Effective permeability of soft magnetic layers adjacent to recorded media

The finite thickness of soft magnetic layers used to "keeper" magnetic recording media results in an effective permeability of the soft magnetic layer which is lower than the bulk material permeability. Multiple imaging techniques are used to create expressions for the effective permeability as a function of soft magnetic film thickness and the magnetic recording frequency. Using measurements of leakage fields through the keeper layer from a magnetized recording media at various recording frequencies and with various keeper thicknesses we can determine the permeability of the soft magnetic layers vs keeper thickness and recording frequency.