Ferromagnetic behavior of the Kondo lattice compoundNp2PtGa3

Abstract

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 $a=0.4409(2)$ 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 ${T}_{C}\ensuremath{\sim}$ 26 K is determined from the magnetization and specific-heat data. An enhanced coefficient of the electronic specific heat $\ensuremath{\gamma}$ = 180 mJ/(mol at. Np K${}^{2}$) and a $\ensuremath{-}$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 ${T}_{K}\ensuremath{\sim}24$ K, Hall mobility of $\ensuremath{\sim}$16.8 cm${}^{2}$/V s, and effective mass of $\ensuremath{\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.