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

Abstract : 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 5$f$ electrons count is close to 3. The orbital (~0.7 $\mu_{\beta}$) and spin (~$-$0.30$\mu_{\beta}$) moments of U at 2.1 K and 17 T (H$\parallel$c) have been determined. Their ratio (~$-$2.3) suggests a significant delocalization of the 5 $f$ electron states. The similar field dependences of the local U/Co and the macroscopic magnetization indicate that the Co moment is induced by the U moment. The XMCD at the Co/Ge $K$ edges reveal the presence of small Co 4$p$ and Ge 4$p$ orbital moments parallel to the macroscopic magnetization. In addition, the Co $3$d moment is estimated to be at most of the order of 0.1$\mu_{\beta}$ 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.