Thermal expansion of the superconducting ferromagnet UCoGe

We report measurements of the coefficient of linear thermal expansion, $\alpha (T)$, of the superconducting ferromagnet UCoGe. The data taken on a single-crystalline sample along the orthorhombic crystal axes reveal a pronounced anisotropy, with the largest length changes along the $b$ axis. The large values of the step sizes $\mathrm{\Delta }\alpha$ at the magnetic and superconducting phase transitions provide solid evidence for bulk magnetism and superconductivity. Specific-heat measurements corroborate bulk superconductivity. Thermal-expansion measurements in magnetic fields $B \parallel a,b$show$\Delta \alpha$at$T_C$ grows rapidly, which indicates the character of the ferromagnetic transition becomes first-order-like.