  The universal criterion for ultrafast vortex-core switching between the up-
and down-core bistates in soft magnetic nanodots was investigated by
micromagnetic simulations along with analytical calculations. Vortex-core
switching occurs whenever the velocity of vortex-core motion reaches the
critical velocity that is expressed as (e.g. m/s for Permalloy), where Aex is
the exchange stiffness, and is the gyromagnetic ratio. On the basis of the
above results, phase diagrams for the vortex-core switching event and switching
times with respect to both the amplitude and frequency of applied circularly
rotating magnetic field were calculated, which offer practical guidance for
implementing nanodots in vortex states into future solid-state
information-storage devices.
