Bloch lines, cross ties, and taffy in permalloy (invited)

Abstract

Cross ties and their attendant Bloch lines have been imaged optically for the first time in permalloy films. Kerr effect images, recorded with our laser magneto-optic microscope (LAMOM), on 50-μm square films (300, 800, and 5800 Å thick), are consistent with wall types seen in large sheet films by other methods. In the 300- and 800-Å samples 180° domain walls are cross-tie type; ties do not occur in the 5800-Å sample. We observe several effects of film-edge proximity on the 180° walls not previously reported. In the thinner films, the density of ties (and Bloch lines) increases as a wall is driven closer to an edge. Magnetization sectoring occurs about the edge side of a wall when it is forced sufficiently close. For all three thicknesses, the 180° walls bow toward the nearest edge, the bow increasing with proximity. The 180° walls arise from the lowest-energy demagnetized state (Kittel configuration for a square) by applying a bias field which displaces the cluster knot formed by two intersecting 90° walls on the square's diagonals. At a threshold on the order of 10 Oe, the cluster splits, forming two triplet knots connected by the 180° wall. Following bifurcation, all walls bisect the angles between adjacent domain magnetizations. This indicates that the energies of formation for the 180° wall and the extra knot stem from magnetic charge accumulated near the walls and film edges prior to the transformation. Knot displacement and bifurcation can be reversed provided no wall makes contact with an edge. Edge contact (followed by bias reduction) nucleates complex domain structures which are not simply returned to the Kittel state. When the drive is cycled, the process is reminiscent of pulling taffy.