Nucleation and annihilation of magnetic vortices in sub-micron permalloy dots

The magnetization reversal processes in Fe20Ni80 dot arrays have been studied both experimentally and theoretically. The circular-shaped dots with thickness of 60 mn and variable diameters of 0.2-0.8 mum were fabricated by using electron-beam lithography and lift-off technique. A size dependent vortex nucleation, annihilation and initial susceptibility were determined from magneto-optical hysteresis loops. With increasing dot diameter the nucleation and annihilation fields decrease, whereas the initial susceptibility exhibits a linear increase. The experimental data were compared to the results of analytical calculations taking into account magnetostatic interactions, exchange and Zeeman energies for a planar array of magnetically soft circular dots.