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 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 to a, b show Delta alpha at T(C) grows rapidly, which indicates the character of the ferromagnetic transition becomes first orderlike.