Application of the underscreened Kondo lattice model to neptunium compounds

Abstract

The coexistence of Kondo e®ect and ferromagnetic order has been observed in many uranium and neptunium compounds such as UTe or Np2PdGa3. This coexistence can be described within the underscreened Anderson lattice model with two f-electrons and S = 1 spins on each site. After performing the Schrie®er-Wol® transformation on this model, we have obtained an e®ective Hamiltonian with a f-band term in addition to the Kondo interaction for S = 1 spins. The results indicate a coexistence of Kondo e®ect and ferromagnetic order, with di®erent relative values of the Kondo TK and Curie TC temperatures. We emphasize here especially the case TK < TC where there is a Kondo behavior below TC and a clear decrease of the magnetization below TK. Such a behavior has been observed in the magnetization curves of NpNiSi2 at low temperatures.