X-ray magnetic circular dichroism studies of 5f magnetism in UCoAl and UPtAl

Experimental and theoretical x-ray magnetic circular dichroism (XMCD) studies of the intermetallic compounds UCoAl and UPtAl at the uranium M₄ and M₅ edges are reported. UPtAl is a 5f-electron ferromagnet, whereas UCoAl exhibits, at low temperatures and in a magnetic field of 0.65 T, a metamagnetic transition from paramagnetic to ferromagnetic ordering of U moments. Applying the XMCD sum rules to the experimental spectra, expectation values of the orbital and spin magnetic moments of the uranium 5f electrons were calculated. The results show that the orbital-to-spin moment ratio is of comparable value, μ_L/μ_S≈-2, for both compounds; however, the moments in UCoAl are strongly reduced with respect to UPtAl. Furthermore, the μ_L/μ_S ratio for UCoAl appears to be conserved both in the ferromagnetic and paramagnetic states, although the μ_S value in the former case is at least five times larger. The theoretical counterparts of the experimental data were obtained from the electronic structure calculated using the FLAPW method within the local spin density approximation. The calculated x-ray absorption and XMCD spectra show a satisfactory agreement with the experimental data and reproduce well the shape and structure of the spectral lines at the M₄ and M₅ edges.