Effect of anisotropy in the $S=1$ underscreened Kondo lattice

TL;DR

This paper investigates how crystal field anisotropy influences the interplay of ferromagnetism and the Kondo effect in an $S=1$ underscreened Kondo lattice, revealing complex phase regimes including re-entrant Kondo phases.

Contribution

It demonstrates that local anisotropy can be mapped to anisotropic Kondo interactions, leading to new insights into coexistence phases of magnetism and Kondo screening in heavy fermion compounds.

Findings

Re-entrant Kondo phase at low temperatures.

Coexistence of ferromagnetism and Kondo effect depending on parameters.

Description of phases with $T_K < T_C$.

Abstract

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 ($J_{\parallel} \neq{J_{\perp}}$). 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 ($T_K<T_C$). We propose that some aspects of uranium and neptunium compounds that present coexistence of Kondo effect and ferromagnetism, can be understood within this model.