Magnetic states and magnetization processes of Ni-Fe/Hf annular dots as candidates of non-volatile memory cells

We have investigated Ni-Fe/Hf annular dots that have closed magnetic circuits as candidates of non-volatile magnetic memory cells. The shapes of the annular dots are circular, square and triangular. We have also investigated ordinary circular, square and triangular dots for comparison. The circular annular dots and the triangular annular dots have the vortical magnetic configurations whose magnetizations are along the sides of the annular dots. On the other hand, the circular dots have the vortex configurations whose cores are at the center of the circular dots. The square annular dots, the square dots and the triangular dots have complicated domain structures. The images of magnetic force microscopy (MFM) for the magnetic dots coincide with the magnetic configurations of 20-50 Oe computed using the Landau-Lifshitz-Gilbert (LLG) equation.