Content area
Abstract
The magnetization reversal in small ferromagnetic particles is investigated theoretically. A numerical model is developed, which represents the particles as a micromagnetic array of interacting cubic elements. Equilibrium and transient states are computed for various magnetic materials, in particular, $\gamma$-Fe$\sb2$O$\sb3$, Co-modified $\gamma$-Fe$\sb2$O$\sb3$, passivated and unpassivated iron particles. It is shown that for particle sizes typical for magnetic recording the topologies of the equilibrium states can be characterized in a simple fashion as generalized curling states. Similarly the irreversible switching mechanisms are a superposition of generalized buckling and curling processes, which become quasi-uniform in the limit of small particle size. The magnetization reversal starts at the particle ends and propagates towards the center of the particle. The relevant length scale determining the uniformity of the reversal processes in the above listed materials is the exchange length. The switching field has been computed as a function of particle size and of the angle of the applied field. The calculations include also the effects of non-uniform applied fields. It has been demonstrated that field gradients typical for recording head fields are significant for the reversal process. In a head field the magnetization reversal proceeds asymmetrically from the high field region to the low field region. Features of transient states as obtained in a uniform applied field may become stabilized by the inhomogeneous applied field. For high exchange coupling irreversible switching is determined by the volume average of the applied field. For relatively lower exchange coupling the magnitude of the applied field near the leading particle end determines the irreversible switching.
