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

The actinide cubic Laves compounds NpAl2, NpOs2, NpFe2, and PuFe2 have been examined by X-ray magnetic circular dichroism (XMCD) at the actinide M4,5 absorption edges and Os L2,3 absorption edges. The XMCD experiments performed at the M4,5 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 SQUID data for NpOs2 and PuFe2 allows a determination of the individual orbital and spin magnetic moments and the magnetic dipole contribution mmd. The resulting orbital and spin magnetic moments are in good agreement with earlier values determined by neutron diffraction, and the values of mmd are non-negligible, and close to those predicted for intermediate coupling. There is a comparatively large induced moment on the Os atom in NpOs2 such that the Os contribution to the total moment per formula unit is ~30% of the total. The spin and orbital moments at the Os site are parallel, in contrast to the anti-parallel configuration of Os impurities in 3d ferromagnetic transition metals. Calculations using the LDA+U technique are reported. The ab initio computed XMCD spectra show good agreement with experimental spectra for small values (0-1eV) of the Hubbard U parameter, which underpins that 5f electrons in these compounds are relatively delocalized.