNOVSKIY AND LARISSA PANINA

University of Plymouth, United Kingdom November 2005

The following work outlines the main program character. The main physical principles, which determine the behaviour of tuneable microwave composites based on ferromagnetic wires, have already been established. At present, it is necessary to focus our attention on the principle ideas and answer some actual technological questions which may inspire future work. We begin with a survey of dispersion characteristics of the wire filled composites and their classification in order to give the reasons for further experimental and theoretical investigations. This new application for ferromagnetic wires as the inclusions in tuneable composites stimulates the unique investigation of their magnetostatic and high frequency properties with a novel perspective. At present the emphasis is on the radiative characteristics of superfine ferromagnetic wires, which are determined, in turn, by their conductive and magnetostatic properties. Such complicated interconnection of different characteristics provides the rich set of behavioural properties of proposed materials, and also their various applications. At the same time, it becomes possible to avoid a number of objections related to glass-coated ferromagnetic wires:

* The large spread of the material parameters in such wires is no longer a critical issue because the microwave response measured from the composite in free space is an averaged characteristic from many wires. In an informal conversation, any specialist will note that the spread of wire parameters makes their practical use quite awkward, especially as a microelement for mass-produced devices. This problem concerns even wire samples from the same bobbin. In addition, their glass-coating hampers soldering.

* Since a large amount of wires is required for the fabrication of composites, this assumes mass production. For comparison: just one wire bobbin will be enough for the fabrication of hundreds of sensors! Of course, a company, which would be ready to invest in the study of wires, cannot grow from such a "modest demand". Admittedly, a large production could be required for the bi-stable wires that are supposed to be used as the magnetic-tags. But even here there are more problems rather than final solutions. In addition, resent progress with active RF tags and their drastic reduction of price makes magnetic tags less favourable, reducing their prospects for wide use.

* Ferromagnetic wires are the principle construction elements in tuneable composites and they cannot be replaced by any other materials. On the contrary, in sensors, thin film elements are the primary competition for wires and will most probably succeed them due to better sensitivity and manufacturability.

Therefore, we have to conclude that the tunable composites are the materials where the specific magnetic effects in glass-coated wires become most needed.

The authors would like to thank:

Vladimir Larin, Director of MFTI Ltd (Chisinau, Republic of Moldova), for funding the experimental investigations on tuneable composites and also for the general technical supervision of the composite project,

Julia Nezvinskaya, Alexey Rata, Ruslan Taucci, Lilian Chicu, Alexander Burunchenco, Tatiana Larina and Roman Alexandrov, MFTI Ltd, for their significant participation in the creation of the free space microwave measuring system, extensive experimental work and also the solution of a whole series of technological problems related with the fabrication of glass-coated wires,

Dr Efim Badinter, Director of ELIRI Institute (Chisinau, Republic of Moldova) and Dr Iulian Colpacovici, ELIRI Institute, for the general support of the composite project and for the provision of microwave apparatuses in ELIRI Institute,

Dr Arkadiy Zhukov and Carlos Garcia, University of San Sebastian (Spain), and also Dr Sergey Sandacci, Sensors Technology Ltd (United Kingdom), for the conducted measurements of the high frequency magnetoimpedance and the dc magnetostatic properties in glass-coated ferromagnetic wires,

Nicholas Fry for his great help in the manuscript preparation.

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