Nonlinear motion of magnetic vortex cores during fast magnetic pulses

The time evolution of vortex spin structures in ferromagnetic FeNi disks (4 μm and 6 μm in diameter and thickness of 30 nm) during fast magnetic field pulses was observed by time-resolved x-ray magnetic circular dichroism with photoelectron emission microscopy. For the smaller disk, the displacement of the vortex core was found to linearly follow the field amplitude as described by an analytical model based on Thiele's equation with boundary conditions for patterned magnets. In contrast, for the larger disk which contains a smaller magnetic restoring force, a nonlinear response of the core displacement to the field amplitude, and also a sub-ns delay of the core motion, was observed. In parallel a deformation of the vortex spin structure was observed. Part of the Zeeman energy is stored in deforming the vortex structure during the rise time of the magnetic field with a large ΔH/Δt.