X-ray magnetic circular dichroism experiments and theory of transuranium Laves phase compounds

Abstract

The actinide cubic Laves compounds NpAl${}_{2}$, NpOs${}_{2}$, NpFe${}_{2}$, and PuFe${}_{2}$ have been examined by x-ray magnetic circular dichroism (XMCD) at the actinide ${M}_{4,5}$ absorption edges and Os ${L}_{2,3}$ absorption edges. They have the interesting feature that the $An\ensuremath{-}An$ spacing is close to the so-called Hill limit so that substantial hybridization between the 5$f$ states on neighboring atoms is expected to occur. The XMCD experiments performed at the ${M}_{4,5}$ absorption edges of Np and Pu allow us to determine the spectroscopic branching ratio, which gives information on the coupling scheme in these materials. In all materials, the intermediate coupling scheme is found appropriate. Comparison with the magnetization data for NpOs${}_{2}$ and neutron results for PuFe${}_{2}$ allows a determination of the individual orbital and spin magnetic moments and the magnetic-dipole contribution $m$${}_{md}$. The resulting orbital and spin magnetic moments are in good agreement with earlier values determined by neutron diffraction, and the values of $m$${}_{md}$ are non-negligible, and close to those predicted for intermediate coupling. There is a comparatively large induced moment on the Os atom in NpOs${}_{2}$ such that the Os contribution to the total moment per formula unit is $\ensuremath{\sim}$30$%$ of the total. The spin and orbital moments at the Os site are parallel, in contrast to the antiparallel configuration that we find for Os impurities in a 3$d$ ferromagnetic transition-metal alloy. Calculations using the LDA+$U$ technique are reported. The ab initio computed XMCD spectra show good agreement with experimental spectra for small values (0--1 eV) of the Hubbard $U$ parameter, which demonstrates that the 5$f$ electrons in these compounds are relatively delocalized. The calculations confirm the sign and magnitude of the experimentally determined induced magnetic moments on the Os site in NpOs${}_{2}$. A posteriori, by comparison of the theoretical and measured XMCD spectra for a given material, we can determine the most appropriate LSDA+$U$ variant and, more importantly, the applicable value of the Hubbard $U$ parameter.