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

Abstract

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. They have the interesting feature that the An−An spacing is close to the so-called Hill limit so that substantial hybridization between the 5f states on neighboring atoms is expected to occur. 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 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 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. The calculations confirm the sign and magnitude of the experimentally determined induced magnetic moments on the Os site in NpOs2. A posteriori, by comparison of the theoretical and measured XMCD spectra, we can determine the most appropriate LSDA+U variant and the value of U.

Ground-state wave function of plutonium in PuSb as determined via x-ray magnetic circular dichroism

“…This is quite different from that found for PuFe 2 , where the value is +0.23 (5). 20 Interestingly, the atomic theory value 13 for intermediate coupling is -0.22, and we have calculated −0.28, so we see that the PuSb value is much closer to atomic theory than in the case of PuFe 2 . This again is ex-pected for the localized 5f electrons in PuSb.…”

“…2-5, we find the values for µ L , µ s,ef f and 2 3 L · S presented in table II. Further, because the total 5f moment is µ = µ s + µ L we can use the total moment measured by neutron scattering µ = 0.75(1) µ B /Pu 6 to deduce 20 the spin moment µ s = −2.0(1). This also allows us to estimate the ratio between the orbital and the total magnetic moments C 2 = µ L µ L +µs , and the magnetic dipole contribution µ md = µ s,ef f − µ s .…”

“…A review of XMCD measurements on U-and Np-compounds in Ref. 20 shows that in all such experiments a larger linewidth of ≈10 eV has been observed. This is also the case of the PuFe 2 M 4 XMCD spectra, and this was reproduced by LSDA+U calculations.…”

“…This is not strictly correct in intermediate coupling, but a good approximation. In fact the branching ratio, B, can be related to the occupation number n 5/2 of the j = 5/2 states, 20 which is over 4 electrons in PuSb (and PuFe 2 ). This implies less than two electron states above the Fermi energy E F .…”

Probing Magnetism in the Vortex Phase of PuCoGa5 by X-Ray Magnetic Circular Dichroism

“…The line is a guide to the eye. Inset: XMCD spectra at the M4 absorption edge of Pu measured for PuCoGa5 (present work) and PuFe2 [24]. The spectra are normalized to their respective maxima.…”