High resolution magnetic force microscopy (MFM) has been carried out on L1 0 -FePt dot arrays patterned by plasma modified nanosphere lithography. An ex situ tip magnetization reversal experiment is carried out to determine the magnetic domains and verify the imaging stability of MFM and the mutual perturbations between the magnetic tip and the sample. We have identified that the critical size for the single domain region is about 90 nm across. Comparison with MFM image simulation also suggests

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... that the magnetizations of the triangular dots in both single and double domain states are parallel to one edge of the dots, indicating the large uniaxial magnetocrystalline anisotropy of the L1 0 -FePt phase and the need for decreasing the magnetostatic energy.