Universal Criterion and Phase Diagram for Switching a Magnetic Vortex Core in Soft Magnetic Nanodots

The universal criterion for ultrafast vortex-core switching between the up- and down-core bistates in soft magnetic nanodots is investigated by micromagnetic simulations along with vortex-core switching that occurs whenever the velocity of vortex-core motion reaches its critical velocity, v(cri) = (1.66 +/- 0.18)gamma root A(ex) (e.g., v(cri) 330 +/- 37 m/s for Permalloy), with the exchange stiffness A(ex) and the gyromagnetic ratio gamma. On the basis of the universality of v(cri), phase diagrams for the vortex-core switching event and switching time with respect to both the amplitude and frequency of a circularly rotating magnetic field are calculated.