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Electromagnetic Waves in Media with Ferromagnetic Losses
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
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: urn:nbn:se:kth:diva-4776ISBN: 978-91-7415-011-7 (print)OAI: oai:DiVA.org:kth-4776DiVA: diva2:13894
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
List of papers
1. Spiral Elements for Broad-Band Phased Arrays
Open this publication in new window or tab >>Spiral Elements for Broad-Band Phased Arrays
2005 (English)In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 53, no 8, 2558-2562 p.Article in journal (Refereed) Published
Abstract [en]

In this paper, we present a numerical analysis of an infinite periodic array of planar spiral elements with octave bandwidth. For off-broadside scan the array is found to exhibit very narrow resonances, which are independent of scan angle. They occur when the spiral arms are multiples of half a wavelength, in which case the current forms a high amplitude standing wave along the spiral arms. The resonances are conveniently suppressed by making the arms unequally long. We also discuss the equivalent 3-port for this nonsymmetrical array element and evaluate the element polarization performance.

Keyword
broad-band arrays, phased arrays, spiral antennas
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-8537 (URN)10.1109/TAP.2005.852309 (DOI)000231156100024 ()2-s2.0-27644455861 (Scopus ID)
Note
QC 20100906Available from: 2008-05-28 Created: 2008-05-28 Last updated: 2017-12-14Bibliographically approved
2. Biased magnetic materials in RAM applications
Open this publication in new window or tab >>Biased magnetic materials in RAM applications
2007 (English)In: Progress in Electromagnetics Research, ISSN 1559-8985, Vol. 75, 85-117 p.Article in journal (Refereed) Published
Abstract [en]

The magnetization of a ferro- or ferri-magnetic material has been modeled with the Landau-Lifshitz-Gilbert (LLG) equation. In this model demagnetization effects are included. By applying a linearized small signal model of the LLG equation, it was found that the material can be described by an effective permeability and with the aid of a static external biasing field, the material can be switched between a Lorentz-like material and a material that exhibits a magnetic conductivity. Furthermore, the reflection coefficient for normally impinging waves on a PEC covered with a ferro/ferri-magnetic material, biased in the normal direction, is calculated. When the material is switched into the resonance mode, two distinct resonance frequencies in the reflection coefficient were found, one associated with the precession frequency of the magnetization and the other associated with the thickness of the layer. The former of these resonance frequencies can be controlled by the bias field and for a bias field strength close to the saturation magnetization, where the material starts to exhibit a magnetic conductivity, low reflection ( around - 20 dB) for a quite large bandwidth ( more than two decades) can be achieved.

Keyword
Bandwidth, Differential equations, Electric conductivity, Random access storage, Saturation magnetization, Switching, External biasing field, Landau-Lifshitz-Gilbert (LLG) equation, Lorentz-like material
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-8538 (URN)10.2528/PIER07052501 (DOI)000248081200006 ()2-s2.0-34547246315 (Scopus ID)
Note
QC 20100906Available from: 2008-05-28 Created: 2008-05-28 Last updated: 2010-09-06Bibliographically approved
3. Scattering from a thin magnetic layer with a periodic lateral magnetization: application to electromagnetic absorbers
Open this publication in new window or tab >>Scattering from a thin magnetic layer with a periodic lateral magnetization: application to electromagnetic absorbers
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.

Keyword
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:nbn:se:kth:diva-8539 (URN)10.2528/PIER08042805 (DOI)000260082000015 ()2-s2.0-49549117107 (Scopus ID)
Note
QC 20100906. Uppdaterad från submitted till published (20100906).Available from: 2008-05-28 Created: 2008-05-28 Last updated: 2010-09-06Bibliographically approved
4. Magnetic Losses in Composite Materials
Open this publication in new window or tab >>Magnetic Losses in Composite Materials
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.

Keyword
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:nbn:se:kth:diva-8540 (URN)10.1088/0022-3727/41/13/135005 (DOI)000256928100037 ()2-s2.0-48249129687 (Scopus ID)
Note
QC 20100906. Uppdaterad från accepted till published (20100906).Available from: 2008-05-28 Created: 2008-05-28 Last updated: 2017-12-14Bibliographically approved
5. On the amount of magnetic material necessary in broadband magnetic absorbers
Open this publication in new window or tab >>On the amount of magnetic material necessary in broadband magnetic absorbers
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.

Keyword
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:nbn:se:kth:diva-8541 (URN)10.1109/APS.2008.4619271 (DOI)000261440600355 ()2-s2.0-55649111439 (Scopus ID)978-1-4244-2041-4 (ISBN)
Note
QC 20100906Available from: 2008-05-28 Created: 2008-05-28 Last updated: 2010-09-06Bibliographically approved
6. Stability and Causality of effective material parameters for biased ferromagnetic materials
Open this publication in new window or tab >>Stability and Causality of effective material parameters for biased ferromagnetic materials
2008 (English)In: XXIXth General Assembly, URSI, 2008Conference paper, Published paper (Refereed)
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-8542 (URN)
Note
QC 20100906Available from: 2008-05-28 Created: 2008-05-28 Last updated: 2010-09-06Bibliographically approved

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