On the amount of magnetic material necessary in broadband magnetic absorbers

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.