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-s), it is well known that the stable remanent state is a vor tex structure where the spins around the center of the dot direct up- or do wn-ward. If only these two states are allowed in the vortex structure, it i s 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 mn in height were fabricated using e lectron-lithography and a lift-off technique. While PMOKE measurements show very low remanence against vertical field, MFM detects distinct spin vorte x structures where spins around the dot centers direct up- or down-ward. Th is result suggests a possibility of spin vortex structures as bi-stable swi tching memory elements, although their averaged remanence is quite small. F rom LLG calculations, it is found that lateral dimension of the spin vortex coincides with the exchange length l(ex) = A(1/2)/M-s similar to 15 run re gardless of the dot diameter. Furthermore we have established a magnetic ph ase diagram of permalloy dots as functions of dot diameter and height.