Abstract:
New concepts for micro-wave antennas with special geometries and improved performance require new materials:
Magneto-photonic arrays (MPA) are built of sheets of anisotropic dielectrics and Faraday rotation layers.
Groundplane antennas (GPA) require materials with the relative dielectric constant preferably equal to the relative magnetic permeability.
In both cases the electromagnetic losses should be low:
For MPAs eps = 20...50; tan(delta) ~10^-4 at 10 GHz
For GPAs eps = 10; mu = 10, tan(delta) <5x10^-3 at 2...4 GHz
The objectives can likely be met for the dielectric properties. For the magnetic part the state-of-the-art is very far from the above requirements.
Faraday materials with a high static permeability still show significant loss at GHz frequencies.
The best high frequency magnetic materials have mu = 9 and tan(delta) = 1 at 1.4 GHz with mu diminishing and tan(delta) increasing at higher frequencies.
The Kramer-Kronig relations completely define the relation between the real and complex part of the magnetic permeability, mu' and mu". This implies that any steep variation of mu' with frequency results in very significant values of mu" around that frequency range.
Steep variations of mu' with frequency can be described in terms of several different resonances: Longitudinal resonances, in turn divided into domain wall and rotation resonances. Transverse resonance, aka gyromagnetic resonance. The characteristic frequencies follow the sequence domain < rotation < gyromagnetic
The occurrence and extent of these resonances depend for a given material in a complex way on:
Intrinsic magnetization and anisotropy (ideal unit cell properties)
Grain morphology and the spatial relation between grains (micro-structural properties).
Presence of an internal and/or external static bias field (partly related to the above).
Intrinsic properties are strongly related to composition and crystal structure: spinel (cube), magneto-plumbite (hexa), and garnet.
Most practical materials have a poorly defined and irregular micro-structure which makes analysis and prediction difficult.
Aspects to be considered include:
Effects of grain size, shape and inter-grain exchange/coupling on anisotropy and resonances.
Demagnetizing field in an irregular grain morphology.
Effects of static bias fields on resonance, and higher frequency mu'.
Critical grain size for super-paramagnetism.
Effects of magneto-striction.
Loss mechanisms by geometric scattering, electrical conduction, and through generation of spin waves.
Improvements are likely expected by a combination of the best intrinsic properties with the best possible micro-structure consisting of one or more phases. Some recent improvements by better micro-structure control will be shown.
The objective of the discussion is to present and discuss possibilities for improving high frequency magnetic response properties.