Hexagonal and Square Flux Line Lattices in CeCoIn5
  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. 
M. R. Eskildsen, C. D. Dewhurst, B. W. Hoogenboom, C. Petrovic, P. C.   Canfield