[show abstract][hide abstract] ABSTRACT: We demonstrate magnetic switching between two $360^\circ$ domain wall vortex
states in cobalt nanorings, which are candidate magnetic states for robust and
low power MRAM devices. These $360^\circ$ domain wall (DW) or "twisted onion"
states can have clockwise or counterclockwise circulation, the two states for
data storage. Reliable switching between the states is necessary for any
realistic device. We accomplish this switching by applying a circular Oersted
field created by passing current through a metal atomic force microscope tip
placed at the center of the ring. After initializing in an onion state, we
rotate the DWs to one side of the ring by passing a current through the center,
and can switch between the two twisted states by reversing the current, causing
the DWs to split and meet again on the opposite side of the ring. A larger
current will annihilate the DWs and create a perfect vortex state in the rings.