A simple approach based on anodic aluminum oxide template-assisted electrodeposition was developed to fabricate large-area Co nanoring arrays on silicon substrate. The ring outer diameter and interspace can be modulated by varying the anodization parameters. Magnetic measurements and micromagnetic simulation revealed that the onion to vortex (O-V) transition is strongly diameter dependent. With increasing the outer diameter from 100 nm to 300 nm, the O-V switching field gradually changes from positive value to negative value. This was also proved by in situ observation of the magnetic states of the rings under different external fields using magnetic force microscope.
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