Interaction-induced anisotropy in the onion-to-vortex transition in dense ferromagnetic nano-ring arrays

We show that the onion-to-vortex switching field in dense arrays of nanostructured ferromagnetic rings is strongly dependent on the angle between the applied magnetic field and the array's main axis. The variations in switching field of up to 8 mT are connected to the anisotropy produced by dipolar interactions between domain walls in the rings. The interactions stabilize the onion state in aligned arrays but assist domain wall rotation and onion-to-vortex switching in rotated arrays. These results are established using magneto optical Kerr effect measurements of major and minor hysteresis loops together with micromagnetic simulations.