Relationship between Ferromagnetic Criticality and the Enhancement of Superconductivity Induced by Transverse Magnetic Fields in UCoGe

We have performed Co-59 NMR experiments on the ferromagnetic superconductor UCoGe under magnetic fields (H) along the -and b-axes to investigate the relationship between ferromagnetic properties and superconductivity. The ferromagnetic ordering temperature T-Curie is suppressed and the nuclear spin-lattice relaxation rate 1/T-1 at 2K is enhanced in H || b, although T-Curie and 1/T-1 are unchanged in H || a, indicating that the ferromagnetic criticality is induced only when H is applied along the b-axis. We show the close relationship between the magnetic anisotropies and the superconducting ones reported by Aoki et al.: the superconductivity is gradually suppressed in H || a, but enhanced in H || b above 5 T. We strongly suggest that the enhancement of the superconductivity observed in H b originates from the field induced ferromagnetic criticality, as pointed out by Aoki et al. and Mineev.