Vortex pinning and lock-in effect in a layered superconductor with large in-plane anisotropy

We use ac susceptibility to study the vortex pinning force anisotropy and the magnetic lock-in effect in the organic superconductor (TMTSF)2ClO4, which is believed to have an in-plane anisotropy of ba10 and a maximum out-of-plane anisotropy ca100. Our measurements show only weak effects of the in-plane anisotropy. The pinning force for Josephson vortices (parallel to the conducting planes) is nearly independent of their orientation, except for a small but narrow peak (full width at half maximum 6°) when the vortices are parallel to the TMTSF stacks (a axis). The pinning force initially decreases as the vortices unlock from the layers, contrary to the behavior previously observed in the organic superconductor (BEDT-TTF)2Cu(SCN)2. The lock-in threshold field is only weakly dependent on the initial angle of the Josephson vortices in the ab plane.