  The morphology of the superconducting flux line lattice (FLL) of Nb comprises
gradual variations with various lock-in transitions and symmetry breaking
rotations. We report a comprehensive small-angle neutron scattering study of
the FLL in an ultra-pure single crystal of Nb as a function of the orientation
of the applied magnetic field. We attribute the general morphology of the FLL
and its orientation to three dominant mechanisms. First, non-local
contributions, second, the transition between open and closed Fermi surface
sheets and, third, the intermediate mixed state between the Meissner and the
Shubnikov phase.
