Configurational stability and magnetization processes in submicron permalloy disks

A finite-element micromagnetic approach is employed to study magnetization reversal processes in submicron permalloy disks of various sizes, with diameter between 50 and 500 nm and thickness between 5 and 200 nm. The reversal is accomplished by a fixed-directional in-plane magnetic field. Depending on which (meta)stable states are accessible in the magnetization path, various types of hysteresis loops are observed. For example, for thin disks (<5 nm), the magnetization remains in an "onion" (almost a single-domain) state throughout the process, resulting in a square loop. For thick disks (>50 nm), the magnetization collapses to a vortex state, resulting in a dumbbell-looking loop. For disks whose diameters are larger than 200 nm, the magnetization can pass through some intermediate buckle state before collapsing to either a vortex or an onion state. In all cases, the reversal process is dictated by the stability of the magnetic configuration. For some disks, a rotational field is used effectively to reverse the magnetization and hence avoid the so-called configurational anisotropy effect. The spread function is introduced to quantify the degree of nonuniformity of a magnetic configuration. This quantity is particularly helpful in studying the evolution of a magnetic pattern by the action of an external field.