  We fabricated a current-perpendicular-to-plane pseudo-spin-valve nanopillar
comprising a thick and a thin Co rings with deep submicron lateral sizes. The
dc current can effectively induce the flux-closure vortex states in the rings
with desired chiralities. Abrupt transitions between the vortex states are also
realized by the dc current and detected with the giant magnetoresistance
effect. Both Oersted field and spin-transfer torque are found important to the
magnetic transitions, but the former is dominant. They can be designed to
cooperate with each other in the vortex-to-vortex transitions by carefully
setting the chirality of the vortex state in the thick Co ring.
