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Bit-patterned media (BPM) samples were fabricated from hard/soft-stacked [exchange-coupled composite (ECC)] base media with a [Co/Pt]-super-lattice hard layer and a Co soft layer. The write margin of synchronous recording was evaluated using a static tester. The write margin of 50 nm dots with a 120 nm period was confirmed to be 65 nm and a large margin loss almost corresponding to the dot diameter was due to the occurrence of multidomain (MD) dots. Behavior analysis of the average MD dot formation rates within the transition regions beside the write window revealed that recording dots with diameters 30 nm, or smaller, are required to eliminate MD dots. A close association between the MD dot formation rate and the write error rate was confirmed. Therefore, a reduction of the large margin loss arising from the MD dots and a resultant improvement in the write error rate is expected when the recording dots become smaller than 30 nm.