Magnetic losses in composite materials

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.