Microscopic Coexistence of Ferromagnetism and Superconductivity in Single-Crystal UCoGe

TL;DR

This study provides clear microscopic evidence that ferromagnetism and superconductivity coexist in UCoGe, revealing inhomogeneous superconductivity and a first-order ferromagnetic transition, advancing understanding of FM-SC coexistence in itinerant magnets.

Contribution

It demonstrates the microscopic coexistence of ferromagnetism and superconductivity in UCoGe using $^{59}$Co-NQR, and reveals a first-order ferromagnetic transition in this material.

Findings

Ferromagnetism and superconductivity coexist microscopically in UCoGe.

Superconductivity in UCoGe is inhomogeneous, indicating a self-induced vortex state.

The ferromagnetic transition in UCoGe is first-order.

Abstract

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.