Here we report on a study of the ternary ${\text{Np}}_2{\text{PtGa}}_3$ compound. The x-ray-powder diffraction analysis reveals that the compound crystallizes in the orthorhombic CeCu${}_2$-type crystal structure (space group $Imma$) with lattice parameters $a=0.4409\left(2\right)$ nm, $b=0.7077\left(3\right)$ nm, and $c=0.7683\left(3\right)$ 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 $T_C\sim$ 26 K is determined from the magnetization and specific-heat data. An enhanced coefficient of the electronic specific heat $\gamma$ = 180 mJ/(mol at. Np K${}^2$) and a $-$lnT dependence of the electrical resistivity indicate the presence of a Kondo effect, which can be described in terms of the $S=1$ underscreened Kondo-lattice model. The estimated Kondo temperature $T_K\sim 24$ K, Hall mobility of $\sim$16.8 cm${}^2$/V s, and effective mass of $\sim$83$m_e$ are consistent with an assumption that the heavy-fermion state develops in ${\text{Np}}_2{\text{PtGa}}_3$ at low temperatures. We compare the observed properties of ${\text{Np}}_2{\text{PtGa}}_3$ to that found in ${\text{Np}}_2{\text{PdGa}}_3$ and discuss their difference in regard to change in the exchange interaction between the conduction and localized 5$f$ electrons. We have used the Fermi wave vector $k_F$ to evaluate the Rudermann-Kittel-Kasuya-Yosida (RKKY) exchange. Based on experimental data of the (U, Np)${}_2$(Pd, Pt)Ga${}_3$ compounds we suggest that the evolution of the magnetic ground states in these actinide compounds can be explained within the RKKY formalism.

• Received 20 September 2013
• Revised 4 February 2014

DOI:https://doi.org/10.1103/PhysRevB.89.054424