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 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 was 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​ show 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.Comment: 5 pages, 5 figure