Irreversible processes in soft-ferromagnetic thin films

By means of Bitter observations, irreversible magnetization transformations have been studied in rectangular Permalloy bars of various sizes which were exposed to a uniform external field. Samples prepared by the lift-off technique showed a remarkable deviation in the domain structure conversions compared with samples prepared by RF-sputter etching. This can be attributed to edge defects. Besides the impact of edge imperfections on the hysteresis of the object, it is shown that even without defects a response of the volume-average magnetization to the external field exhibits hysteresis as soon as one single irreversible transformation has taken place. The functional dependence of the value of the external field at which the irreversible transformations reveal themselves on the dimensions of the rectangular bars is derived by using a surface-charge model. The predictions agree very well with the observations. The irreversible behavior of these particular transformations is explained by considering the free energy of the object, which involves magnetostatic and exchange energy.