2 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 t wo distinctly different quantum oscillation (''rapid oscillation'' RO I and II) wave forms in the magnetization. Both exhibit the same frequ ency for B parallel to c but the phase is different by pi. RO I is do minant above similar to 20 T in a high-temperature region above 3 K. T he amplitude has a maximum at similar to 4 K and steeply decreases wit h 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 oscillatio n in the Fermi surface reconstructed by the spin-density-wave formatio n and the anion order, but that the mechanism of RO I remains an open question.