Dimensional crossover of vortex state and peak effect in magnetization in organic superconductors

Magnetization measurements on the single crystals of the organic superconductor κ-(BEDT-TTF)2Cu(NCS)2 are performed in the magnetic field perpendicular to the conducting layers. We find an anomalous second peak on magnetization curves which is almost independent of the temperature, similar to the case of high-Tc superconductor Bi2Sr2CaCu2O8+y. The peak field Hp-150 Oe well coincides with the dimensional crossover field Bcr = φ0/(γs)2. The temperature dependence of the irreversibility field in the quasi two dimensional (2D) region above Bcr is different from those in the three dimensional (3D) region below Bcr. At the 3D region, the irreversibility field is described by Hirr (1−t)n with n = 1.9, however Hirr increases exponentially with 1/T at the 2D region. The dimensionality of the vortex state and the vortex pinning property are discussed.