8.6 MAGNETIC RAM

So far in discussing RAM design we have limited ourselves to materials whose relative complex permeability is ? _r=1+i0. For that case, the intrinsic impedance of the material is simply 377/ , and the index of refraction is . Magnetic materials are those whose relative permeabilities are different than that of free space. Clearly, the presence of permeability values greater than 1 gives us significantly more freedom in tailoring the intrinsic impedance and index of refraction of a material to meet our needs for absorber performance. However, although we will focus in this section on the magnetic properties of materials, the magnetic materials available for use in RAM generally have relative permittivities higher than their relative permeabilities. Therefore, in practice we do not deal with purely magnetic absorbers, that is, those with =1+i0, but with materials that contain both magnetic and electric loss properties.

The catalog of materials available for use as magnetic absorbers at microwave frequencies is large, but iron or compounds of iron are most often exploited. The two most common magnetic materials employed in RAM are carbonyl iron and ferrites.

Carbonyl iron is a pure iron powder (the carbonyl term relates to how it is produced), with particle diameters ranging from a few micrometers to tens of micrometers. RAM performance is a function of particle size, and in the ideal situation individual particles contain a sufficient number of magnetic domains that they are isotropic, but small enough that self-shielding is not a problem...