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 piezo-electric 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 $B \parallel b$ 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 $B \parallel b$.