Influence of nonlocal electrodynamics on the anisotropic vortex pinning in $YN{i}_2{B}_{2}C$

We have studied the pinning force density Fp of YNi_2B_2C superconductors for various field orientations. We observe anisotropies both between the c-axis and the basal plane and within the plane, that cannot be explained by usual mass anisotropy. For magnetic field $H\parallel c$, the reorientation structural transition in the vortex lattice due to nonlocality, which occurs at a field $H_1\sim 1kOe$, manifests itself as a kink in Fp(H). When $H\mathrm{\perp }c$, Fp is much larger and has a quite different H dependence, indicating that other pinning mechanisms are present. In this case the signature of nonlocal effects is the presence of a fourfold periodicity of Fp within the basal plane.