5f States In Uga2 Probed By X-Ray Spectroscopies

The 5f-based ferromagnet UGa2 with the Curie temperature T-C = 125 K was investigated by x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) experiments at the U-M-4.5 and Ga-K edges. The position of the U-M-4 white line, determined in the high-energy resolution fluorescence detection XAS, suggests that UGa2 is neither a localized 5f(2) nor an itinerant system with 5f occupancy close to n(5f )= 3. The analysis of the acquired M-4.5 XANES and XMCD spectra indicates the 5f occupancy close to 2.5 and a large orbital magnetic moment of the uranium 5f states (3.18 mu(B)) that is partly compensated by the antiparallel spin moment (1.31 mu(B)). Thus, the total 5f magnetic moment of 1.87 mu(B) is obtained, which is smaller than the known bulk magnetization of 3.0 mu(B) per formula unit, while the magnetic moments of the Ga atoms are negligible. Several methods based on density-functional theory were applied and the obtained results were compared with XAS spectral features, the Sommerfeld coefficient of the electronic specific heat, and the size of the U moments and 5f occupancies. A clear correlation is revealed between the U-M-4 white-line position of three metallic uranium compounds and the calculated uranium ionicity. It is demonstrated that only electronic structure methods taking appropriate care of orbital magnetism and related atomic multiplet effects can successfully describe all considered properties.