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Magnetic Losses in Composite Materials
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
Department of Electrical and Information Technology, Faculty of Engineering, Lund University.
2008 (English)In: Journal of Physics D: Applied Physics, ISSN 0022-3727, E-ISSN 1361-6463, Vol. 41, no 13, 135005- p.Article in journal (Refereed) Published
Abstract [en]

We discuss some of the problems involved in homogenization of a composite material built from ferromagnetic inclusions in a nonmagnetic background material. The small signal permeability for a ferromagnetic spherical particle is combined with a homogenization formula to give an effective permeability for the composite material. The composite material inherits the gyrotropic structure and resonant behaviour of the single particle. The resonance frequency of the composite material is found to be independent of the volume fraction, unlike dielectric composite materials. The magnetic losses are described by a magnetic conductivity which can be made independent of frequency and proportional to the volume fraction by choosing a certain bias. Finally, some concerns regarding particles of small size, i.e. nanoparticles, are treated and the possibility of exciting exchange modes are discussed. These exchange modes may be an interesting way to increase losses in composite materials.

Place, publisher, year, edition, pages
2008. Vol. 41, no 13, 135005- p.
Keyword [en]
Capillarity, Composite materials, Composite micromechanics, Dielectric materials, Ferromagnetic materials, Ferromagnetism, Liquids, Magnetic domains, Magnetic materials, Resonance, Titration, Effective permeability, Gyrotropic structure, Magnetic (CE), Magnetic conductivities, Non magnetic, Resonance frequencies, Single particles, Small signals, Small size, Spherical particles
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
URN: urn:nbn:se:kth:diva-8540DOI: 10.1088/0022-3727/41/13/135005ISI: 000256928100037Scopus ID: 2-s2.0-48249129687OAI: oai:DiVA.org:kth-8540DiVA: diva2:13891
Note
QC 20100906. Uppdaterad från accepted till published (20100906).Available from: 2008-05-28 Created: 2008-05-28 Last updated: 2017-12-14Bibliographically 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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