  Using small-angle neutron scattering, we have imaged the magnetic flux line
lattice (FLL) in the d-wave heavy-fermion superconductor CeCoIn5. At low fields
we find a hexagonal FLL. Around 0.6 T this undergoes what is very likely a
first-order transition to square symmetry, with the nearest neighbors oriented
along the gap node directions. This orientation of the square FLL is consistent
with theoretical predictions based on the d-wave order parameter symmetry.
