In-plane reversal mechanisms in circular Co dots

The reversal process of polycristalline circular Co dots is investigated for dot sizes for which the vortex state corresponds to the ground state configuration. Using magneto-optic Kerr magnetometry and magnetic force microscopy imaging, it is found that during the reversal of the magnetization, the system does not necessarily pass through its ground state configuration. For large dot diameters and thickness, the reversal occurs via nucleation, propagation, and expulsion of a single vortex, whereas for a dot thickness below 20 nm the reversal process may occur by a coherent rotation of the quasi-single-domain configuration. Furthermore, a double-vortex state formation has been evidenced for 1000-nm-large dots.