59Co-NMR/nuclear-quadrupole-resonance (NQR) Studies on Ferromagnetic Superconductor UCoGe

We have performed NMR and nuclear-quadrupole-resonance (NQR) on U-based ferromagnetic superconductor UCoGe and a reference metallic compound YCoGe without 5f electrons, in order to investigate the normal and superconducting-state properties. From Co-59-NQR measurements, we give clear evidence that ferromagnetism and superconductivity coexist microscopically, both of which are ascribed to the U 5f-electrons. In addition, direction-dependent Knight shift and nuclear spin-lattice relaxation rate measurements provide microscopic evidence that both static and dynamic susceptibilities are ferromagnetic with strong Ising anisotropy. We point out that superconductivity induced by such magnetic fluctuations prefers spin-triplet pairing state