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Scattering from a thin magnetic layer with a periodic lateral magnetization: application to electromagnetic absorbers
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.ORCID iD: 0000-0001-9241-8030
Department of Electrical and Information Technology, Electromagnetic Theory, Lund University.
2008 (English)In: Progress in Electromagnetics Research, ISSN 1559-8985, Vol. 83, 199-224 p.Article in journal (Refereed) Published
Abstract [en]

A magnetized thin layer mounted on a PEC surface is considered as an alternative for an absorbing layer. The magnetic material is modeled with the Landau-Lifshitz-Gilbert equation, with a lateral static magnetization having a periodic variation along one lateral direction. The scattering problem is solved by means of an expansion into Floquet-modes, a propagator formalism and wave-splitting. Numerical results are presented, and for parameter values close to the typical values for ferro- or ferrimagnetic media, reflection coefficients below -20 dB can be achieved for the fundamental mode over the frequency range 1-4 GHz, for both polarizations. It is found that the periodicity of the medium makes the reflection properties for the fundamental mode almost independent of the azimuthal direction of incidence, for both normally and obliquely incident waves.

Place, publisher, year, edition, pages
2008. Vol. 83, 199-224 p.
Keyword [en]
Electromagnetism, Ferromagnetism, Magnetic devices, Magnetization, Magnets, Reflection, Absorbing layers, Azimuthal directions, Electromagnetic absorbers, Floquet, Frequency ranging, Fundamental modes, Landau-Lifshitz-Gilbert equations, Magnetic layers, Numerica l results, Obliquely incident waves, Parameter values, Periodic variations, Reflection coefficients, Reflection properties, Scattering problem, Static magnetization, Thin layering, Typical values
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
URN: urn:nbn:se:kth:diva-8539DOI: 10.2528/PIER08042805ISI: 000260082000015Scopus ID: 2-s2.0-49549117107OAI: oai:DiVA.org:kth-8539DiVA: diva2:13890
Note
QC 20100906. Uppdaterad från submitted till published (20100906).Available 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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