Characterization of keepers for film memories

A permeability measurement technique is discussed for evaluating the frequency response of sintered and cast ferrite keepers. Real and imaginary parts of the relative permeability for several ferrite materials in both sintered and cast form are compared over a large frequency range. The large difference between sintered and cast keepers at low frequencies and the difference in their resonance behavior at high frequencies is to be noted in particular. Optimization of the cast keeper permeability is investigated through the parameters of ferrite particle permeability and porosity. The dependence of particle permeability on particle size, and of porosity on particle size distribution is established and then exploited to obtain a higher permeability keeper.