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, such as UTe or $\mathrm{U}{\mathrm{Cu}}_{0.9}{\mathrm{Sb}}_{2}$, this kind of coexistence has been experimentally observed: they order ferromagnetically with a Curie temperature of order ${T}_{C}\ensuremath{\sim}100\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ 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.''