Commensurate vortex-core switching in magnetic nanodisks at gigahertz frequencies

The development of future spintronic applications requires a thorough and fundamental understanding of the magnetization dynamics. Of particular interest are magnetic nanodisks, in which the vortex state emerges as a stable spin configuration. Here, we focus on how the vortex-core polarization can be reversed periodically by an oscillating magnetic field, applied perpendicularly to the disk's surface. By means of micromagnetic simulations, we demonstrate the presence of several subharmonic switching modes, i.e., the commensurate ratio between the switching frequency of the core and the driving frequency. The underlying mechanism of this periodic behavior depends on the disk thickness. For thin disks, the core switches periodically due to resonant excitation of radial spin wave modes, while it is due to the breathing mode in the case of thick disks. However, overlap of both modes impedes periodic vortex-core switching. For thin disks, the threshold field amplitude required for periodic switching can be substantially lowered by increasing the disk diameter. For thick disks, in contrast, the diameter is less decisive for the minimal field amplitude, as only the average energy density in a central region around the vortex core is relevant to excite the breathing mode. Our results contribute to the understanding of the switching mechanisms in magnetic nanodisks, which are of technological interest due to their potential in nonvolatile memory devices.