Element-specific insight into ferromagnetic stability in UCoGe revealed by soft x-ray magnetic circular dichroism

We investigated the element-specific electronic states and magnetic properties of ferromagnetic superconductor UCoGe and Ru-substituted $\mathrm{U}({\mathrm{Co}}_{0.88}{\mathrm{Ru}}_{0.12})\mathrm{Ge}$ using soft x-ray magnetic circular dichroism (XMCD) experiments at the absorption edges of U ${N}_{4,5}$, Co ${L}_{2,3}$, and Ge ${L}_{2,3}$. YCoGe, a nonmagnetic compound without the $5f$ electrons, was also examined as a reference material. The shapes of the XMCD spectra of UCoGe at the U ${N}_{5}$ and Ge ${L}_{2,3}$ edges are indistinguishably similar to those of $\mathrm{U}({\mathrm{Co}}_{0.88}{\mathrm{Ru}}_{0.12})\mathrm{Ge}$. On the other hand, the XMCD spectral shape at the Co ${L}_{2}$ edge is very peculiar and is changed by the presence of the U $5f$ electrons and the Ru substitution. From the sum rule analysis for the XMCD spectra, it was found that the magnetic moments of the U $5f$ and Co $3d$ electrons in UCoGe and $\mathrm{U}({\mathrm{Co}}_{0.88}{\mathrm{Ru}}_{0.12})\mathrm{Ge}$ are aligned in the same direction under the present experimental conditions: $T$ = 5.5--50 K and ${\ensuremath{\mu}}_{0}H$ = 0--10 T. Furthermore, not only in UCoGe and $\mathrm{U}({\mathrm{Co}}_{0.88}{\mathrm{Ru}}_{0.12})\mathrm{Ge}$ but even in the case of YCoGe, the Co $3d$ electrons have a large orbital magnetic moment, suggesting that the Co $3d$ electrons in these compounds are in a low-symmetry electronic environment originally. Through the $T$ and ${\ensuremath{\mu}}_{0}H$ dependence of the XMCD intensity, it was revealed that the magnetic properties at the Co site are not simply induced by the magnetism at the U site and that the Co $3d$ electrons play an important role in the stability of ferromagnetism. The present findings clearly demonstrate that the contribution of not only the U $5f$ electrons but also the Co $3d$ electrons is very crucial to understanding the complex physical properties of the $\mathrm{U}({\mathrm{Co}}_{1\ensuremath{-}x}{\mathrm{Ru}}_{x})\mathrm{Ge}$ system.