Micromagnetics of Domains and Walls in Soft Ferromagnetic Materials

Magnetization processes in soft ferromagnetic materials consist of complex domain wall movements and domain structure transitions. A micromagnetic model is developed to numerically simulate such dynamic behaviors in order to achieve fundamental understanding of the switching mechanisms involved in magnetic recording sensors, such as thin film inductive heads and magnetoresistive heads. Various energy minimization schemes have been utilized to study the quasi-static equilibrium properties of these micro -structures, whereas the phenomenological Landau-Lifshitz equation is solved to follow the dynamic magnetization response to external driving fields. It is observed that the intrinsic gyromagnetic damping effect introduces dissipation loss in domain wall motion, and induces dynamic transitions of wall structures at high amplitude and frequency driving fields. The dynamics of the flux-closure asymmetric vortex wall in permalloy thin films with thicknesses ranging from 500 A to several microns has been studied in detail. The effective wall mass and viscosity coefficients are obtained to characterize wall motions in a simple way. The effect of eddy current damping on thick film wall motions is discussed within the micromagnetic context. The magnetic reversals of very thin permalloy platelets have also been investigated. The interactions of magnetostatic and exchange effects introduce formations of transient vortices and multi-domain states for these small (micron-scale) devices, which in turn act as the sources of hysteretic and noisy response to external fields.