  Magnetic skyrmion motion induced by an electric current has drawn much
interest because of its application potential in next-generation magnetic
memory devices. Recently, unidirectional skyrmion motion driven by an
oscillating magnetic field was also demonstrated on large (20 micrometer)
bubble domains with skyrmion topology. At smaller length scale which is more
relevant to high-density memory devices, we here show by numerical simulation
that a skyrmion of a few tens of nanometers could also be driven by
high-frequency field oscillations but with the motion direction different from
the tilted oscillating field direction. We found that high-frequency field for
small size skyrmions could excite skyrmion resonant modes and that a
combination of different modes would result in the final skyrmion motion with a
helical trajectory. Because this helical motion depends on the frequency of the
field, we can control both the speed and the direction of the skyrmion motion,
which is a distinguishable characteristic compared with other methods.
