Superconductivity on the border of weak itinerant ferromagnetism in UCoGe

We report the coexistence of ferromagnetic order and superconductivity in UCoGe at ambient pressure. Magnetization measurements show that UCoGe is a weak ferromagnet with a Curie temperature T-C=3 K and a small ordered moment m(0)=0.03 mu(B). Superconductivity is observed with a resistive transition temperature T-s=0.8 K for the best sample. Thermal-expansion and specific-heat measurements provide solid evidence for bulk magnetism and superconductivity. The proximity to a ferromagnetic instability, the defect sensitivity of T-s, and the absence of Pauli limiting, suggest triplet superconductivity mediated by critical ferromagnetic fluctuations.