Magnetization Creep in Thin Nickel—Iron Films

Abstract

The initial magnetization creep threshold of vacuum-deposited thin nickel—iron films has been investigated as a function of film thickness, magnitude of the anisotropy field, and the amount of ``negative'' anisotropy present. The creep threshold, normalized to the transverse field where the probability density function is a maximum, shows a large improvement as the anisotropy field is decreased by uniformly straining the film; this process makes ``negative'' anisotropy regions appear. For isotropic films this normalized creep threshold is inversely proportional to the film thickness for thicknesses above 200 Å. The creep threshold for anisotropic films also decreases with increasing thickness, except for a dip in the thickness range where crosstie walls appear, 250–900 Å. Because the Bloch—Néel—Bloch transition occurs at larger transverse fields for thicker films, this mechanism is rejected. Another theory is proposed for a cause of creep that yields the correct thickness dependence: When the transverse field is zero, the net magnetic charge on the wall is zero. When a transverse pulse is applied, domains with M antiparallel to the applied dc field rotate more than do those with M parallel, creating a net magnetic charge on the wall. The stray field from this charge together with the applied field causes wall motion.