The domain pattern evolution in magnetic fields along the diagonal of a square Permalloy film element (edge length 57 μm, thickness 207 nm) is studied theoretically and by Kerr-microscopic observation. Equilibrium configurations and magnetization curves are computed using a reduced two-dimensional model, based on minimization of the micromagnetic free-energy. Two main hysteretic events are found. The first is associated with domain walls reaching the sample edge, the second one with the expulsion of vortices from the interior of the sample. Only the second event leads to a remanent state that is different from the initial ac-demagnetized state. Our analysis shows that the first event corresponds to the nucleation of two regions of field penetration within the sample, while the second event is related to the merging of such regions causing a change of topology of the nonpenetrated part in the center region of the sample.
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