ELECTRONIC-PROPERTIES AND MAGNETIC-STRUCTURE OF U3X4 (X=P,AS,SB)

We report results of first-principles calculations of the electronic p roperties and magnetic structure of U3X4 (X=P,As,Sb) which were carrie d but using a generalization of the local density approximation to den sity functional theory to apply to the case of noncollinear magnetic c onfigurations which allows a determination of both length and directio n of the magnetic moments. Spin-orbit coupling and an effective orbita l field responsible for Hund's second rule are taken into account. The calculated trends in the variation of the magnetic anisotropy and the values of the uranium magnetic moments are in agreement with experime ntal trends in the series. In particular, the experimentally observed change of the axis of easy magnetization from (111) in U3P4 and U3As4 to (001) in U3Sb4 is born out by our calculations. For all three compo unds, our calculations result in a noncollinear magnetic structure. We show that this noncollinearity is a direct consequence of the spin-or bit coupling and supply corresponding symmetry arguments based on the generalized spin-space groups. For U3P4 and U3As4 our results concerni ng the noncollinearity of the magnetic structure are in qualitative ag reement with experimental findings. However, for U3Sb4 our conclusion disagrees with previous treatments of the magnetic structure as collin ear. We supply detailed arguments in support of our point of view. For all three compounds we obtain the interesting property that the spin and orbital moments of the same U atom are noncollinear and we formula te the symmetry criterion for the appearance of this effect.