Driven dynamic mode-splitting of magnetic vortices

It has been established theoretically and experimentally that a magnetic vortex in restricted geometry possesses a translational excitation that corresponds to circular motion of the vortex core at a characteristic frequency. Here we explore the effect of increased driving-field amplitude on this dynamic mode using a microwave reflection technique. We find a new effect - the vortex translational eigenmode splits into two peaks. The splitting in frequency is $>$25\% for driving magnetic fields $<$25 Oe for micron-sized permalloy ellipses that are 40-nm thick. Splitting effects were detected for driving fields as low as 3 Oe in circular dots. Micromagnetic modeling suggests this effect could be indicative of nonlinear fold-over but further theoretical work will be required to develop a full understanding of these observations.