  Using small-angle neutron scattering, we have studied the flux-line lattice
(FLL) in superconducting CeCoIn5. The FLL is found to undergo a first-order
symmetry and reorientation transition at ~0.55 T at 50 mK. The FLL form factor
in this material is found to be independent of the applied magnetic field, in
striking contrast to the exponential decrease usually observed in
superconductors. This result is consistent with a strongly field-dependent
coherence length in CeCoIn5, in agreement with recent theoretical predictions
for superclean, high-kappa superconductors.
