The underscreened Kondo lattice model applied to heavy fermion uranium compounds

TL;DR

This paper models the coexistence of ferromagnetism and Kondo effect in uranium compounds using an underscreened Kondo lattice model, providing a theoretical framework that aligns with experimental observations.

Contribution

It introduces a theoretical model for the coexistence of ferromagnetism and Kondo behavior in heavy fermion uranium compounds, expanding the understanding of their magnetic and electronic properties.

Findings

Large $J_K$ leads to ferromagnetic order and a high $T_K$

The model explains coexistence of Kondo effect and ferromagnetism

Proposes a new ferromagnetic Doniach diagram

Abstract

We present theoretical results for the underscreened Kondo lattice model with localized S=1 spins coupled to a conduction band through a Kondo coupling, $J_K$, and interacting among them ferromagnetically. We use a fermionic representation for the spin operators and expand the Hamiltonian in terms of bosonic fields. For large values of $J_K$, we obtain a ferromagnetically ordered solution and a Kondo regime with a Kondo temperature, $T_K$, larger than the Curie temperature, $T_C$. This finding suggests a scenario for a coexistence of Kondo effect and ferromagnetic order. In some uranium compounds, like $UTe$ or $UCu_{0.9}Sb_{2}$, this kind of coexistence has been experimentally observed: they order ferromagnetically with a Curie temperature of order $T_C \sim 100K$ and exhibit a Kondo behavior for $T > T_C$. The proposed underscreened Kondo lattice model accounts well for the coexistence between magnetic order and Kondo behavior and yields to a new ``ferromagnetic Doniach diagram''.