The underscreened Kondo lattice model applied to heavy fermion uranium compounds

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 $UC{u}_{0.9}S{b}_2$, 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''.

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