  We report on the evolution with magnetic field and temperature of the vortex
lattice (VL) in fully-oxygenated YBa2Cu3O7 as studied by time-of-flight small
angle neutron scattering. Using the HFM/EXED beamline, we have obtained data up
to 25.9 T - much higher than that available previously. Our VL structure
results indicate the progressive suppression by field of the superconductivity
along the crystallographic b (CuO chain) direction. The intensity of the
diffracted signal reveals the spatial variation of magnetization caused by the
VL (the "form factor"). Instead of a rapid fall-off with field, as seen in
superconductors with smaller upper critical fields, we find that the form
factor is almost constant with field above about 12 T. We speculate that this
is due to Pauli paramagnetic moments, which increase at high fields due to
alignment of the spins of quasiparticles in the vortex cores.
