Superconducting and ferromagnetic phase diagram of UCoGe probed by thermal expansion

We report thermal expansion measurements on a single crystal of the superconducting ferromagnet UCoGe for magnetic fields applied along the main orthorhombic axes. The thermal expansion cell was mounted on a piezoelectric rotator in order to fine-tune the magnetic field angle. The superconducting and magnetic phase diagram has been determined. With our bulk technique we confirm the S shape of the upper-critical field, B_c2 , for B ∥ b and reinforcement of superconductivity above 6 T. At the same time the Curie point shifts towards lower temperatures on increasing the field along the b axis. This lends further support to theoretical proposals of spin-fluctuation-mediated reinforcement of superconductivity for B ∥ b.