Two distinct magnetic oscillations in a high-field spin-density-wave phase for the organic conductor (TMTSF)(2)ClO4

Magnetic torque measurements have been performed in the field induced spin-density-wave phase of the quasi-one-dimensional conductor (TMTSF)(2)ClO4. Over a wide temperature and magnetic field region, we found two distinctly different quantum oscillation (''rapid oscillation'' RO I and II) wave forms in the magnetization. Both exhibit the same frequency for B parallel to c* but the phase is different by pi. RO I is dominant above similar to 20 T in a high-temperature region above 3 K. The amplitude has a maximum at similar to 4 K and steeply decreases with decreasing temperature. RO II is observable below similar to 4 K and its amplitude monotonically increases with decreasing temperature. We conclude that RO II is the conventional de Haas-van Alphen oscillation in the Fermi surface reconstructed by the spin-density-wave formation and the anion order, but that the mechanism of RO I remains an open question.