Instability of square vortex lattice in d-wave superconductors is due to   paramagnetic depairing
  Effects of the paramagnetic depairing on structural transitions between vortex lattices of a quasi two dimensional d-wave superconductor are examined. It is found that, in systems with Maki parameter $\alpha_{\rm M}$ of order unity, a square lattice induced by a d-wave pairing is destabilized with {\it increasing} fields, and that a reentrant rhombic lattice occurs in higher fields. Further, a weak Fermi surface anisotropy competitive with the pairing symmetry induces another structural transition near $H_{c2}$. These results are consistent with the structure changes of the vortex lattice in CeCoIn$_5$ in ${\bf H} \parallel c$ determined from recent neutron scattering data. 
Norihito Hiasa and Ryusuke Ikeda