Rotation of the onion state in micromagnetic rings

Abstract

The behavior of the movement of the two domain walls that make up the onion state in 5 μm Permalloy rings have been studied under rotation using longitudinal magneto-optical Kerr effect. Resistance to the domain wall motion was added by making rings with a cleaved edge known as D-rings. The longitudinal magneto-optical Kerr effect signal contrast was selectively enhanced for different parts of the rings by layers of ZnS optical coating. The contrast enhancement was used to get more information from the captured signal. The micromagnetic rings were fabricated using electron beam lithography and physical vapor deposition.To support the experimental results simulations of the magnetization dynamics were performed and a model predicting the captured signal curves for different setups was developed.The onion state was rotated using an external magnetic field. Together with quarter-coating of the ring this showed two distinct domain walls moving around the ring. When the strength of the external driving field was decreased below a certain threshold there was a sudden change in Kerr contrast consistent with a transition to the vortex state.D-rings with increasing amounts of cutoff showed an increasing resistance to the movement of the domain walls through increased pinning. This also increased the field strength for which the onion state could no longer be rotated without collapsing into the vortex state.