Spin waves in cylindrical nanowires in the vortex state

Due to the reduced dimensions of magnetic nanowires (NWs), the possibility to control spin wave (SW) confinement and to couple electromagnetic waves to the magnetization textures with non-trivial topologies (e.g. vortex state), makes these structures good candidates for SW based information processing technologies. Here, we present experimental results of dynamic stimulation of Fe28Co63Cu10 NWs with 120 nm diameter and 25 micron length. Microwave permeability with DC and microwave magnetic fields perpendicular to the NW axis shows enhanced losses in the low frequency range for magnetic field below 2 kOe. In simulations, we observed the formation of a vortex state on the NW ends for a field close to the one with experimental losses. Investigations on how the modes depend on the distance from the NW end and on the product of vortex polarity and chirality were carried out. We distinguish two types of SW modes: lower frequency modes localized close to the NW ends and higher frequency delocalized modes, which are described as plane waves with a finite pinning at the NW ends. Simulation results are in agreement with the analytical model based on the generalized Thiele equation for the vortex core string, which accounts for the exchange and non-local magnetostatic interactions.