We have developed a new and nonlithographic process for fabricating large numbers (10 billion) of 100-nm nanoring magnets over a macroscopic area (4 cm^2) using monodispoersive polystyrene (PS) spheres as templates (left panel in Fig. 1). We deposit a Co layer, above, beside, and below the PS spheres. We then use ion beam etching in normal incidence to remove all Co except those under and protected by the PS spheres, resulting in nanorings. The PS spheres can be left over the rings or be chemically removed. SEM image (right panel of Fig. 1) show the Co nanorings. The yellow box is 1 µm x 1 µm enclosing 45 nanorings. This is the highest areal density of nanorings reported.

Fig. 1: (Left) Using PS spheres as templates for making nanorings, (right) SEM image of 100 nm Co nanorings with density 45/µm^2 (yellow box: 1 µm^2).

The magnetic properties can be readily measured using a common magnetometer because of the large number of Co nanorings. The hysteresis loop shown on the left of Fig. 2 exhibits several unusual features. The remanence is about 65%, which is just (2/pi) due to the formation of the "onion" state with two domain walls at the opposite sides of the rings. There are two switching fields due to two distinct switching processes. From the onion state, when the two domain walls move in the same direction towards each other, after annihilation, one obtains the vortex state with no net magnetization. When the two domain walls move in opposite direction, the process is that of onion rotation reversal. The actual hysteresis is the superposition of these two reversal processes. The micromagnetic simulation shown on the right of Fig. 2 agrees closely with the experimental results.

Fig. 2: Hysteresis loop (left) and micromagnetic simulation (right) of arrays of 100-nm Co nanorings showing the two switching fields and demonstrating quantitative agreements.