Nonlocal effects and vortex lattice transitions in YNi2B2C

High resolution, neutron small angle scattering studies have been performed to investigate the low field (B < 250 mT) region of the phase diagram for the vortex lattice in the superconducting state of YNi2B2C. The data present clear evidence for a vortex lattice reorientation transition from a state with the diagonal of the rhombic unit cell along a [110] direction to a [100] direction. Above this transition the lattice distorts under the influence of the applied field until the apex angle becomes constant at some higher field. For B parallel to c a square lattice configuration is formed. These experiments confirm qualitatively many of the predictions of a general model based on nonlocal corrections to the London model as applied to this and similar materials.