Anomalous magnetic properties in the single-crystalC superconductor

Magnetization measurements have been performed on single crystals of the organic superconductor (Formula presented)-(Formula presented)(Formula presented) with the field parallel to the (Formula presented) axis (perpendicular to the conducting plane). At high temperatures, the magnetization (Formula presented) displays a power-law dependence on (Formula presented) with (Formula presented) and (Formula presented). At low temperatures ((Formula presented) K), an anomalous field dependence in (Formula presented) is observed. The irreversibility field determined from magnetic hysteresis loops displays a universal temperature dependence, i.e., from a high-temperature power-law dependence to a low-temperature exponential dependence. The results are qualitatively similar to the fishtail magnetization reported for the highly anisotropic oxide superconductors.