Domain Wall Structure and Magnetization Creep in Ni–Fe Films 1200–2000 Å Thick

Abstract

The wall structure and magnetization-creep behavior of homogeneous polycrystalline films 1200–2000-Å thick were examined by Lorentz microscopy. The films were then annealed in the electron microscope to enlarge the crystallites and increase the inhomogeneity. Both the wall structure and the creep process are radically different in the two cases. Homogeneous films thicker than ∼1000 Å have uniform, cross tie-free walls, but the annealed films have very pronounced cross ties, of a different variety than those in thinner films: either the cross tie or the wall is discontinuous so that the wall runs through a hole in the cross tie or the cross tie runs through a hole in the wall. In one degenerate variety, the wall consists only of Bloch lines and cross ties: the magnetization curls 360° around each Bloch line and the cross ties separate the curls. The inhomogeneous films crept by the distorting hump mechanism previously reported for conventional cross tie walls. The homogeneous films have no cross ties, but smaller humps are induced by two processes: (1) variations of coercivity, and (2) anisotropy dispersion which causes the magnetization to rotate more in some parts of the film than in others, thus causing magnetostatic charges to form on a small segment of the wall, causing local movement. Micrographs of an 1800-Å film are presented showing four different kinds of cross ties, the creep process in annealed films and the two different kinds of hump formation and creep in the homogeneous films. Creep measurements from loop apparatus confirm that the extra easy-axis field induced by magnetization rotation near the creeping walls (the field that causes creep) is much greater in inhomogeneous films in this thickness range than in homogeneous films.