Wall clusters in thin soft ferromagnetic configurations

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Abstract
The wall cluster, which is a new concept in ferromagnetic domain theory, is introduced as a tool for getting a grip on the development of domain structures. A wall cluster consists of an arbitrary number of walls, which have a common intersection line. This paper is confined to wall clusters in thin ferromagnetic layers, such as the permalloy overlayers in bubble devices. Two classes of edge clusters, the intersection of which coincides with one of the edges of the ferromagnetic configuration, are distinguished. The odd-edge cluster bridges the outermost domains by an odd number of walls. The magnetization directions in these exterior domains are parallel. In contradistinction, the outermost domains, in which the tangential components to the edge are opposite, require an even-edge cluster in between. It is shown that the magnetization direction in each of the domains is determined by the position of the walls. The clock sense of the rotation of each successive wall in an edge cluster changes the sign. The transformations of edge clusters are also extensively studied. In particular, the annihilation and the creation of complete clusters are discussed. Cluster creation is a process in which a cluster is generated by a continuous succession of equilibrium states. In this context the definition of the creation line, one from which all the walls of the clusters grow, is of significance. The experimental evidence is given for all aspects of the theory.

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