Fast magnetization switching of Stoner particles: A nonlinear dynamics picture

The magnetization reversal of Stoner particles is investigated from the point of view of nonlinear dynamics within the Landau-Lifshitz-Gilbert formulation. The following results are obtained. (1) We clarify that the so-called Stoner-Wohlfarth (SW) limit becomes exact when the damping constant is infinitely large. Under the limit, the magnetization moves along the steepest-energy-descent path. The minimal switching field is the one at which there is only one stable fixed point in the system. (2) For a given magnetic anisotropy, there is a critical value for the damping constant, above which the minimal switching field is the same as that of the SW limit. (3) We illustrate how fixed points and their basins change under a field along different directions. This change explains well why a nonparallel field gives a smaller minimal switching field and a short switching time. (4) The field of a ballistic magnetization reversal should be along a certain direction window in the presence of energy dissipation. The width of the window depends on both the damping constant and magnetic anisotropy. The upper and lower bounds of the direction window increase with the damping constant. The window width oscillates with the damping constant for a given magnetic anisotropy. It is zero for both zero and infinite damping. Thus, the perpendicular field configuration widely employed in the current experiments is not the best one since the damping constant in a real system is far from zero.