Microscopic Coexistence of Ferromagnetism and Superconductivity in Single-Crystal UCoGe

Abstract

Unambiguous evidence for the microscopic coexistence of ferromagnetism and superconductivity in UCoGe ($T_{\mathrm{Curie}}\sim 2.5$ K and $T_{\mathrm{SC}}$ $\sim$ 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 $T_{\mathrm{SC}}$ 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 $T_{\mathrm{Curie}}$ 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.

5 pages, 5 figures