Microscopic coexistence of ferromagnetism and superconductivity in single-crystal UCoGe

Unambiguous evidence for the microscopic coexistence of ferromagnetism and superconductivity in UCoGe (TCurie-2.5 K and TSC-0.6 K) is reported from 59Co nuclear quadrupole resonance (NQR). The 59Co-NQR signal below 1 K indicates ferromagnetism throughout the sample volume, while the nuclear spin-lattice relaxation rate 1/T1 in the ferromagnetic (FM) phase decreases below TSC due to the opening of the superconducting (SC) gap. The SC state is found to be inhomogeneous, suggestive of a self-induced vortex state, potentially realizable in a FM superconductor. In addition, the 59Co-NQR spectrum around TCurie shows that the FM transition in UCoGe possesses a first-order character, which is consistent with the theoretical prediction that the low-temperature FM transition in itinerant magnets is generically of first-order. (author)