DIMENSIONAL CROSSOVER OF VORTEX STATE AND PEAK EFFECT IN MAGNETIZATION IN ORGANIC SUPERCONDUCTORS

Magnetization measurements on the single crystals of the organic super conductor kappa-(BEDT-TTF)(2)Cu(NCS)(2) are performed in the magnetic field perpendicular to the conducting layers. We find an anomalous sec ond peak on magnetization curves which is almost independent of the te mperature, similar to the case of high-T-c superconductor Bi2Sr2CaCu2O 8+y. The peak field H-p similar or equal to 150 Oe well coincides with the dimensional crossover field B-cr = phi(0)/(gamma s)(2). The tempe rature dependence of the irreversibility field in the quasi two dimens ional (2D) region above B-cr is different from those in the three dime nsional (3D) region below B-cr. At the 3D region, the irreversibility field is described by H-irr proportional to (1 - t)(n) with n = 1.9, h owever H-irr increases exponentially with 1/T at the 2D region. The di mensionality of the vortex state and the vortex pinning property are d iscussed.