Kondo Lattice Models for Rare-Earth and Actinide Systems

We present here some works on the strong competition between the Kondo e ect, magnetic order and eventually spin glass or frustration e ect in anomalous rare-earth and actinide systems. First, we develop an underscreened Kondo lattice model with Sf = 1 spins for the 5f -electrons and we have recently improved it by deriving, by the Schrie er Wol transformation, a 5f -band with a nite bandwidth. The underscreened Kondo lattice model can account for properties of some uranium and neptunium compounds, like UTe, Np2PdGa3 or UCu2Si2 which have a large Curie temperature Tc of order 100 K and present also a Kondo behavior. In particular, we can account for the observed maximum of Tc under pressure in UTe and the magnetization curves of NpNiSi2 showing the occurrence of the Kondo e ect at low temperatures below Tc. Second, we have studied the properties of disordered cerium alloys like CeCuxNi1−x or CeRhxPd1−x by considering the Kondo e ect, a ferromagnetic order and a spin glass behavior described by several approaches. The van Hemmen approach gives a good explanation of the properties of cerium alloys and we are describing the magnetic glass clusters which occur in both spin glass and ferromagnetic phases. Third, we present a new description of a frustrated Kondo lattice model, which can account for the behavior under pressure or doping of some ytterbium compounds like Yb2Pd2Sn and YbAgGe.