Worm motion of domain walls in permalloy films

Abnormal wall motion in Permalloy films was observed by Kerr magneto-optic apparatus under hard-axis pulse drive with nanosecond rise time. Although this phenomenon is like creeping, the observed domain tip and sidewise motion is not monotonous, but looks like worm motion. Each domain expands and shrinks quasi-periodically, and the net magnetization along the easy direction is never constant. This phenomenon depends closely on the field strength and pulse width as well as the rise time of the drive pulse. The thickness range of the films observed was from 330 to 3500 Å, and for the films of thicknesses below 860 Å, worm motion never occurred. This suggests that the Bloch wall configuration is essential for such motion, and this was also verified by Bitter patterns. Worm motion was detected not only by Kerr magneto-optic apparatus, but also as a change of amplitude of transverse induced voltage measured with a strip line and sampling oscilloscope system. A theoretical velocity of walls during worm motion was estimated assuming a simple model for Bloch walls and compared with experimental results. The so-called creeping in the Bloch wall thickness region was found to be closely related to the worm motion of domain walls.