Anomalous magnetization in single-crystal κ-[bis(ethylenedithiotetrathiafulvalene)] 2 Cu[N(CN) 2 ]Br superconductors

We report detailed magnetization studies on single-crystal superconductors of {kappa}-[bis(ethylenedithiotetra- thiafulvalene)]{sub 2}Cu[N(CN){sub 2}]Br with the field {ital H} parallel to the {ital b} axis (perpendicular to the conducting plane). At high temperatures, the magnetization, {ital M}, displays a power-law dependence on {ital H} with {ital M}{proportional_to}{ital H}{sup {ital n}} and {ital n}={minus}2/3. The critical current extrapolated from the width of the hysteresis loop depends on {ital H} exponentially. At low temperatures ({ital T}{lt}7 K), an anomalous field dependence with a change of sign in the curvature of {ital M}({ital H}) is observed. The anomalous field dependence is qualitatively similar to the fishtail magnetization reported for the oxide superconductors. Possible mechanisms giving rise to this effect are discussed.