Effect of Asymmetric Shape on Vortex Control in Permalloy Rings

A method to control the vortex state (flux-closure state) nucleation and annihilation field in asymmetric permalloy (Py) rings is presented. By introducing an inner circle shifted from the center of outer circle by a shift length, samples of asymmetric permalloy rings with different asymmetric ratios were fabricated by using E-beam lithography, sputtering, lift-off techniques, and measured by focused magneto-optic Kerr effect magnetometry. The vortex nucleation (H_n) and vortex annihilation (H_an) fields, analyzed from the simulated and experimental hysteresis loops of 500 nm and 1000 nm outer diameters, have linear relations to the asymmetric ratio. When the lateral size reduces down to nanosize, two switching behaviors for the simulated hysteresis loops were observed. The single switching loop indicates that the switching process is single-domain switching and the all the spins rotate coherently. The double switching loop corresponds to the switching process via the formation of a vortex state. Both analyzed switching fields from the experiment and simulation present a linear relation with the aspect ratio in the small range of variation. This relation indicates that the H_n and H_an of asymmetric ring arrays are mainly controlled by the shape anisotropy. It is observed that the switching behavior is in agreement with the experimental results.