Memory effects and nucleation of domain structures in small Permalloy shapes

The processes of nucleation and stabilization of domain structures were studied by means of the reflection Kerr effect in small Permalloy discs subjected to alternating demagnetization. Patterns with vortex and composite topologies were observed with probabilities depending on the disc radius. Demagnetizations below a critical amplitude were unable to erase the memory of the previous state; on the contrary, demagnetizing above that threshold yielded statistically independent results, that is, complete erasure. Image-processing techniques and dynamical observations during demagnetization allowed analysis of the random nucleation of a domain structure and its subsequent stabilization inside an energy basin of given topology.