Arrays of 100 nm Magnetic Nanorings with High Areal Densities

Arrays of magnetic nanorings with high areal densities have special attributes. To date only a small number of μ m-size rings have been made by lithography. Here we present a new lithography-less process for fabricating a large number of 100 nm Co nanorings over a macroscopic area with an extremely high areal density (45 rings/μ m^2). Hysteresis loops show two switching fields at about 150 Oe and 2 kOe. Micromagnetics simulations reveal two reversal processes. In the first process two domain walls in the onion state attract and annihilate to form a vortex state. The stable vortex state requires 2 kOe to induce two domain walls and form the reversed onion state. The second process with a switching field of about 50 Oe involves the rotation of the onion state by 180r with the two domain walls at the opposite ends. Both reversal processes exist simultaneously in our sample, and the hysteresis behavior is the superposition of two such hysteresis loops. Work supported by NSF and DARPA.