Vertical magnetization process in sub-micron permalloy dots

For a circular dot with the lateral size larger than the exchange length l_ex(=A^1/2/M), it is well known that the stable remanent state is a vortex structure where the spins around the center of the dot direct up- or down-ward. If only these two states are allowed in the vortex structure, it is possible to use it as a bi-stable memory element. In order to check this possibility, we have investigated vertical magnetization process of various sub-micron permalloy dots by PMOKE and MFM. Polycrystalline circular dots with 180-4000 nm in diameter and 40-80 nm in height were fabricated using electron-lithography and a lift-off technique. While PMOKE measurements show very low remanence against vertical field, MFM detects distinct spin vortex structures where spins around the dot centers direct up-or down-ward. This result suggests a possibility of spin vortex structures as bi-stable switching memory elements, although their averaged remanence is quite small. From LLG calculations, it is found that lateral dimension of the spin vortex coincides with the exchange length l_ex=A^1/2/M_s~15 nm regardless of the dot diameter. Furthermore we have established a magnetic phase diagram of permalloy dots as functions of dot diameter and height