X-ray magnetic circular dichroism studies of5fmagnetism in UCoAl and UPtAl

Experimental and theoretical x-ray magnetic circular dichroism (XMCD) studies of the intermetallic compounds UCoAl and UPtAl at the uranium $M_4$ and $M_5$ 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, ${\mu }_L/{\mu }_S\approx -2,$ for both compounds; however, the moments in UCoAl are strongly reduced with respect to UPtAl. Furthermore, the ${\mu }_L/{\mu }_S$ ratio for UCoAl appears to be conserved both in the ferromagnetic and paramagnetic states, although the ${\mu }_S$ 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 $M_4$ and $M_5$ edges.