  We fabricated Co nano-rings incorporated in the vertical pseudo-spin-valve
nanopillar structures with deep submicron lateral sizes. It is shown that the
current-perpendicular-to-plane giant magnetoresistance can be used to
characterize a very small magnetic nano-ring effectively. Both the onion state
and the flux-closure vortex state are observed. The Co nano-rings can be
switched between the onion states as well as between onion and vortex states
not only by the external field but also by the perpendicularly injected dc
current.
