Anomalous magnetization and dimensional crossover of the vortex system in the organic superconductor kappa-(BEDT-TTF)(2)Cu(NCS)(2)

Magnetization measurements on high-quality single crystals of kappa-(BEDT-TTF)(2)Cu(NCS)(2) [where BEDT-TTF denotes bis(ethylenedithio)tetrathiafulvalene] are performed in the magnetic field perpendicular to the conducting layers. An anomalous second peak on magnetization curves observed at H-p(approximate to 150 Oe) is attributed to the dimensional crossover in the vortex system. At the dimensional crossover field B-2D=phi(0)/(gamma(2)s(2)) the temperature dependence of the irreversibility field changes from the power-law dependence in the three-dimensional (3D) regime below B-2D to the exponential dependence with 1/T in the quasi-2D regime above B-2D It is also found that the different magnetic field dependence of the critical current density above and below B-2D is well described by the collective pinning theory.