Role of the dimerized gap due to anion ordering in spin-density wave phase of (TMTSF)$_2$ClO$_4$ at high magnetic fields

Magnetoresistance measurements have been carried out along the highly conducting a axis in the FISDW phase of hydrogened and deuterated (TMTSF)$_2$ClO$_4$ for various cooling rates through the anion ordering temperature. With increasing the cooling rate, a) the high field phase boundary $beta_{rm {HI}}$, observed at 27 T in hydrogened samples for slowly cooled, is shifted towards a lower field, b) the last semimetallic SDW phase below $beta_{rm {HI}}$ is suppressed, and c) the FISDW insulating phase above $beta_{rm {HI}}$ is enhanced in both salts. The cooling rate dependence of the FISDW transition and of $beta_{rm {HI}}$ in both salts can be explained by taking into account the peculiar SDW nesting vector stabilized by the dimerized gap due to anion ordering.