Magnetization density in URhAl-evidence for hybridization effects

URhAl is one of a large class of uranium compounds with the hexagonal ZrNiAl structure. It becomes ferromagnetic at 27 K and the ordered moment is found to be 0.94 mu _B per formula unit. The authors have performed experiments with polarized neutrons in order to measure the magnetization density in the unit cell. A crystallographic study gives atomic positions in agreement with previous work, but they find that the crystals, which were grown by a Czochralski method, are highly perfect and exhibit a large amount of extinction. Corrections for these effects are discussed. The magnetization density shows that the total moment as measured by magnetization is made up of four contributions. The moments (in mu _B) on uranium, Rh_I, Rh_II, and from the conduction electrons are, respectively 0.94(3), 0.28(2), 0.03(2), and -0.11(3). The observation of a large moment at the Rh_I site, which lies in the (001) plane with the U atoms shows that strong anisotropic hybridization occurs between the U and Rh_I. This is the cause of the very large bulk anisotropy. At the uranium site they have analysed the contributions from the spin and orbital moments and find - mu _L/ mu _S=1.81(7). The free ion 5f³ value for this ratio if 2.6. The reduction gives one a quantitative measure of hybridization in the system. The results are compared with those calculated from density functional theory.