Underscreened Kondo lattice model at finite temperature

Abstract We have developed an underscreened Kondo lattice model with an intra-site Kondo exchange interaction J K between the conduction band and localized 5f electrons—described by S = 1 spins in the f 2 configuration—and an inter-site ferromagnetic exchange f–f interaction J H . Using a fermionic representation for the spin operators and a mean-field approximation we obtain, from T = 0 up to a critical temperature, a Kondo-ferromagnetic coexistence. This is in agreement with the experimental situation of some Uranium compounds, such as UTe or UCu 0.9 Sb 2 , which order ferromagnetically with a Curie temperature of order 100 K and present also a Kondo behavior. The Curie and Kondo temperatures are also determined.