Underscreened Kondo lattice model versus underscreened Anderson lattice model: Application to uranium compounds

We present here two theoretical models, the underscreened Kondo lattice (UKL) model and the underscreened Anderson lattice (UAL) model, which are appropriate models for a description of the coexistence of Kondo effect and ferromagnetic order experimentally observed in various uranium compounds. First we discuss a Kondo-ferromagnetic diagram (KFD), obtained in the framework of the UKL model with S = 1 [N.B. Perkins, M.D. Nunez-Regueiro, J.R. Iglesias, B. Coqblin, Phys. Rev. B 76 (2007) 125101]. The KFD differs significantly from the analogous Doniach diagram for the regular Kondo lattice with S = frac(1, 2). While both the Kondo and Curie temperatures in KFD increase with an increase of the intrasite Kondo coupling, J_K, the Néel temperature in the Doniach diagram decreases with increase of J_K. Ferromagnetism is not suppressed by the Kondo interaction, but instead the coexistence between the two phenomena takes place. Then, we compare the obtained phase diagram with available experimental data. Indeed, the predicted behavior has been observed in some uranium compounds. However, there are certain difficulties in describing the pressure dependence of the Curie temperature of UTe, which firstly passes through a maximum and then decreases at higher pressures. This pressure dependence of the Curie temperature clearly indicates that the character of 5f electrons changes with pressure, becoming more and more itinerant. Then, the UKL model cannot capture the change of localization of the 5f electrons. In this case, the underscreened Anderson lattice (UAL) model, taking explicitly into account the possibility of the formation of a f-band, becomes more appropriate. It allows to treat the case of a non-integer number of f electrons and a finite f bandwidth. We present here the two-orbital UAL model and discuss some preliminary results.