Pinning of magnetic vortices in microfabricated permalloy dot arrays

Temperature dependent magnetic properties of vortices trapped in a lithographically patterned Permalloy disk were examined. A large residual magnetization at 5 K was observed in hysteresis curves unlike theoretical prediction. The residual magnetization, coercive field, and initial susceptibility were found to be dependent on temperature. Escaping from the pinning potential was facilitated by the increase of temperature, and the pinning temperature T _pin was 9.6 K. The vortex is effectively pinned at the pinning potential when T<T _pin. This physical picture is well supported by the temperature variations of ac susceptibility for the biased dc field. The energy barrier is probably originated from defects such as the edge and surface roughnesses, and irregular grain boundaries in the disk.