Application of the underscreened Kondo lattice model to neptunium compounds

The coexistence of Kondo effect 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 Schrieffer-Wolff transformation on this model, we have obtained an effective Hamiltonian with a f-band term in addition to the Kondo interaction for S = 1 spins. The results indicate a coexistence of Kondo effect and ferromagnetic order, with different relative values of the Kondo T-K and Curie T-C temperatures. We emphasize here especially the case T-K < T-C where there is a Kondo behavior below T-C and a clear decrease of the magnetization below T-K. Such a behavior has been observed in the magnetization curves of NpNiSi2 at low temperatures.