Tuesday, November 25, 2014
335 West Hall
Ferromagnetic nanostructures exhibit unique magnetic states and have potential applications in the data storage industry. Structures with circular geometries can exist in the vortex state, in which the magnetic moments align circumferentially in the clockwise or counterclockwise direction. For a symmetric ring in a uniform field, these states are energetically degenerate and cannot be selected experimentally. A circular field allows us to study the switching behavior between these vortex states. We have developed an experimental technique to apply a local circular field by passing current through the tip of an atomic force microscope. Using this technique, we have demonstrated the switching of symmetric and asymmetric rings between the vortex states and investigate the controllable motion of individual domain walls (DWs). Our simulations show that for rings with thicknesses on the order of an exchange length, the switching process will occur through the formation of pairs of 360o DWs of opposite topological index when the field is positioned at the center of the ring. The 360o DW with the same circulation as the ring vortex will annihilate first as the external field is increased. If the field is not centered, we are able to manipulate individual DWs in rings and wires, moving 180o DWs together to form 360o DWs and selectively annihilating walls. Some of these states do not exist under a uniform in-plane field, and are relevant to proposed data storage devices.