Crystal-field effects and intersite exchange interactions in localized f-electron compounds

Orateur : Leonid POUROVSKII

Rare-earth and actinide compounds are studied by an ab initio framework that combines the density-functional theory (DFT) with a quas i-atomic (Hubbard-I) dynamical treatment for the on-site Coulomb repulsion between localized f states. We first discuss applications of this approach to the crystal-field (CF) splitting and single-ion magnetic anisotropy of rare-earth ions in hard magnetic intermetallics. We implement an averaging scheme [1] to remove the contribution of DFT self-interaction error from the CF splitting. The CF parameters of SmCo5 are obtained in good agreement with experiment. We also explain a puzzling reduction of the 4f moment observed experimentally [2] in NdCo5 by a significant rank-6 CF parameter induced by the 4f-3d hybridization in a hexagonally coordinated local environment of the Nd ion. The enhancement of rank-6 CF parameter is found to be quite general along the RCo5 series ; however, the magnitude of its impact on magnetic properties is ion-depended, as we show by comparing NdCo5 with TbCo5. In the second part of the talk we present a linear-response approach for extracting intersite exchange interactions between localized shells. This approach [3] is implemented in the same DFT+Hubbard-I framework and applied to magnetic and quadrupole orders in the uranium dioxide UO2 [4]. The frustration of face-centered-cubic U sublattice in UO2 results in several magnetic structures being degenerate with respect to the usual magnetic superexchange. The calculated quarupole-quadrupole superexchange interactions are shown to lift this degeneracy stabilizing the experimentally observed non-collinear 3k-magnetic order. This stabilization stems from a particular anisotropy of the quandrupole-quadrupole superexchange in the undistorted parent cubic structure of UO2. Hence, the observed non-collinear magnetic order can be explained by a purely electronic superexchange mechanism.

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