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.