D ec 2 01 4 Ferromagnetic behavior of the Kondo lattice compound Np 2 PtGa

In this study we report the results of study of novel ternary Np2PtGa3 compound. The x-raypowder diffraction analysis reveals that the compound crystallizes in the orthorhombic CeCu2-type crystal structure (space group Imma) with lattice parameters a = 0.4409(2) nm, b = 0.7077(3) nm and c = 0.7683(3) nm at room temperature. The measurements of dc magnetization, specific heat and electron transport properties in the temperature range 1.7 300 K and in magnetic fields up to 9 T imply that this intermetallic compound belongs to a class of ferromagnetic Kondo systems. The Curie temperature of TC ∼ 26 K is determined from the magnetization and specific heat data. An enhanced coefficient of the electronic specific heat of γ = 180 mJ/(mol at. Np K) and -lnT dependence of the electrical resistivity indicate the presence of Kondo effect, which can be described in terms of the S = 1 underscreened Kondo-lattice model. The estimated Kondo temperature TK ∼ 24 K, Hall mobility of ∼ 16.8 cm/Vs and effective mass of ∼ 83 me are consistent with assumption that the heavy-fermion state develops in Np2PtGa3 at low temperatures. We compare the observed properties of Np2PtGa3 to that found in Np2PdGa3 and discuss their difference in regard to change in the exchange interaction between the conduction and localized 5f electrons. We have used the Fermi wave vector kF to evaluate the Rudermann-Kittel-Kasuya-Yosida (RKKY) exchange. Based on experimental data of the (U, Np)2(Pd, Pt)Ga3 compounds we suggest that the evolution of the magnetic ground states in these actinide compounds can be explained within the RKKY formalism.