Domain Nucleation and Boundary Effects in Thin Uniaxial Plates

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Abstract
In high-mobility materials with large uniaxial anisotropy (K>2πMs2) the magnetostatic wall-energy model as applied to an infinite lamina apparently provides quantitative explanation of most observed domain structures. However, the presence of boundaries and nucleation energy barriers usually prevent the spontaneous development of the equilibrium (lowest energy)1 structures. To understand this, we argue that the confluence of any two domains (or a domain and an edge boundary), or the reverse process of separation, always involves an intermediate state of higher energy. Consequently, for example, the periodic stripe array should not be expected to spontaneously divide into a lattice of bubbles when the latter is a lower energy structure. Similarly, the repulsive interaction with the boundaries prevents the equilibrium field dependence of the lattice nearest-neighbor distance. Our observations on an epitaxial Ga–YIG film2 are shown to be entirely consistent with these qualitative considerations, while the observed domain critical fields, sizes, and spacings are quantitatively predicted by the wall-energy model. Calculations of the nucleation energy for radial growth were carried out for a semicircular (half bubble) domain at a crack and at a free edge. The latter process is even more costly energetically than volume nucleation while the former requires less than half the volume nucleation energy. At crack intersections the barrier is reduced still further. Our observations on the Ga–YIG film show that domain nucleation occurs readily at cracks and rarely at edges in agreement with the theoretical suggestions. A more complete discussion of this work will be published elsewhere.

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