Fluss-rückschluss in magnetischen dünnfilmspeichern

An isotropic soft magnetic layer as a "flux keeper" on top of a magnetic thin-film memory matrix reduces the stray fields of both the film spots and the drive lines. The magnetostatic field distribution around and within a homogeneously magnetized rectangular film spot near a homogeneous, isotropic magnetic layer of a) infinite and b) finite thickness is calculated. Problem a has been solved in three dimensions by introducing image magnetizations. A solution of problem b in two dimensions has been derived using a special mathematical method to obtain and to superpose particular solutions of Poisson\´s equation. As results, the demagnetizing field and the part of the flux closing through the substrate are given in dependence on the flux keeper\´s permeability, thickness, and distance from the film. The field of a stripline below the flux keeper is also calculated with some simplifications. Increase of usable drive field and decrease of stray field of the lines dependent on geometry and keeper\´s permeability are shown. As shown by measurements, the modification of the static and dynamic fields of spots and lines leads to considerable improvements in sense voltages as well as in disturb and switching thresholds of the film spots. The results are discussed with respect to the contributions of the various field effects.