Magnetization reversal in elliptical-ring nanomagnets

The magnetization reversal exhibited by elliptical-ring-shaped magnets has been investigated using magnetic force microscopy and micromagnetic simulations. Arrays of cobalt elliptical-ring structures of thickness 12 nm, long axis 930 nm, short axis 620 nm and ring widths 110, 140 and 200 nm were investigated. The rings show transitions between onion (bidomain) states, twisted states (containing 360degrees walls) and vortex (flux-closed) magnetization states with increasing field. The switching fields are lower for wider rings, and depend on the applied field direction. When the field is parallel to the short axis, the vortex state is stable over a smaller range of fields, intermediate states such as 360degrees walls are more commonly observed, and the rings show greater reversible magnetization.