Vortex-lattice transitions inYNi2B2C:Nature of the45-degree reorientation

Abstract

High-resolution small-angle neutron-scattering (SANS) studies of the vortex lattice (VL) in single-crystal ${\mathrm{YNi}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ allows us to separate Bragg scattered intensities from the multidomain VL that exists for $B\ensuremath{\Vert}c.$ A precise determination of the VL unit-cell apex angle, \ensuremath{\beta}, shows that there is a finite transition width associated with the field-driven $45\ifmmode^\circ\else\textdegree\fi{}$ 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).