Effect of anisotropy in the S=1 underscreened Kondo lattice

We study the effect of crystal field anisotropy in the underscreened S=1 Kondo lattice model. Starting from the two orbital Anderson lattice model and including a local anisotropy term, we show, through Schrieffer–Wolff transformation, that local anisotropy is equivalent to an anisotropic Kondo interaction (J_∥≠J_⊥). The competition and coexistence between ferromagnetism and Kondo effect in this effective model is studied within a generalized mean-field approximation. Several regimes are obtained, depending on the parameters, exhibiting or not coexistence of magnetic order and Kondo effect. Particularly, we show that a re-entrant Kondo phase at low temperature can be obtained. We are also able to describe phases where the Kondo temperature is smaller than the Curie temperature (T_KC). We propose that some aspects of uranium and neptunium compounds that present coexistence of Kondo effect and ferromagnetism can be understood within this model. - Highlights: • We apply the Schrieffer–Wolff transformation to the two orbital Anderson lattice. • Anisotropy breaks the symmetry between parallel and perpendicular Kondo interactions. • Two regimes are described: a re-entrant Kondo phase and a region with T_KC. • Our results are in agreement with the experimental evidences of coexistent phases