MEMORY EFFECTS AND NUCLEATION OF DOMAIN-STRUCTURES IN SMALL PERMALLOYSHAPES

The processes of nucleation and stabilization of domain structures wer e studied by means of the reflecion Kerr effect in small Permalloy dis cs 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, demagnetiz ing above that threshold yielded statistically independent results, th at is, complete erasure. Image-processing techniques and dynamical obs ervations during demagnetization allowed analysis of the random nuclea tion of a domain structure and its subsequent stabilization inside an energy basin of given topology.