Roles of Antiferromagnetic Fluctuation in High Field Phase Diagram of Superconductors with Strong Paramagnetic Depairing

The high field phase diagram and magnetic properties of CeCoIn_5 below H_c2(0) are examined from the picture regarding the high field and low temperature (HFLT) phase as a possible Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) vortex lattice. Crucial roles of antiferromagnetic (AFM) fluctuations enhanced close to H_c2(0) are stressed. The FFLO vortex lattice with a longitudinal modulation parallel to the field is stabilized compared with those with lateral modulations as a consequence of the presence of AFM fluctuations. Further, an unusual field-induced enhancement of the flux distribution is argued to be a consequence of interplay between the paramagnetic depairing and AFM fluctuation, both of which are enhanced with increasing field.