Evolution of disorder in magnetic stripe domains. I. Transverse instabilities and disclination unbinding in lamellar patterns

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Abstract
Video polarization microscopy, optical diffraction, and digital image analysis have been employed to investigate the transformation of a well-ordered lamellar (‘‘smectic’’) domain phase of ferrimagnetic garnet films into a globally disordered, ‘‘labyrinthine’’ pattern. Surprisingly, in view of the presence of nonlocal interactions, the ensuing pattern evolution in response to temperature-induced strain is characterized, in its first stages, by a (local) transverse elastic response which manifests itself in the form of ‘‘smectic’’ instabilities and generates undulation and chevron (‘‘zigzag’’) patterns. At a characteristic limit of accumulated strain, the nucleation of topological defects in the form of disclination dipoles initiates the second stage of the evolution. The subsequent continuous, topologically constrained ‘‘unbinding’’ of these dipoles represents the essential mechanism mediating the loss of global orientational order in the pattern. The application of a set of algorithms for line-pattern analysis to track the motion of individual topological charges permits the quantitative description of this process. The emerging labyrinthine patterns represent the result of a constrained optimization and, while globally disordered, have in fact been shown to exhibit a robust and well-defined local structure in the form of ‘‘cybotactic’’ clusters.