Muon Spin Rotation and Relaxation in the Superconducting Ferromagnet UCoGe

We report zero-field muon-spin rotation and relaxation measurements on the superconducting ferromagnet UCoGe. Weak itinerant ferromagnetic order is detected by a spontaneous muon-spin precession frequency below the Curie temperature T(C)=3 K. The mu(+) precession frequency persists below the bulk superconducting transition temperature T(sc)=0.5 K, where it measures a local magnetic field B(loc)=0.015 T. The amplitude of the mu SR signal provides unambiguous proof for ferromagnetism present in the whole sample volume. We conclude ferromagnetism coexists with superconductivity on the microscopic scale.