Dielectric effects of ferrite flux keepers in thin-film memories

Flexible ferrite keepers may show relatively high dielectric constants (\varepsilon_{r} \approx 10). In a magnetic thin-film memory, both permeability and permittivity are distributed discontinuously. The distribution affects the transmission and coupling properties of the drive lines. A simple model permits separate calculation of inductive and capacitive coupling between two parallel wires above ground. The keeper reduces the inductive and enhances the capacitive part of coupling. It depends on keeper properties and line spacing whether the resulting coupling is increased or decreased. Experiments confirmed the calculated predictions in general. With both wires and striplines, the keeper will increase coupling in most cases of practical interest. A good keeper should have a dielectric constant as low as possible. Among the samples investigated, those containing the Ni-Zn ferrite proved to be most suitable.