Underscreened Kondo lattice model applied to heavy fermion uranium compounds

We present theoretical results for the underscreened Kondo lattice model with localized S=1 spins coupled to a conduction band through a Kondo coupling, JK, 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 JK, we obtain a ferromagnetically ordered solution and a Kondo regime with a Kondo temperature, TK, larger than the Curie temperature, TC. This finding suggests a scenario for a coexistence of Kondo effect and ferromagnetic order. In some uranium compounds, such as UTe or U Cu0.9 Sb2, this kind of coexistence has been experimentally observed: they order ferromagnetically with a Curie temperature of order TC ∼100 K and exhibit a Kondo behavior for T> TC. 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."