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Bit-patterned media (BPM) with 20 nm diameter dots and a 72 nm dot period were fabricated from hard/soft-stacked 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. All recorded dots were in single domain state, as expected from previous experiments. The write margin was about 50 nm, and there was a total margin loss almost corresponding to the dot diameter. According to an analysis of margin loss factors based on the slope recording model, the margin loss factors for compensating effect of multidomain dots decreased drastically. Therefore it was confirmed that the slope recording model well illustrated the recording process of small dots in single-domain states. The margin loss factor due to the dot position jitter was 17.7 nmp-p and became the dominant loss factor. The loss factor attributable to the switching field distribution also increased rapidly to 11.3 nmp-p and was the second largest loss factor. To secure a sufficient write margin for a robust recording system, suppression of these degradation factors in the BPM fabrication process is required.