Pinning of Vortices Nearly Parallel to the Superconducting Plane in κ-(ET)2Cu[N(CN)2]Br and κ-(ET)2Cu(NCS)2

The vortex state under a magnetic field applied nearly parallel to the superconducting plane has been studied by means of an ac susceptibility measurement around 2 MHz in the organic superconductors κ-(ET)2Cu[N(CN)2]Br and κ-(ET)2Cu(NCS)2. The diamagnetic response, due to pinning of the staircase vortices for the inclined field, was suppressed by the increase in the magnetic field. This finding indicates that the parallel vortices become freely movable as a result of either decoupling between layers or depinning of Abrikosov vortices in the layer plane. We found that the suppression of the diamagnetic response was promoted by the parallel field component.