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Domain structures in CoFeB-MgO thin films with a perpendicular easy magnetization axis were observed by magneto-optic Kerr-effect microscopy at various temperatures. The domain-wall surface energy was obtained by analyzing the spatial period of the stripe domains and fitting established domain models to the period. In combination with superconducting quantum interference device measurements of magnetization and anisotropy energy, this leads to an estimate of the exchange stiffness and domain-wall width in these films. These parameters are essential for determining whether domain walls will form in patterned structures and devices made of such materials.