Relationship between ferromagnetic criticality and the enhancement of superconductivity induced by transverse magnetic fields in UCoGe

We have performed⁵⁹Co NMR experiments on the ferromagnetic superconductor UCoGe under magnetic fields (H) along the a- and b-axes to investigate the relationship between ferromagnetic properties and superconductivity. The ferromagnetic ordering temperature TCurie is suppressed and the nuclear spin-lattice relaxation rate 1/T₁at 2K is enhanced in H ∥ b, although TCurie and 1/T₁are 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.