We have investigated the magnetic domains of high coercive FePt/MgO films with various morphologies by magnetic force microscopy (MFM). The film morphology changes from particulate to continuous depending on the film thickness (t), accompanied by a drastic decrease in coercivity when t exceeds the critical thickness, tc = 45 nm. The film with t = 10 nm is comprised of only single-domain particles of approximately 50 nm, which has a huge coercivity as high as 40kOe. The isolated particles larger than 200 nm are multi-domain, and the films containing these isolated particles show slightly lower coercivity. The coercivity drastically decreases with the percolation of the particles.
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