Optimal control of vortex-core polarity by resonant microwave pulses.

In a vortex-state magnetic nanodisc, the static magnetization curls in the plane, except in the core region, where it points out of plane, either up or down, leading to two possible stable states of opposite core polarity p. Dynamical reversal of p by large-amplitude motion of the vortex core has recently been demonstrated experimentally, raising the prospect of practical applications, in particular in magnetic-storage devices. Here we demonstrate coherent control of p by single- and double-microwave-pulse sequences, taking advantage of the resonant vortex dynamics in a perpendicular-bias magnetic field. Experimental optimization of the microwave-pulse duration required for switching p also yields information about the characteristic decay time of the vortex core in the large-oscillation regime. This time is found to be less than half the length seen in the small-oscillation regime, suggesting a nonlinear behaviour of magnetic dissipation. [ABSTRACT FROM AUTHOR]