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On the amount of magnetic material necessary in broadband magnetic absorbers
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
Department of Electrical and Information Technology, Faculty of Engineering, Lund University.
2008 (English)In: 2008 IEEE International Symposium on Antennas and Propagation (AP-S 2008), 2008, 1412-1415 p.Conference paper, Published paper (Refereed)
Abstract [en]

In this paper we have examined the use of ferromagnetic particles for thin broadband RAMs. The permeability of the composite material was obtained from a well tested model for ferromagnetic materials and a result from homogenization theory. It was found that the resonance frequency of the composite is independent of the volume fraction of the particles and that a magnetic conductivity proportional to the volume fraction exists. Furthermore, we showed that a certain amount of magnetic material is required in order to maintain a low reflection and that there exists a lower bound on how thin we could expect the absorber to be. Numerical calculations show that, though thin, the RAM tends to be quite heavy, which might prevent the use of ferromagnetic material in RAM applications. These results may serve as guidelines when designing thin broadband RAMs.

Place, publisher, year, edition, pages
2008. 1412-1415 p.
Keyword [en]
absorbing media, composite materials, ferromagnetic materials, radar cross-sections, broadband magnetic absorbers, composite material, ferromagnetic materials, ferromagnetic particles, homogenization theory, magnetic conductivity, magnetic material necessary, random access memory, resonance frequency, thin broadband radar absorbing material, volume fraction
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
URN: urn:nbn:se:kth:diva-8541DOI: 10.1109/APS.2008.4619271ISI: 000261440600355Scopus ID: 2-s2.0-55649111439ISBN: 978-1-4244-2041-4 (print)OAI: oai:DiVA.org:kth-8541DiVA: diva2:13892
Note
QC 20100906Available from: 2008-05-28 Created: 2008-05-28 Last updated: 2010-09-06Bibliographically approved
In thesis
1. Electromagnetic Waves in Media with Ferromagnetic Losses
Open this publication in new window or tab >>Electromagnetic Waves in Media with Ferromagnetic Losses
2008 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

The operation of a wide variety of applications in today's modern society are heavily dependent on the magnetic properties of ferromagnetic materials and their interaction with electromagnetic fields.

The understanding of these interactions and the associated loss mechanisms is therefore crucial for the improvement and future development of such applications.

This thesis is concerned with electromagnetic waves in media with ferromagnetic losses. We model the dynamics of the magnetization of a ferromagnetic material with the nonlinear Landau-Lifshitz-Gilbert (LLG) equation and study stability conditions on static solutions. Furthermore, with the aid of a small signal analysis this equation is linearized around a stable static solution. From this analysis we obtain a small signal permeability, which shows that ferromagnetic material in general are gyrotropic with a resonant frequency behavior similar to that of a Lorentz material. In difference to dielectric Lorentz material, this resonance frequency can be shifted with the aid of a bias field. For a specific bias field we obtain a frequency behavior that mimics that of a material with electric conductivity losses. In terms of losses per unit volume it is then possible to define a magnetic conductivity which is independent of frequency.

We treat composite materials built from ferromagnetic inclusions in a nonmagnetic and nonconductinig background material. The composite material inherits the gyrotropic structure and resonant behavior of the single particle. The resonance frequency of the composite material is found to be independent of the volume fraction, unlike dielectric composite materials. For small enough particles, typically around 100 nm, it becomes energetically favorable to form a single domain in the particle, where disturbances in the magnetization can propagate in the form of spin waves. We study the possibility of exciting spin waves and derive a susceptibility that takes spin waves into account. It is found that spin wave resonances are excited in the gigahertz range and this could offer a way to increase the losses in a composite material. We also discuss some concerns regarding stability and causality of effective material parameters for biased ferromagnetic materials.

Finally, we discuss the possibility of using magnetic materials in absorbing applications. We analyze the scattering of electromagnetic waves from a metal surface covered with a thin magnetic lossy sheet. It is found that very thin magnetic layers can provide substantial specular absorption over a wide frequency band. However, magnetic specular absorbers, where the waves propagates just a fraction of the wavelength in the material, seem to require a certain amount of ferromagnetic material which make them quite heavy and thereby limit its practical use. On the other hand, for nonspecular absorbers where the waves propagates several wavelengths in the material, the amount of magnetic material required for efficient absorption seems to be substantially less than for specular absorbers. Thus, as nonspecular absorbers, magnetic lossy materials could offer very thin and light designs.

Place, publisher, year, edition, pages
Stockholm: KTH, 2008. xii, 30 p.
Series
Trita-EE, ISSN 1653-5146 ; 2008:029
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-4776 (URN)978-91-7415-011-7 (ISBN)
Public defence
2008-06-05, D3, D, Lindstedtsv 5, Stockholm, 13:15 (English)
Opponent
Supervisors
Note
QC 20100906Available from: 2008-05-28 Created: 2008-05-28 Last updated: 2010-09-06Bibliographically approved

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