Unambiguous evidence for the microscopic coexistence of ferromagnetism and superconductivity in UCoGe ($T_{ \rm Curie} \sim 2.5$ K and $T_{ \rm SC}$ $\sim$ 0.6 K) is reported from $^{ 59}$Co nuclear quadrupole resonance (NQR). The $^{ 59}$Co-NQR signal below 1 K indicates ferromagnetism throughout the sample volume, while nuclear spin-lattice relaxation rate $1/T_1$ in the ferromagnetic (FM) phase decreases below $T_{ \rm SC}$ 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 $^{ 59}$Co-NQR spectrum around $T_{ \rm Curie}$ 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.
Categories: cond-mat.supr-con cond-mat.str-el