Microscopic magnetic properties of the ferromagnetic superconductor UCoGe reviewed by x-ray magnetic circular dichroism

The ferromagnetic superconductor UCoGe has been investigated by high field x-ray magnetic circular dichroism (XMCD) at the U M_4,5 and Co/Ge K edges. The analysis of the branching ratio and XMCD at the U M_4,5 edges reveals that the U 5f electrons count is close to 3. The orbital (∼0.70μ_B) and spin (∼-0.30μ_B) moments of U at 2.1 K and 17 T (H∥c) have been determined. Their ratio (∼-2.3) suggests a significant delocalization of the 5f electron states. The similar field dependences of the local U/Co and the macroscopic magnetizationindicate that the Co moment is induced by the U moment. The XMCD at the Co/Ge K edges reveal the presence of small Co 4p and Ge 4p orbital moments parallel to the macroscopic magnetization. In addition, the Co 3d moment is estimated to be at most of the order of 0.1μB at 17 T. Our results rule out the possibility of an unusual polarizability of the U and Co moments as well as their antiparallel coupling. We conclude that the magnetism which mediates the superconductivity in UCoGe is driven by U.