Vortex-lattice transitions in YNi2B2C:: Nature of the 45-degree reorientation -: art. no. 014515

High-resolution small-angle neutron-scattering (SANS) studies of the vortex lattice (VL) in single-crystal YNi2B2C allows us to separate Bragg scattered intensities from the multidomain VL that exists for Bparallel toc. A precise determination of the VL unit-cell apex angle, beta, shows that there is a finite transition width associated with the field-driven 45degrees reorientation of the VL at a field H-1. Low- and high-field rhombic VL phases coexist over a finite range of applied field with no continuous distortion of the VL between the two phases. The smooth variation in scattered intensity from each phase through the transition indicates a redistribution of domain populations between the low- and high-field vortex structures. Our data supports the notion of a first-order reorientation phase transition in the VL at H-1 in the presence of weak static disorder (vortex pinning).