FREQUENCY-DEPENDENCE OF THE TRANSVERSE BIASED PERMEABILITY IN THIN PERMALLOY-FILMS

The frequency dependence of the transverse biased permeability mu(t)'(f ) has been measured up to 250 MHz in thin (0.044 mum) and thick (2.1 mum) unpatterned sputtered permalloy films with uniaxial anisotropy. In these films, the initial permeability along the hard axis can be described by the classical eddy current damping model assuming uniform rotation. For both films, mu(t)' at low frequency agrees reasonably well with Hoffman's ripple model assuming appropriate values for the structure factor. In the thin film, mu(t)'(f) is independent of frequency within experimental error up to at least 250 MHz. However, in the thick film mu(t)'(f) decreases more rapidly with frequency than can be explained by classical eddy currents, when the transverse bias field H(B) is not very large compared to the net anisotropy field. The value of H(B) at which mu(t)'(f) reaches a maximum amplitude is also found to increase monotonically with increasing frequency, also inconsistent with simple eddy current predictions. It is suggested that the formation of ripple walls leads to anomalous eddy current losses similar to that observed in bulk materials.