Instantaneous reversible permeability in amorphous ferromagnets

The instantaneous reversible permeability is the permeability measured, by an alternating test field of small amplitude and high frequency, on the various points of the hysteresis loop during the slow traversing of the magnetization. We report here the behaviour of this permeability for tape-wound ribbons of amorphous alloys 2605 and 2826 in various conditions. As for polycrystalline materials, this permeability is generally much higher than that measured at the same points of the hysteresis loop under static fields, particularly in the region of the ac hysteresis loop where large irreversible magnetization jumps occur. The characteristics of the real part of the instantaneous permeability and those of the loss factor suggest that the permeability can increase because the test field is able to produce irreversible displacement of some of the walls which are removed from their minimum energy sites during the reversal of the magnetization. The behaviour of the instantaneous permeability compared to that of the gross magnetization rate indicates that an additional contribution to the instantaneous permeability can arise from an increased wall mobility due to the hindering of the after-effect.